Position Sensors

• Position sensors report the physical position of an object with respect to a

reference point.
• The information can be an angle, as in how many degrees a radar dish has turned,
or linear, as in how many inches a robot arm has extended.
Potentiometers
• A potentiometer (pot) can be used to convert rotary or linear displacement to a
voltage.
• Figure P.1(a) illustrates how the pot works. Actually, the pot itself gives resistance,
but as we will see, this resistance value can easily be converted to a voltage.
• resistive material, such as conductive plastic, is formed in the shape of a circle
(terminating at contacts A and C).
• This material has a very uniform resistivity so that the ohms-per-inch value along
its length is a constant.
• Connected to the shaft is the slider, or wiper, which slides along the resistor and
taps off a value.
 Position Sensors
• Example P.1 (a) shows a pot that detects the angular position of a robot arm.
• In this case, the pot body is held stationary, and the pot shaft is connected directly
to the motor shaft. Ten volts is maintained across the (outside) terminals of the
pot. Look at Figure example P.1 (b) and imagine how the voltage is changing
evenly from 0 to 10 Vdc along the resistive element.
• The wiper merely taps off the voltage drop between its contact point and ground.
For example, if the wiper is at the bottom, the output is 0 V corresponding to 0°.
When the wiper is at the top, the output is 10 V corresponding to 350°; in the
exact middle, a 5-V output indicates 175° (350°/2 = 175°). Example P.1
demonstrates how to calculate the pot voltage for any particular angle.

• Figure P.1
 Position Sensors
EXAMPLE P.1
• A pot is supplied with 10 V and is set at 82°. The range of this single-turn pot is
350°. Calculate the output voltage.
SOLUTION
• If the pot is supplied 10 V, then the maximum angle of 350° will produce a 10-V
output.

10𝑉𝑑𝑐
Pot voltage (at 82°) = x82°=2.34 Vdc//
350°
 Position Sensors
PROXIMITY SENSORS
• A limit switch is a mechanical/electrical device which can be used to determine the
physical position of equipment.
• A limit switch is an example of a proximity sensor. A limit switch is a mechanical
push-button switch that is mounted in such a way that it is actuated when a
mechanical part or lever arm gets to the end of its intended travel.
• For example, an extension on a valve shaft mechanically trips a limit switch as it
moves from open to shut or shut to open. The limit switch gives ON/OFF output
that corresponds to valve position. Normally, limit switches are used to provide full
open or full shut indications as illustrated in Figure P.2

Figure P.2 Limit Switch

 Position Sensors
• Another example is in an automatic garage-door opener, all the controller
needs to know is if the door is all the way open or all the way closed. Limit
switches can detect these two conditions.
Switches are fine for many applications, but they have at least two
drawbacks:
• (1) Being a mechanical device, they eventually wear out, and
• (2) they require a certain amount of physical force to actuate.
• Limit switch failures are normally mechanical in nature.
 Position Sensors
• Optical proximity sensors, sometimes called interrupters, use a light source and a
photo sensor that are mounted in such a way that the object to be detected cuts the
light path.
• Figure P.3 illustrates an application of using photo detectors.
• Here, a photo detector counts the number of cans on an assembly line.

Figure P.3 Counting Cans on a Conveyor Belt

 Position Sensors
photo detectors
• Four types of photo detectors are in general use: photo resistors,
photodiodes, photo transistors, and photovoltaic cells.

Figure P.4: Various Types of Photo detectors

 Position Sensors
• A photo resistor, which is made out of a material such as cadmium sulfide (CdS),
has the property that its resistance decreases when the light level increases.
• It is inexpensive and quite sensitive—that is, the resistance can change by a factor
of 100 or more when exposed to light and dark.
• Figure P.4 (a) shows a typical interface circuit—as the light increases, Rpd
decreases, and so Vout increases.

• A photodiode is a light-sensitive diode. A little window allows light to fall directly

on the PN junction where it has the effect of increasing the reverse-leakage current.
Figure P.4 (b) shows the photodiode with its interface circuit.
 Notice that the photodiode is reversed-biased and that the small reverse-leakage
current is converted into an amplified voltage by the op-amp.

