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LINEAR POSITION
SENSORS
LINEAR POSITION
SENSORS
Theory and Application

DAVID S. NYCE

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright © 2004 by John Wiley & Sons, Inc. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.

Published simultaneously in Canada.

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in

any form or by any means, electronic, mechanical, photocopying, recording, scanning, or
otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright
Act, without either the prior written permission of the Publisher, or authorization through
payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222
Rosewood Drive, Danvers, MA 01923, 978-750-8400, fax 978-750-4470, or on the web at
www.copyright.com. Requests to the Publisher for permission should be addressed to the
Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030,
(201) 748-6011, fax (201) 748-6008, e-mail: permreq@wiley.com.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best
efforts in preparing this book, they make no representations or warranties with respect to the
accuracy or completeness of the contents of this book and specifically disclaim any implied
warranties of merchantability or fitness for a particular purpose. No warranty may be created
or extended by sales representatives or written sales materials. The advice and strategies
contained herein may not be suitable for your situation. You should consult with a professional
where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any
other commercial damages, including but not limited to special, incidental, consequential, or
other damages.

For general information on our other products and services please contact our Customer Care
Department with the U.S. at 877-762-2974, outside the U.S. at 317-572-3993 or fax 317-572-4002.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in
print, however, may not be available in electronic format.

Library of Congress Cataloging-in-Publication Data:

Nyce, David S.
Linear position sensors: theory and application / David S. Nyce.
p. cm.
Includes bibliographical references and index.
ISBN 0-471-23326-9 (cloth)
1. Transducers. 2. Detectors. I. Title.
TK7872.T6N93 2003
681¢.2—dc21
2003053455

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1
To Gwen, and our children Timothy, Christopher, and Megan,
whose love and support helped me complete this project
CONTENTS

PREFACE xi

1 SENSOR DEFINITIONS AND CONVENTIONS 1

1.1 Is It a Sensor or a Transducer? / 1
1.2 Position versus Displacement / 3
1.3 Absolute or Incremental Reading / 5
1.4 Contact or Contactless Sensing and Actuation / 5
1.5 Linear and Angular Configurations / 8
1.6 Application versus Sensor Technology / 8

2 SPECIFICATIONS 10
2.1 About Position Sensor Specifications / 10
2.2 Measuring Range / 10
2.3 Zero and Span / 11
2.4 Repeatability / 12
2.5 Nonlinearity / 13
2.6 Hysteresis / 19
2.7 Calibrated Accuracy / 21
2.8 Drift / 23
2.9 What Does All This about Accuracy Mean to Me? / 23
2.10 Temperature Effects / 25
vii
viii CONTENTS

2.11 Response Time / 26

2.12 Output Types / 28
2.13 Shock and Vibration / 32
2.14 EMI/EMC / 34
2.15 Power Requirements / 37
2.16 Intrinsic Safety, Explosion Proofing, and Purging / 38
2.17 Reliability / 45

3 RESISTIVE SENSING 47
3.1 Resistive Position Transducers / 47
3.2 Resistance / 48
3.3 History of Resistive Linear Position Transducers / 49
3.4 Linear Position Transducer Design / 49
3.5 Resistive Element / 52
3.6 Wiper / 54
3.7 Linear Mechanics / 55
3.8 Signal Conditioning / 55
3.9 Advantages and Disadvantages / 57
3.10 Performance Specifications / 57
3.11 Typical Performance Specifications and Applications / 60

4 CAPACITIVE SENSING 62
4.1 Capacitive Position Transducers / 62
4.2 Capacitance / 63
4.3 Dielectric Constant / 65
4.4 History of Capacitive Sensors / 66
4.5 Capacitive Position Transducer Design / 67
4.6 Electronic Circuits for Capacitive Transducers / 70
4.7 Guard Electrodes / 74
4.8 EMI/RFI / 75
4.9 Typical Performance Specifications and Applications / 76

5 INDUCTIVE SENSING 78
5.1 Inductive Position Transducers / 78
5.2 Inductance / 79
5.3 Permeability / 83
5.4 History of Inductive Sensors / 84
5.5 Inductive Position Transducer Design / 85
5.6 Coil / 86
 CONTENTS ix

5.7 Core / 89
5.8 Signal Conditioning / 89
5.9 Advantages / 92
5.10 Typical Performance Specifications and Applications / 92

6 THE LVDT 94
6.1 LVDT Position Transducers / 94
6.2 History of the LVDT / 95
6.3 LVDT Position Transducer Design / 95
6.4 Coils / 97
6.5 Core / 98
6.6 Carrier Frequency / 100
6.7 Demodulation / 101
6.8 Signal Conditioning / 104
6.9 Advantages / 106
6.10 Typical Performance Specifications and Applications / 108

7 THE HALL EFFECT 109

7.1 Hall Effect Transducers / 109
7.2 The Hall Effect / 110
7.3 History of the Hall Effect / 112
7.4 Hall Effect Position Transducer Design / 113
7.5 Hall Effect Element / 115
7.6 Electronics / 116
7.7 Linear Arrays / 118
7.8 Advantages / 119
7.9 Typical Performance Specifications and Applications / 120

8 MAGNETORESISTIVE SENSING 122

8.1 Magnetoresistive Transducers / 122
8.2 Magnetoresistance / 123
8.3 History of Magnetoresistive Sensors / 129
8.4 Magnetoresistive Position Transducer Design / 130
8.5 Magnetoresistive Element / 131
8.6 Linear Arrays / 131
8.7 Electronics / 133
8.8 Advantages / 134
8.9 Typical Performance Specifications and Applications / 134
x CONTENTS

9 MAGNETOSTRICTIVE SENSING 136

9.1 Magnetostrictive Transducers / 136
9.2 Magnetostriction / 137
9.3 History of Magnetostrictive Sensors / 139
9.4 Magnetostrictive Position Transducer Design / 140
9.5 Waveguide / 140
9.6 Position Magnet / 142
9.7 Pickup Devices / 144
9.8 Damp / 145
9.9 Electronics / 145
9.10 Advantages / 147
9.11 Typical Performance Specifications / 148
9.12 Application / 149

