Mechatronics I Laboratory

MOTOR CHARACTERISTICS
FORCE/TORQUE SENSORS AND DYNAMOMETER PART 1

Force Sensors
Force and torque are not measured directly. Typically, the deformation (strain) of some
material is what is measured and the force or torque is inferred from that measurement. The
deformation can be measured in many ways. If the displacement is large as with a spring, the
displacement can be read directly on a scale. If the displacement is smaller, an LVDT or other
sensitive displacement measuring transducer can be used. If the deformation is very small, strain
gages can be used. In this laboratory experiment you will use strain gages to measure the
deformation of a cantilever beam. You will construct and calibrate a cantilever beam type force
sensor. Strain gages will be applied to one end of the beam. The beam will be clamped on one
end, and deflected by suspending known weights from the free end. The force exerted by the
weights is proportional to the strain along the beam. This strain will be measured using two or four
strain gauges. You will calibrate (equation and plot) the output of the strain gages as a function of
the applied force.

Strain Gages
A strain gage creates an electrical resistance that is proportional to the strain in the beam on
which it its mounted. It is important that the gage adheres well to the beam to obtain accurate
results. One or two gages will be mounted below, and one or two above the beam. They will be
wired together to form a bridge circuit which provides maximum sensitivity to the very slight
changes in resistance. The bridge will be explained in lab. Think about how to mount these gauges
on the beam to obtain the most sensitive response. Where on beam is the highest strain?

Laboratory Exercise
This is the first of a two part lab. You will use your force measurement setup you build in
this lab to later construct a dynamometer. If you are careful and take your time with this part of the
lab then your next lab will go much more smoothly. Do not tear down your circuit when you are
done! Instead, demonstrate to your T.A. that it works and then let them store for you until the next
lab.
The gages in the laboratory are of the etched foil variety. There is one gage on each plastic
backing. The gages must be applied carefully for them to work properly. Ask the T.A. for a data
booklet on the strain gauges if you have any questions about mounting them or how to wire them.
Steps:
1. The surface of the beam is first cleaned with emery paper and a solvent such as alcohol or
acetone.
2. Next use scotch tape to position the gauge over the beam by allowing one end of the tape to
stick, make sure the gauge will be glued with the right side up.
3. Apply a small amount of catalyst to the underside of the strain gauge (the side to be glued).
Apply a small amount of glue to the beam where the tape touches and pull the tape over and
down while pressing the glue along underneath the strain gauge. Hold or clamp tightly for 2
minutes.
4. Pull back the tape and gage should stay put, be careful at this stage not to peel off the gauge
or you will need to start over.
5. Solder wires on each of the gage wires and firmly tape or tie the wires to the beam so that
they will not pull on the gages.
6. Connect the wires in a bridge configuration, as seen in the left half of figure 1.
7. The strain gauges will provide a very weak signal. You will want to boost this signal to
obtain usable results. Connect the bridge to the instrumentation amplifier shown in the right
half of figure 1. Additionally, you may need to apply one or two amplifier stages after the
stage shown below in order to boost the signal. Don’t be afraid to apply capacitors to your
amplifier stages to filter the signal.

C = .1uF
+5 Volts
1K or less
pot
R1
Strain Gauges

Resistors
R2
V1 -
-
+
+
Vout

R2
-
V2
+
R1
Vout=(R1/R2)*(V1-V2)

Figure 1. Strain-Gauge and Amplifier Circuit.

Mass Voltage

Obtain a calibration constant (volts/kg) for your setup by determining the ratio of output
voltage to applied weight.

Calibration Constant: ______________________________________

2) Plot of the output voltage versus load.

1.2

0.8

0.6

0.4

0.2

3) Explain any anomalies - differences from theory or what you thought should happen.