• A photo transistor [Figure P.4(c)] has no base lead. Instead, the light effectively
creates a base current by generating electron-hole pairs in the CB junction—the
more light, the more the transistor turns on.
 Position Sensors
• The photovoltaic cell is different from the photo sensors discussed
so far because it actually creates electrical power from light—the
more light, the higher the voltage.
• (A solar cell is a photovoltaic cell.) When used as a sensor, the
small voltage output must usually be amplified, Optical sensors
enjoy the advantage that neither the light source, the object to be
detected, nor the detectors have to be near each other. An example
of this is a burglar alarm system.
 Position Sensors
Linear Variable Differential Transformers(LVDT)
• The linear variable differential transformer is a high-resolution position sensor that
outputs an AC voltage with a magnitude proportional to linear position.
• The LVDT is composed of a primary coil (terminals a-b) and two secondary coils
(c-e and d-e) connected in the series-opposing manner as shown in figure.
• The coupling between these two coils is changed by the motion of high-
permeability alloy slug between them.
• The primary coil is sinusoidal excited, with a frequency between 60Hz and 20kHz.
• The alternating magnetic field induces nearly equal voltages Vce and Vde in the
secondary coils.
 Position Sensors
• The LVDT is an electromechanical transducer whose input is the mechanical
displacement of a core and whose output is a pair of ac voltages proportional to
core position.
• Figure p.5 (a) illustrates that the unit consists of three windings and a movable iron
core.
• The center winding, or primary, is connected to an AC reference voltage. The outer
two windings, called secondaries, are wired to be out of phase with each other and
are connected in series.
 Position Sensors
• If the iron core is exactly in the center, the voltages induced on the secondaries by
the primary will be equal and opposite, giving a net output (Vnet) of 0 V [as shown
in Figure p.5 (c)].
• Consider what happens when the core is moved a little to the right. Now there is
more coupling to secondary 2 so its voltage is higher, while secondary 1 is lower.
Figure p.5(d) illustrates the waveforms of this situation.
• If the core is moved a little left of center, then secondary1 has the greater
voltage, producing a net output that is in phase with secondary1 [Figure p.5 (b)].
• The magnitude represents the distance that the core is off center, and the
phase angle represents the direction of the core (left or right.)
• The extension valve shaft, or control rod,
is made of a metal suitable for acting as
the movable core of a transformer.
• Extension valve shaft using an LVDT
 Position Sensors
• The only moving part in an LVDT is the central iron core. As the core is only
moving in the air gap between the windings (Almost contactless) there is no
friction or wear during operation. For this reason,
• Linear over a large range (0, 25%) and has a quoted life expectancy of 200
years. The typical inaccuracy is 0.5% of full-scale reading
• Instruments are available to measure a wide span of measurements from 100
mm to 100 mm.
• The instrument can be made suitable for operation in corrosive environments
by enclosing the windings within a nonmetallic barrier, which leaves its Is
insensitivity to mechanical shock and vibration
• High sensitivity
• High range: from 0,1mm up to 250mm
• High life expectancy
• Some problems that affect the accuracy of the LVDT are the presence of
harmonics in the excitation voltage and stray capacitances, both of which cause
a nonzero output of low magnitude when the core is in the null position.
 Position Sensors
• Optical Rotary Encoders
• An optical rotary encoder produces angular position data directly in
digital form, eliminating any need for the ADC converter.
• The concept is illustrated in Figure p.6, which shows a slotted disk
attached to a shaft.
• A light source and photocell arrangement are mounted so that the slots pass
the light beam as the disk rotates.
• The angle of the shaft is deduced from the output of the photocell.
• There are two types of optical rotary encoders: the absolute encoder and
the incremental encoder.

Figure p.6: An optical

rotary encoder
 Position Sensors
Absolute Optical Encoders
• Absolute optical encoders use a glass disk marked off with a pattern of
concentric tracks (Figure p.7).
• A separate light beam is sent through each track to individual photo sensors.
• Each photo sensor contributes 1 bit to the output digital word.
• The encoder in Figure p.7 outputs a 4-bit word with the LSB coming from the
outer track. The disk is divided into 16 sectors, so the resolution in this case is
360°/16 = 22.5°. For better resolution, more tracks would be required. For
example, eight tracks (providing 256 states) yield 360°/256 = 1.4°/state, and ten
tracks (providing 1024 states) yield 360°/1024 = 0.35°/state.

Figure p.7: An absolute optical

encoder using straight binary code
 Position Sensors

• Figure p.8: An absolute optical encoder showing how an out-of-alignment

photocell can cause an erroneous state.
(Note: Dark areas produce a 1, and light areas produce a 0.)
• An advantage of this type of encoder is that the output is in straightforward
digital form and, like a pot, always gives the absolute position.
• It is used for angular motion measurement.
• A disadvantage of the absolute encoder is that it is relatively expensive
because it requires that many photocells be mounted and aligned very
precisely.
• If the absolute optical encoder is not properly aligned, it may occasionally
report completely erroneous data.
 Position Sensors
• Figure p.8 illustrates this situation, and it occurs when more than 1 bit changes at
a time, say, from sector 7 (0111) to 8 (1000).
• In the figure, the photo sensors are not exactly in a straight line. In this case,
sensor B1 is out of alignment (it’s ahead) and switches from a 1 to a 0 before the
others. This causes a momentary erroneous output of 5 (0101).
• If the computer requests data during this “transition” time, it would get the
wrong answer.
• One solution is to use the Grey code on the disk instead of the straight binary
code (Figure p.9). With the Grey code, only 1 bit changes between any two
sectors. If the photocells are out of line, the worst that could happen is that the
output would switch early or late. Put another way, the error can never be more
than the value of 1 LSB when using the Grey code.