10 ENCODERS 151
10.1 Linear Encoders / 151
10.2 History of Encoders / 151
10.3 Construction / 152
10.4 Absolute versus Incremental Encoders / 153
10.5 Optical Encoders / 154
10.6 Magnetic Encoders / 155
10.7 Quadrature / 156
10.8 Binary versus Gray Code / 157
10.9 Electronics / 158
10.10 Advantages / 159
10.11 Typical Performance Specification and Applications / 160

REFERENCES 162
INDEX 165
PREFACE

Society and industry worldwide continue to increase their reliance on the

availability of accurate and current measurement information. Timely access
to this information is critical to effectively meet the indication and control
requirements of industrial processes, manufacturing equipment, household
appliances, onboard automotive systems, and consumer products. A variety of
technologies are used to address the specific sensing parameters and configu-
rations needed to meet these requirements.
Sensors are used in cars to measure many safety- and performance-related
parameters, including throttle position, temperature, composition of the
exhaust gas, suspension height, pedal position, transmission gear position, and
vehicle acceleration. In clothes-washing machines, sensors measure water level
and temperature, load size, and drum position variation. Industrial process
machinery requires the measurement of position, velocity, and acceleration, in
addition to chemical composition, process pressure, temperature, and so on.
Position measurement comprises a large portion of the worldwide require-
ment for sensors. In this book we explain the theory and application of the
technologies used in sensors and transducers for the measurement of linear
position.
There is often some hesitation in selecting the proper word, sensor or trans-
ducer, since the meanings of the terms are somewhat overlapping in normal
use. In Chapter 1 we present working definitions of these and other, some-
times confusing, terms used in the field of sensing technology. In Chapter 2 we
explain how the performance of linear position transducers is specified. In the
remaining chapters we present the theory supporting an understanding of the
prominent technologies in use in linear position transducer products. Appli-
cation guidance and examples are included.
xi
xii PREFACE

The following are the owners of the trademarks as noted in the book:

CANbus Robert Bosch GmbH, Stuttgart, Germany

HART HART Communications Foundation, Austin, TX
Lincoder Stegmann Corporation, Germany
NiSpan C Huntington Alloys, Incorporated
Permalloy B&D Industrial Mining Services, Inc.
Profibus PROFIBUS International
Ryton Phillips Petroleum Company
SSI Stegmann Corportation, Germany
Temposonics MTS Systems Corporation, Eden Prairie, MN
Terfenol D Extrema Products, Inc., Ames, IA
Torlon Amoco Performance Products, Inc.
CHAPTER 1

SENSOR DEFINITIONS
AND CONVENTIONS

1.1 IS IT A SENSOR OR A TRANSDUCER?

A transducer is generally defined as a device that converts a signal from one

physical form to a corresponding signal having a different physical form [29,
p. 2]. Energy can be converted from one form into another for the purpose
of transmitting power or information. Mechanical energy can be converted
into electrical energy, or one form of mechanical energy can be converted
into another form of mechanical energy. Examples of transducers include a
loudspeaker, which converts an electrical input into an audio wave output; a
microphone, which converts an audio wave input into an electrical output; and
a stepper motor, which converts an electrical input into a rotary position
change.
A sensor is generally defined as an input device that provides a usable
output in response to a specific physical quantity input. The physical quantity
input that is to be measured, called the measurand, affects the sensor in a way
that causes a response represented in the output. The output of many modern
sensors is an electrical signal, but alternatively, could be a motion, pressure,
flow, or other usable type of output. Some examples of sensors include a ther-
mocouple pair, which converts a temperature difference into an electrical
output; a pressure sensing diaphragm, which converts a fluid pressure into a
force or position change; and a linear variable differential transformer
(LVDT), which converts a position into an electrical output.

Linear Position Sensors: Theory and Application, by David S. Nyce

ISBN 0-471-23326-9 Copyright © 2004 John Wiley & Sons, Inc.

1
2 SENSOR DEFINITIONS AND CONVENTIONS

Obviously, according to these definitions, a transducer can sometimes be a

sensor, and vice versa. For example, a microphone fits the description of both
a transducer and a sensor. This can be confusing, and many specialized terms
are used in particular areas of measurement. (An audio engineer would
seldom refer to a microphone as a sensor, preferring to call it a transducer.)
Although the general term transducer refers to both input and output devices,
in this book we are concerned only with sensing devices. Accordingly, we will
use the term transducer to signify an input transducer (unless specified as an
output transducer).
So, for the purpose of understanding sensors and transducers in this book,
we will define these terms more specifically as they are used in developing
sensors for industrial and manufacturing products, as follows:

An input transducer produces an electrical output, which is representative of

the input measurand. Its output is conditioned and ready for use by the receiv-
ing electronics.

The receiving electronics can be an indicator, controller, computer, program-

mable logic controller, or other. The terms input transducer and transducer can
be used interchangeably, as we do in this book.

A sensor is an input device that provides a usable output in response to the

input measurand.

The sensing part of a transducer can also be called the sensing element, primary
transducer, or primary detector. A sensor is often one of the components of a
transducer.
Sometimes, common usage will have to override our theoretical defini-
tion in order to result in clear communication among engineers in a
specific industry. The author has found, for instance, that automotive engi-
neers refer to any measuring device providing information to the onboard
controller, as a sensor. In the case of a position measurement, this includes
the combination of sensing element, conditioning electronics, power supply,
and so on. That is, the term sensor is used to name exactly what our defin-
ition strives to call a transducer. In automotive terminology, the word
sender is also commonly used to name a sensor or transducer. In any case,
we rely on the definition presented here, because it applies to most industrial
uses.
An example of a sensor as part of a transducer may help the reader under-
stand our definition. The metal diaphragm shown in Figure 1.1a is a sensor
that changes pressure into a linear motion. The linear motion can be changed
into an electrical signal by an LVDT, as in Figure 1.1b. The combination of the
diaphragm, LVDT, and signal conditioning electronics would comprise a pres-
sure transducer. A pressure transducer of this description, designed by the
author, is shown in Figure 1.2.
 POSITION VERSUS DISPLACEMENT 3

Housing
Metal
diaphragm

LVDT
Signal-conditioning
Actuator rod
electronics
Output

Pressure
Linear
motion

Core

(a) (b)
Figure 1.1 (a) The circular diaphragm (shown edgewise, cutaway) changes pressure
into linear motion. (b) An LVDT changes linear motion to an electrical signal, com-
prising a transducer with the addition of signal-conditioning electronics.

Zero and span

adjustment cap
Printed Pressure tube
circuit
Cable Housing cover
Core
LVDT
Cover supports
Reference pressure
Input pressure
port
port

Pressure cavity Pressure

capsule
Housing base
Figure 1.2 Commercially available pressure transducer according to Figure 1.1.
Cutaway view with diaphragm in the lower cavity, and LVDT, core, and signal-
conditioning electronics in the upper cavity.

1.2 POSITION VERSUS DISPLACEMENT

Since linear position sensors and transducers are presented in this work and
many manufacturers confuse the terms position and displacement, the differ-
ence between position and displacement should be understood by the reader.
4 SENSOR DEFINITIONS AND CONVENTIONS

Permanent
magnet

Measured
position
Measuring
range
Figure 1.3 Magnetostrictive linear position transducer with position magnet. (Cour-
tesy of MTS Systems Corporation.)

Mounting flange
End caps (2)

Encoder scale
inside housing Read head
Cable
Figure 1.4 Incremental magnetic linear encoder.

A position transducer measures the distance between a reference point and

the present location of the target. The word target is used in this case to mean
that element of which the position or displacement is to be determined. The
reference point can be one end, the face of a flange, or a mark on the body of
the position transducer (such as a fixed reference datum in an absolute trans-
ducer), or it can be a programmable reference datum. As an example, consider
Figure 1.3, which shows the components of the measuring range of a magne-
tostrictive absolute linear position transducer. This transducer measures the
location of a permanent magnet with reference to a fixed point on the trans-
ducer. (More details on the magnetostrictive position transducer are presented
in Chapter 9.)
Conversely, a displacement transducer measures the distance between the
present position of the target and the position recorded previously. An
example of this would be an incremental magnetic encoder (see Figure 1.4).
Position transducers can be used as displacement transducers by adding cir-
cuitry to remember the previous position and subtract the new position, yield-
ing the difference as the displacement. Alternatively, the data from a position
transducer may be recorded into memory by a microcontroller, and differ-
ences calculated as needed to indicate displacement. Unfortunately, and con-
 CONTACT OR CONTACTLESS SENSING AND ACTUATION 5

stituting another assault against clarity, it is common for many manufacturers

of position transducers to call their products displacement transducers.
To summarize, position refers to a measurement with respect to a constant
reference datum; displacement is a relative measurement.

1.3 ABSOLUTE OR INCREMENTAL READING

An absolute-reading position transducer indicates the measurand with respect

to a constant datum. This reference datum is usually one end, the face of a
flange, or a mark on the body of a position transducer. For example, an
absolute linear position transducer may indicate the number of millimeters
from one end of the sensor, or a datum mark, to the location of the target (the
item to be measured by the transducer). If power is interrupted, or the posi-
tion changes repeatedly, the indication when normal operation is restored will
still be the number of millimeters from one end of the sensor, or a datum mark,
to the location of the target. If the operation of the transducer is disturbed by
an external influence, such as by an especially strong burst of electromagnetic
interference (EMI), the correct reading will be restored once normal operat-
ing conditions return.
To the contrary, an incremental-reading transducer indicates only the
changes in the measurand as they occur. An electronic circuit is used to keep
track of the sum of these changes (the count) since the last time that a reading
was recorded and the count was zeroed. If the count is lost due to a power
interruption, or the sensing element is moved during power-down, the count
when normal operating conditions are restored will not represent the present
magnitude of the measurand. For example, if an incremental encoder is first
zeroed, then moved upscale 25 counts, followed by moving downscale 5 counts,
the resulting position would be represented by a count of 20. If there are 1000
counts per millimeter, the displacement is 0.02 mm. If power is lost and
regained, the position would probably be reported as 0.00 mm. Also, if the
count is corrupted by an especially strong burst of EMI, the incorrect count
will remain when normal operation is restored.

1.4 CONTACT OR CONTACTLESS SENSING AND ACTUATION

One classification of a position transducer pertains to whether it utilizes a

contact or noncontact (also called contactless) type of sensing element. With
contactless sensing, another aspect is whether or not the transducer also uses
contactless actuation. In a contact type of linear position sensor, the device
making the conversion between the measurand and the sensor output
incorporates a sliding electrical and/or mechanical contact. The primary
example is the linear potentiometer, (see Figure 1.5). The actuator rod is
connected internally to a wiper arm. The wiper arm incorporates one or more
Other documents randomly have
different content
 This is a smaller animal than the common or Virginian Opossum,
but its tail is long in proportion to its body. It is the South American
representative of its larger fellow species, and is found over a very
wide extent of country. It was noticed by the celebrated naturalist
D’Azara in Paraguay; Mr. Darwin found it at Maldonado, La Plata; and
specimens have been obtained from the Brazils, Santa Fé de Bogota,
and Bolivia. This is because it is not entirely a forest animal, but is
found occasionally in the open country. It may be distinguished from
the common Opossum by three distinct black marks on its head, by
its large tail, one-third of which is covered with fur like that on the
body. The rest of this important member is scaly, with small hairs
springing from between, the scales being black in the second third,
and white at the tip in colour. The habits of this Opossum are
nocturnal, and it lies concealed by day in burrows in the ground or in
thickets. At night it climbs trees to feed upon fruits and birds’ eggs.
It will chase and catch sleeping birds, and suck their blood like a
Weasel.