Figure p.9
An absolute optical
encoder using a grey code.
 Position Sensors
• Incremental Optical Encoders
• The incremental optical encoder (Figure p.10) has only one track of equally
spaced slots.
• Position is determined by counting the number of slots that pass by a photo
sensor, where each slot represents a known angle.
• This system requires an initial reference point, which may come from a second
sensor on an inner track or simply from a mechanical stop or limit switch.
• In many applications, the shaft being monitored will be cycling back-and-forth,
stopping at various angles. To keep track of the position, the controller must know
which direction the disk is turning as well as the number of slots passed.

Figure p .10
An incremental
optical encoder.
 Position Sensors
EXAMPLE p.2
• An incremental encoder has 360 slots. Starting from the reference point, the photo
sensor counts 100 slots clockwise (CW), 30 slots counterclockwise (CCW), then 45
slots CW. What is the current position?
SOLUTION
• If the disk has 360 slots, then each slot represents 1° of rotation. Starting at the
reference point, we first rotated 100° CW, then reversed 30° to 70°, and
• finally reversed again for 45°, bringing us finally to 115° (CW) from the reference
point.
• A single photo sensor cannot convey which direction the disk is rotating; however,
a clever system using two sensors can.
• Note that,
• if more than half of the light is blocked the output is zero, and
• If less than half of light is blocked the output is one (5v, 6v,10v etc.) .
• As Figure p.11(a) illustrates, the two sensors, 𝑉1 and 𝑉2 , are located slightly apart
from each other on the same track.
• For this example,𝑉1 is initially off (well, almost-you can see it is half-covered up),
and 𝑉2 is on.
• Now imagine that the disk starts to rotate CCW. The first thing that happens is that
𝑉1 comes completely on (while 𝑉2 remains on).
• After more rotation, 𝑉2 goes off, and slightly later 𝑉1 goes off again.
 Position Sensors

Figure p.11
An incremental
optical encoder

• Figure p.11(b) shows the waveform for 𝑉1 and 𝑉2 . Now consider what happens when
the disk is rotated in the CW direction [starting again from the position shown in Figure
p.11(a)]. This time V1 goes off immediately, and V2 stays on for half a slot and then
goes off. Later 𝑉1 comes on, followed by 𝑉2 coming on.
• Figure p.11(c) shows the waveforms generated by 𝑉1 and 𝑉2 . Compare the two sets of
waveforms-notice that in the CCW case 𝑉2 leads 𝑉1 by 90°,whereas for the CW case 𝑉1
is leading 𝑉2 by 90°. This difference in phase determines which direction the disk is
turning.
 Applications of Position Sensors
• Production Line Conveyor Shift Unit
• Proximity switch is used as a conveyor shift unit. A sensor detects
when a product has arrived, and then sends signals to stoppers and
the Conveyer Shifting Unit to shift the product to another conveyor.
This is depicted in the figure shown below.
 Applications of Position Sensors

Overhead Crane Collision Avoidance

At an iron fabrication plant, two overhead cranes operate on one
runway; at times, the manufacturing process requires the cranes to
operate in overlapping areas of the bay. If an operator becomes
distracted or is concentrating on the load being transported, a collision
with the opposing crane is possible.
 Applications of Position Sensors

• complete system using non-contacting proximity sensors for the

purpose of material handling system collision avoidance could be
used.
• The addition of these sensors to the cranes provides the operator with
an audible warning when one crane comes within 40 ft of the
opposing crane.
• The warning alerts the operator that the cranes are in close proximity.
If the operator continues movement toward the opposing crane, the
braking system will activate at 10 ft.
• Pot sensor position system for robot arm:
• Servo motors are closed-loop systems, and they are inherently
useful in arm systems where precise position control of the joints is
necessary.
 Applications of Position Sensors
• Although the LVDT is a displacement sensor, many other physical
quantities can be sensed by converting displacement to the desired
quantity via thoughtful arrangements. One application is fluid level
position sensing in hydraulic cylinders. This is shown in the figure
below.