THE CRAB-EATING OPOSSUM.[125]

A small Opossum, with a long black tail tipped with white, and a
dull-coloured fur to its body, lives in Brazil and Guiana, and has a
very omnivorous disposition. Preferring swampy situations, it lives
mostly on the trees, hunts small birds and insects, and even catches
a reptile now and then, but its fondness for the Crustacea of the
swamps is proverbial, and hence its name of Crab-eater.
Another species is interesting from being found in the part of
California which adjoins Mexico. The Short-headed Opossum also
belongs to this group, and is from the same locality. Besides these,
there are several smaller pouch-bearing Opossums, without the long
hair of those just mentioned, and they are from Brazil, Guiana, and
Surinam—for instance, the Quica, the Naked-tailed, and the Four
Spotted kinds. The Philander Opossum is a bird-hunter, and lives in
Surinam.
 The next group of Opossums have no pouch, but there may be
folds of the skin protecting the mammæ.

THE THICK-TAILED OPOSSUM.[126]

As its name implies, this pouchless Opossum has a very thick tail.
Moreover, it has smaller ears than the other Opossums, and has a
short head and short legs. The fur is made up of harsh hairs, which
are close to the body, and there is but little under fur. Its colour is
yellow-brown, but the eye and muzzle are brownish, and the tail,
with the terminal two-thirds, is black, with the exception of a small
white spot at the end. It inhabits Brazil and Paraguay, and extends
southwards to the River Plate. One of the Opossums was kept by
D’Azara, who found it quiet, tame, and stupid; but having been fed
on raw meat, and a parrot happening to come too close, it killed the
bird in a moment. There are folds of skin in the lower part of the
abdomen, but no pouch, and there are six mammæ.
Another of the Opossums is called Merian’s Opossum, or
Didelphys dorsigera, and it inhabits Surinam. It was described by
Madame Merian in 1717, who represented it in her great book on
insects with its young clustered on its back and hanging on to the
mother’s tail, which was curved over its back, with their little tails.
 MERIAN’S OPOSSUM.

It is very curious that the young of these pouchless Opossums

should resemble those of the whole order in being comparatively
little advanced in their development at the time of their birth. The
young are at first strongly attached to the teats of the mother, and
when they are sufficiently strong and grown to leave them,
occasionally she takes them off from the nipples and places them on
her back. Here they cling on with their tails to hers. Hence the name
of back-bearing, or Dorsigera, which is given to this kind.
 YAPOCK.

It was at first supposed that this method of carrying the young

was restricted to this species, but subsequent experience has shown
that several kinds do the same thing.
Two or three other species of Opossum are interesting from their
small size and habits. Thus the Murina Opossum (Didelphys murina),
with a very long tail, inhabits Guiana, Brazil, Peru, and Mexico. The
body is about five inches in length, and the tail is either slightly
longer or about the same. Yet this little thing attacks birds and
insects; it burrows in the ground, and climbs trees to get its insect
food.
The Elegant Opossum (Didelphys elegans), of Chili, is still smaller
than the last, and frequents the thickets growing on the rocky hills
near Valparaiso. They are numerous, or were so when Mr. Darwin
observed them, and are easily caught in traps baited with cheese or
meat. The tail appeared to be rarely, if at all, used as a prehensile
organ; yet they could run up trees with some degree of facility. It is
an interesting fact that some of the smallest Opossums prey upon
Lizards and Snakes as large, and even heavier, than themselves.
The last section of the Opossums contains the Water Opossum.

THE YAPOCK.[127]

This animal has a perfect pouch, and has large hind feet, the
toes of which are united by a web. The fore feet are moderate-sized,
and the pisiform bone is unusually long. Its habits are aquatic. The
Yapock has large naked ears, and a long, almost naked, tail, and is
altogether rather larger than the common Rat. Its method of life is
very much the same as that of the Otter. It is a good diver, and
feeds upon crustaceous and other aquatic animals. It is a native of
Guiana and Brazil.

The Marsupial animals assume the general shape and habits of

many orders of Mammalia which have no marsupium, and which live
in the other great natural history provinces. Thus there are Marsupial
animals like Dogs, Rats, Squirrels, Flying Squirrels, Deer, &c. They
have, therefore, many methods of life as a group, and, as might be
expected, the brain and nervous system present many differences in
them. In all, the front lobes of the brain which deal with the sense of
smell are very large, and in some, such as in the Carnivorous
Marsupials, they are exposed, and not covered by the main mass of
the brain. In the Kangaroos, however, these olfactory lobes are
hidden more or less. These last also have well-marked convolutions
on the brain which are nearly wanting in those first mentioned.
The Marsupial animals just considered have been classified to a
certain extent during their descriptions, but it is necessary to
recapitulate. They are arranged in groups of genera or species, or
into families. They are as follows:—

ORDER MARSUPIALIA.—SUB-ORDER MARSUPIATA.

Genus Macropus Kangaroos.[128]

„ Dendrolagus Tree Kangaroos.
Family MACROPODIDÆ „ Hypsiprymnus Potoroos.
The
„ Hypsiprymnodon
Hypsiprymnodon.
„ PHASCOLOMYIDÆ „ Phascolomys The Wombat.
„ Phascolarctus The Koala.
The Cuscus.
Dormouse
„ Phalangista
„ PHALANGISTIDÆ Phalanger.
Phalangers.
„ Petaurus Flying Phalangers.
„ Tarsipes Tarsipes.
„ Perameles Bandicoots.
„ PERAMELIDÆ
„ Chœropus Chœropus.
„ Myrmecobius Ant-eaters.
„ Phascogale Phascogale.
„ DASYURIDÆ „ Dasyurus Dasyures.
Dog-
„ Thylacinus
headed Thylacinus.
„ Didelphys Opossum.
„ DIDELPHIDÆ
„ Chironectes Yapock.

The Macropodidæ, Phalangistidæ, Peramelidæ, and Dasyuridæ

are found living somewhere or other in the Australian distributional
province, which includes the mainland, Tasmania to the south, and
the Molucca and Arru Islands to the north, bounded by the Straits of
Lombok, and Celebes, New Guinea, New Ireland, Timor, Amboyna,
Banda, and Waigeoe. Each family is not represented fully, however,
in all the remarkably separated divisions of the province. Thus the
genera Macropus and Dendrolagus of the first family, Petaurus and
Phalangista of the third, Perameles of the fourth, and Phascogale of
the Dasyuridæ have been found in New Guinea; but in other islands,
such as Celebes, and in those from Lombok to Timor, the genus
Cuscus alone is represented. In the Moluccas, Cuscus and the genus
Petaurus are found. In Van Diemen’s Land about one-half of the
species are peculiar to the island, and the remainder are found also
on the eastern districts of the mainland. It has Kangaroos, Potoroos,
Wombats, Phalangers, Bandicoots, and three out of the four genera
of Dasyuridæ. Western Australia, which is such a remarkable
botanical province, and is so separated by desert and sand from the
east, has numerous Kangaroos, Potoroos, Phalangers, Bandicoots,
Phascogales, Dasyures; and, in common with South Australia, a
Chœropus, whilst the genus Tarsipes is peculiar to it. The Wombat is
found in Van Diemen’s Land and some of the islands in Bass Strait. It
is found in the south and east of the mainland of Australia, but not
to the west and north. Mr. Waterhouse notices that the Marsupials of
the eastern districts are for the most part distinct from those of the
opposite side of the continent, there being, when his great work,
which has been so constantly referred to in this description, was
written, but eight species out of upwards of sixty inhabiting the two
provinces. South Australia is the habitat of more common species
than elsewhere. The northern part of Australia has more species
peculiar to it than the other divisions, and some of its Dasyuridæ
especially, and species of Cuscus also, are found in the Arru and
other islands to the north. The metropolis of the sub-genus Cuscus
is in the Moluccas, where two species are widely distributed, or one
is restricted to certain islands.
The other divisions of the genus are represented by the Vulpine
Phalanger, an animal with long loose fur, which inhabits New South
Wales, Western Australia, and North Australia; by Cook’s Phalanger,
of New South Wales and Van Diemen’s Land. The genus Perameles,
the Bandicoots, has species in Van Diemen’s Land, Australia, New
Guinea, and in the Arru Islands, and the genus Petaurus has a
corresponding distribution. The Didelphidæ are found in the United
States, California, Mexico, Peru, Guiana, Brazil, Paraguay, Banda
Oriental, and Chili; and Brazil is the country where they abound the
most in species and individuals, the number diminishing to the north
and south.
The Marsupials have a great ancestry, and some of them lived
when the continents and oceans of the earth were in very different
relative positions to those they now occupy. Indeed, it is most
probable that the fossil remains of the most ancient mammal belong
to this order. There is a small double-fanged molar tooth of a
mammal which was found by Plieninger, in 1847, contained in a
jumble of shells and of the remains of reptiles and fishes in strata
beneath the Lias formation of Diegerloch, near Stuttgart. It and
another which was discovered close by, by the same professor,
belonged to animals which were dead when this topmost stratum of
the Trias, immediately beneath the Lias, was being formed. They are
Triassic in age, therefore, and they somewhat resemble the back
teeth of a fossil which was found subsequently in the Purbeck strata
of England, and which evidently belonged to a Marsupial more or
less resembling the existing Kangaroo-Rats or Potoroos, of the genus
Hypsiprymnus. Later on, Professor W. Boyd Dawkins, F.R.S.,
discovered a small tooth belonging to the same extinct genus as that
which included Plieninger’s fossil, namely, Microlestes; and its
resemblance to one of Hypsiprymnus is even greater. Its position
was high up in the Trias of Watchet in Somersetshire. Mr. Charles
Moore, of Bath, had previously found many specimens of teeth of
the same family in a fissure, down which they had been washed by
the Triassic sea.
A lower jaw of a small Mammal was found in the Trias of North
America by Emmons; and it has on one side three incisors, one long
canine, then a diastema, three premolars, and seven molars with
three points. It is therefore one of the Myrmecobius group.
 After the age of the Trias, when there was much continuous land
surface, Europe was broken up into a coral island tract, during the
age of the collection of the Jurassic deposits. The islands were
tenanted by many small Marsupials, four species of which have been
discovered in the deposits of Stonesfield slate at the bottom of the
Great Oolite. They belong to the extinct genera Amphitherium,
Phascolotherium, and Stereognathus, and the first somewhat
resembled the Myrmecobius of recent times; but all that can be said
is that they belonged to Marsupial animals. Piled on the Stonesfield
slates are many hundred feet of strata, and high up amongst them,
in the Swanage and Purbeck districts, are deposits in which Messrs.
Brodie and Beckles have found portions of the skeletons of
numerous insectivorous Marsupials, of which the genera
Spalacotherium, Plagiaulax, Triconodon, and Galestes are the most
important. They were small, as a rule, and there has been much
debate regarding their affinities with modern insectivorous forms,
and they are still surrounded with doubt.
The appearance of the Mammalia without pouches took place in
the Eocene age, and in the Old and New World, and
contemporaneously with them lived in France a kind of Opossum,
some of whose bones were found in the strata of Montmartre, near
Paris; and in later Tertiary strata other relics have been found. These
are the only instances of a fossil Didelphid occurring out of the New
World; and there, where the Opossums are now characteristic
animals, they were present in the last geological age, for in the
Brazilian latest deposits remains of several species of Didelphys have
been found. Remains of these fossil Opossums have been found in
the North American Pliocene deposits. The more ancient deposits of
Australia have not yielded the remains of any of the animals which
are now so peculiar to the province, but in the bone caves of the
Wellington Valley, some two hundred and ten miles west of Sydney,
Sir Thomas Mitchell discovered a mass of bones, forming a breccia
with limestone, which contained numerous and most interesting
Marsupial remains. In deposits of the same late age, and in bogs
and gravels in Queensland, other remains were found. They were
described by Sir R. Owen in one of his greatest works, and they
belong to the Australian families of Marsupials, and not to the
American Didelphidæ. As was usual elsewhere before the
appearance of man on the earth, and contemporaneously with him
for awhile, many of the kinds which resemble more or less those
now living, or would be classified in the same family, and perhaps in
the same genus, are gigantic. Owen distinguished among the bones
those of large fossil Marsupials which belong to the Macropodidæ,
and which may be arranged as subdivisions of the genus Macropus
or Kangaroos, and of a powerful creature called Thylacoleo, or
Pouched Lion, which must be admitted as a new section of the
Macropodidæ, and whose habits were probably carnivorous,
although there is much diversity of opinion on the subject, some of
the most distinguished anatomists believing the creature to have
been of an innocent disposition, although appearances are much
against it. It is more closely allied to Plagiaulax, of the English
Purbeck beds, than to any other form, and they well fit in between
the genera Macropus and Hypsiprymnus.
A huge Marsupial, with a skull three feet in length, with teeth, in
front especially, on the Kangaroo plan, and with longer fore limbs
and shorter hind ones than the last-named animal, was described by
Owen. The pelvis, however, has but two sacral vertebræ, and its ilio-
pubic process would ally it with the Macropodidæ. This Diprotodon
was an herbivorous animal, and was of the size of a Rhinoceros. This
great Marsupial had fore limbs which possessed the power of
rotation, and it was not without some characters which are seen
amongst the Wombats. It appears to have had a great range, for its
remains have been found in the caverns in the Wellington Valley, at
Welcome Springs, South Australia, Hergolt’s Springs, 500 miles north
of Adelaide, near Melbourne, in the valley of the Condamine River,
and widely over Queensland. A slightly smaller animal, called the
Nototherium, also existed with the larger one.
The species of this genus have no lower incisive tusks, and a
very short chin; the angle of the jaw is curved inwards, and there
were only four molar teeth on each side in both jaws, and they were
with two strong roots or fangs. It was probably one of the
Macropodidæ. Others of this family are allied to Dendrolagus, and
form the genera Protemnodon and Sthenurus. The Wombat was
represented in the age of the great Marsupials; and both large and
small species, one being of the size of the Tapir, have been described
from bones and teeth which were found in the cave deposits of
Australia. Remains of a Marsupial animal, probably of the Vulpine
Phalanger, were found in the same caves, as were also some
referable to the genus Perameles, or Bandicoots, and to the
Potoroos. Several fossil species of the family Dasyuridæ have been
found in the Australian caves, and one of them is referable to a
section of the genus Dasyurus, which at present is restricted to Van
Diemen’s Land, it being somewhat like Dasyurus ursinus; moreover,
probably, there was a species of Thylacinus present also. So far as is
known from the researches of Owen amongst this wonderful cave
fauna, no members of the family Didelphidæ occur there. They were
American then, as they are now.
 CHAPTER IV.
SUB-ORDER—MONOTREMATA.[129]

THE PORCUPINE OR LONG-SPINED ECHIDNA AND

DUCK-BILLED PLATYPUS.[130]
Why the Monotremata are formed into a Sub-order—The lowest of the
Mammalian Class—THE PORCUPINE OR LONG-SPINED ECHIDNA—An Ant-
eater, but not an Edentate—Its Correct Name—Description of the
Animal—Habits and Disposition—Manner of Using the Tongue—Where
it is Found—Anatomical Features: Skull, Brain, Marsupial Bones—The
Young—Species of Van Diemen’s Land and New Guinea—THE WATER-
MOLE, OR DUCK-BILLED PLATYPUS—The most Bird-like Mammal—Various
Names—Description—Their Appearance and Movements in Water—
Their Burrows—Habits of an Individual kept in Confinement—Used by
Natives as Food—How they are Captured—The Young—A Family in
Captivity—The Snout—Jaws—Teeth—Tongue—Fore and Hind Feet—
Heel—Spur—The Shoulder Girdle—Breastbone—Concluding Remarks
on the Sub-orders—Postscript on the Monotremes.

THE PORCUPINE OR LONG-SPINED ECHIDNA.

THIS animal is the first example of some Marsupial beasts which are
separated into a sub-order, because, in addition to the marsupial
bones, there are some internal points of construction which are
more bird- and lizard-like than those of the Kangaroo tribes. It
contains animals which are the lowest of the Mammalian class, and
are found only in the Australian natural history province. The
Porcupine Ant-eater, as its name implies, has somewhat the shape of
a Hedgehog or Porcupine, and it is fond of burrowing with its
peculiar limbs, as well as of eating Ants with the assistance of its
long tongue. But its internal anatomy and the construction of the
skeleton differ from those of the true Ant-eaters, which belong to
the order Edentata. It was called Ant-eater by its first describer
(Shaw) in 1792, but a few years afterwards it was decided to belong
to the same group as an animal about to be described—the Duck-
billed Platypus, or Water Mole—and Cuvier, whilst believing that they
both belonged to a peculiar order, separated this false Ant-eater
from the Water Mole as a species and genus. He called this
Hedgehog-like creature Echidna, from the presence of a spur on the
heel, which is perforated, and which was erroneously supposed to be
poisonous, like the fang of a Viper (Ἔχιδνα). The correct name is the
Long-spined Echidna, or the Porcupine Echidna (Echidna hystrix).

PELVIC ARCH OF THE ECHIDNA.

(a a) Marsupial Bones.

The creature greatly resembles a Hedgehog with a very long

snout, at first sight, but a slight examination will show that it differs
much from the insect-eating and spiny little Hystrix. The Echidna is
about a foot in length, and the upper part of its short body is
covered with strong spines, and the rest is hairy, the front of the
head, and the long, slender, and tapering snout being naked. The
legs are short and strong, and the five toes of the fore leg have
large and strong claws. This is in order to permit the creature to
bury itself in sand and soft earth quickly, and this operation is
assisted by a broad and rounded nail on the inner toe of the hind
foot and by large claws on the other toes, and especially by a long
nail to the second toe. A very long and flexible tongue enables the
creature to catch prey. There are no teeth. The skull, when the skin
and flesh have been removed, has a very pear-like appearance. It is
a great burrower, and manages to get out of the way of observers as
soon as is possible, for working actively with its strong limbs and
claws, it pokes its snout into the earth and soon gets out of view.
Ants are its favourite food, and they are captured in the same way
as by the Great Ant-eaters belonging to the Edentata: for in both
there is a long slimy tongue, which can be poked far out of the
mouth into Ants’ nests. The saliva required to make the tongue
sticky comes from large glands under the lower jaw from the ear on
to the fore part of the chest. When the Ants have collected on the
sticky tongue it is taken into the mouth, and they are swallowed.
The absence of teeth is made up by the presence of horny spines on
the palate and tongue, which look backwards, and these crush and
direct the food to the throat. It is an apathetic and stupid animal,
and usually tries to get out of the light, and it will lie and roll itself
up, but not so successfully as a Hedgehog. One of the first which
was seen was attacked by the Dogs of two of the travellers, Bass
and Flinders, whose names are so familiar from places having been
named after them in Australia. The Dogs did not come off victorious,
for the new animal burrowed in the loose sand, but not head
foremost; it sank itself directly downwards, and left its prickly back
just on a level with the surface.
An Echidna was watched, so that the manner in which it could
use its tongue was observed. Ants could not be had, but a diet of
chopped-up eggs, liver, and meat was readily received, and it was
noticed that the tongue was used in the same manner as that of the
Chameleon, by simple protrusion and bringing in, and also as a
mower moves his scythe, it being curved sideways, and the food
swept into the mouth. The Echidna is fond of water and milk, which
are licked up by a rapid putting out and drawing in of the long
tongue.

PORCUPINE ECHIDNA. (After Gervais.)

Gerard Krefft says that they are usually found in mountain

ranges, and among rocks in the Lower Murray district. He failed to
feed them on Ants and their eggs. On hen’s eggs they fed for a time,
and liked bread-and-milk. He has reason to believe that they live on
grass also, as those whose stomachs and intestines he examined
had fed on herbs and grasses. The spur on the heel is not used as a
weapon of offence.
It inhabits Australia, and has been found as far north as the
Bellenden Plains, Queensland, about 18° south latitude. A specimen
has also been captured at Cape York, and others at Plain Creek,
Queensland. It is not found in Van Diemen’s Land.
With regard to the anatomy of the Echidna, it may be said that
the long muzzle and the very slender lower jaw give the skull a bird-
like look which is increased by the swollen and ball shape of the
brain-case. The bones of the skull remain imperfectly united for
some time, and then they are united by plain lines of junction, and
not by jagged sutures. The shoulder and the bones of the upper part
of the chest resemble those of the Water Mole, and will be noticed in
its description. The brain of the Echidna weighs about one-fiftieth of
the whole body, and the hemispheres do not conceal the cerebellum.
There are three convolutions behind, and in front of them is a large
one bent on itself, and on its outside are some oblique folds. The
sense of smell, evidently acute in the Echidna, is assisted by a large
development of the olfactory lobes of the brain and their nerves.
The Echidnas have large marsupial bones. They have not a true
pouch, but only a rudimentary one, or rather an infolding of the skin,
during the breeding season, in the female. The orifices of the teats
are situated beneath the level of the skin, and inverted; and as the
surrounding parts swell under the influence of suckling, there is a
little cavity made, at the bottom of which are the so-called nipples.
They are really little depressions with hair around them. The young
Echidnas are placed in this temporary cavity by the mother, and help
themselves by placing their snouts in the small depressions leading
to the milk gland. Captain Armit says that some force is required to
get the young out of the pouch, and that there is probably a
muscular ring to it. They are at first very small. When about a month
or so old, the hinder parts of the young may be seen sticking out of
the region of the fold, and at three months the body may be
observed, the animal still adhering by its snout. When the prickles of
the young begin to harden, the old one turns them out into the
world. (But see Postscript, p. 234.)
A short-spined Echidna (Echidna setosa) inhabits Van Diemen’s
Land, whose hair is sufficiently long to hide most of the spines, but
little is known regarding its habits. Quoy and Gaimard, two French
naturalists, kept one for a month, and it took no food, but after that
time it began to lap and to eat a mixture of flour, sugar, and water. It
burrowed very rapidly, and got to the bottom of a large can full of
earth and plants in the course of a few minutes, and it was assisted
in this by its snout.

MOUTH (A) AND NOSE-SNOUT (B) OF

ECHIDNA.

A species of Echidna has been found in the north of the Island of

New Guinea, at the Mont des Karous and Mount Arfak, at an altitude
in the first place of 1,150 yards. It likes the rocky broken ground,
and is unknown on the sea coast. The natives call it “Nokdiak,” and
hunt it for the flesh. As the animal burrows well, the natives dig
down about a yard in different places, and generally cut across one
of the underground runs. It has been described, and has been
named after the explorer, M. Brujn. It is more robust and larger than
the species from Australia and Van Diemen’s Land, has a very long
snout—three times the length of the head—a short tail, and is black
in colour with white points. The fur is plentiful, and like velvet, whilst
the spines are scanty, and about midway in strength between those
of the two Australian kinds. The number of nails on the fore and
hind feet is singular in this New Guinea Echidna, for there are three
on each instead of five. The tongue of the species is longer and
more spiny; moreover, the number of vertebræ differs in this new
kind. There are seventeen dorsal instead of fifteen, and there is one
caudal more than in the others. The spiny pimples on the tongue
and palate, so well developed in this Echidna, have tempted
Professor Gervais to include it in a new genus, Acanthoglossus; but
it is as well to retain the old name, so that the creature is called
Echidna Brujnii. Another species has been found in the south of New
Guinea, at Port Moresby, which is distinguished chiefly by the long,
thin, cylindrical form of the quills, and the stiff, flat, hair-like bristles
on the face. The tint of the flattish bristles covering all the body and
limbs, except the back, is brown; on the back are long cylindrical
spines, some white and others black. There are five claws to each
foot, and the second hind toe is said to be the largest. The fore
limbs are short, stout, and strong. It has been named Echidna
Lawesii (Ramsay), after its discoverer. All these animals can roll
themselves up.

THE WATER MOLE, OR DUCK-BILLED PLATYPUS.[131]

Like most of the other objects of natural history found in
Australia and the neighbouring islands, the Water Mole is very
singular in its construction, nature, and habits. It is of all animals
that suckle their young the most like a bird, and it really deserves
the title, from its external appearance of half beast, half bird. As its
shape and method of life are peculiar, it has received several names,
such as the Water Mole, the Flat-footed, Duck-billed Platypus, the
Bird-beaked quadruped, and the Paradoxical Bird-beaked animal. It
is very fond of the water and also of burrowing in the ground, and,
of course, is admirably adapted for these pursuits: hence its
construction relates to them to a certain extent, and also to that of
the animals of which it was, as it were, a continuation in the scheme
of nature.
The Ornithorhynchus anatinus has a rather flat body of about
eighteen inches in length, and the head and snout greatly resemble
those of a Duck, whilst the tail is short, broad, and flat, and
resembles that of a small Beaver, but is shorter. The feet are webbed
and flat, and the greater part of the creature is covered with a short
dense fur of a dusky brown colour, darker on the upper and paler on
the under parts of the body. A slight examination of the habits of the
animal will explain the necessity for observing it a little more closely.
Mr. Bennett describes his first interview with one shortly after his
arrival in Australia. He writes: “We soon came to a tranquil part of
the river, such as the colonists call a ‘pond,’ on the surface of which
numerous aquatic plants grew. It is in places of this description that
the Water Moles are most commonly seen, seeking their food among
the aquatic plants, whilst the steep and shaded banks afford them
excellent situations for excavating their burrows. We remained
stationary on the banks, waiting their appearance with some degree
of impatience, and it was not long before my companion quietly
directed my attention to one of these animals, paddling on the
surface of the water, not far distant from the bank on which we were
then standing. In such circumstances they may be readily recognised
by their dark bodies, just seen level with the surface, above which
the head is slightly raised, and by the circles made in the water
round them by their paddling action. On seeing them, the spectator
must remain perfectly stationary, as the slightest noise or movement
of his body would cause their instant disappearance, so acute are
they in sight or hearing, or perhaps both; and they seldom appear
when they have been frightened.” On ordinary occasions they do not
remain more than a minute or two at a time on the surface of the
water.
A burrow of an Ornithorhynchus, which Mr. Bennett opened, had
its entrance on a steep part of a bank, situated about one foot from
the water’s edge, and concealed among the long grass and other
Plants. “This burrow ran up the bank in a serpentine course,
approaching nearer to the surface of the earth towards its
termination, at which part the nest is situated. No nest had yet been
made in the termination of the burrow, for that appears to be
formed about the time of bringing forth the young, and consists
merely of dried grass, weeds, &c., strewed over the floor of this part
of the habitation.” The expanded termination measured one foot in
length and six inches in breadth, and the whole length of the burrow
was twenty feet. Besides the entrance before alluded to, it appears
there is usually a second opening into the burrows below the surface
of the water, communicating with the interior, just within the upper
aperture. A burrow subsequently examined by Mr. Bennett
terminated at a distance of thirty-five feet from the entrance; and
that gentleman stated that they have been found fifty feet in length.
From the burrow first opened by Mr. Bennett a living female was
taken, and placed in a cask, with grass, mud, water, &c., and in this
situation it soon became tranquil, and apparently reconciled to its
confinement. On his return home to Sydney, Mr. Bennett determined
to indulge it with a bathe; and with this view, when he arrived in the
vicinity of some ponds, he tied a long cord to its leg. “When placed
on the bank, it soon found its way into the water, and travelled up
the stream, apparently delighting in those places which most
abounded in aquatic weeds. When diving in deep and clear water, its
motions were distinctly seen: it sank speedily to the bottom, swam
there for a short distance, and then rose again to the surface. It
appeared, however, to prefer keeping close to the bank, occasionally
thrusting its beak into the mud, from whence it evidently procured
food, as, on raising the head, after withdrawing the beak, the
mandibles were seen in lateral motion, as is usual when the animal
masticates. The motions of the mandibles were similar to those of a
Duck under the same circumstances. After feeding, it would lie
sometimes on the grassy bank, and at others partly in and partly out
of the water, combing and cleaning its coat with the claws of the
hind feet. This process occupied a considerable time, and greatly
improved its sleek and glossy appearance.”
The Water Moles are said to have a peculiarly fishy smell, more
especially when wet, which probably proceeds from an oily
secretion. They are used by the aborigines for food; “but it is no
particular recommendation of them,” Mr. Bennett remarks, “to say
they are eaten by the native Australian, as nothing in the shape of
provender comes amiss to him, whether it be Snakes, Rats, Frogs,
Grubs, or the more delicate Opossum, Bandicoot, and Flying
Squirrel.”
The Ornithorhynchus is captured by the natives when in its
burrow. They first examine the neighbourhood of the burrow, to
ascertain, by the presence of recent footmarks on the soil, whether
it is inhabited, and if the examination proves satisfactory, they
proceed to dig holes with pieces of sticks from the surface of the
ground into the burrow, at distances from each other, until they
discover its termination, when the Australians consider themselves
exceedingly fortunate should they find the young, since they are
regarded as a great delicacy.
The young have been found in their nests by Mr. Bennett about
one inch and seven-eighths in length, in the early part of December,
and near the end of the same month he found young Water Moles of
ten inches in length. These latter were kept alive for nearly five
weeks, and their habits whilst in captivity are described in detail in
his paper, which is illustrated by some admirable figures, showing
their various attitudes, &c. The young were allowed to run about the
room; but an old Ornithorhynchus in the possession of our author
was so restless, and damaged the walls of the room so much by her
attempts at burrowing, that it was found necessary to confine her to
the box. “During the day she would remain quiet, huddled up with
her young ones; but at night she became very restless, and eager to
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