STRAIN GAUGE FORCE AND TORQUE SENSORS
Measurement Objectives:
1) Learn the function principle of the tensometric (strain gauge) sensor and the IRC
optoelectrical angular position sensor (incremental optical encoder) – consult respective
lecture materials. Also learn the properties of industrial remote measurement system
Compact FieldPoint and its input/output modules – see [1].
2) Measure the masses of available weights and their combinations.
3) Measure the torque Mk and the twist angle (torsion) ϕ of the round-profile steel rod.
Make the measurement for five different values of the weights. (Use combinations of
weights.) The torque Mk is to be measured both using the strain gauge measurement card
and through twist angle using the optoelectrical angle transducer.
4) From the measured value of the twist angle compute the shear modulus G (torsion
elasticity modulus) of the material.
Useful physical equations:
ϕ=
Mk=Z.g.R ,
where:
is the shear modulus of the material [N.m-2.rad-1],
G
Jp =
r
X
R
ϕ
g
Z
M k .X
,
G.J p
π .r 4
2
is the polar quadratic moment of the round profile rod [m4],
is the rod radius (0.005 m),
is the length of rod exposed to the torsion (0.5 m),
is the lever length, i.e. the arm of force (0.21 m),
is the twist angle of the rod [rad],
is the Earth’s gravity acceleration (9.81 m.s-2 in our location),
is the mass of the weight [kg].
Compact Field Point
cFP is an industrial measurement system by National Instruments company. The base of the
system is “chasis” that provide supply voltage and communication means to individual
input/output modules. Usually, the chasis contains a simple microcomputer that performs part of
the local control algorithm of the connected industrial process. In our case a simplified chasis is
used that contains only communication interface (Ethernet with TCP/IP) to the host PC that runs
the control algorithm. Two I/O modules are plugged into the chasis: strain gauge module cFPSG-140 and quadrature input module (for incremental optical encoder) cFP-QUAD-510.
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cFP-SG-140 can measure up to 8 channels of full bridge or half bridge strain gauges – see Fig.1.
The individual channels have configurable parameters (bridge supply voltage, input range, filter).
The input sigma-delta AD converter has 16 bit resolution. The measurement is by impulse (to
suppress DC offset voltage instability) with repetition rate of about 1s.
Fig. 1 (from [1])
cFP-QUAD-510 can measure 4 channels of reversible counters (each measure A, B and Index
signals, differential or TTL voltage levels, up to 250 kHz). In our case, the rotation angle is small,
thus Index signal (once per rotation) need not be used to reset counters to zero.
Measurement Procedure
Strain gauge
bridge
Fig. 2
Measurement of the mass on the strain gauge weight scale
a) Connect the strain gauge scale device to the strain gauge measurement module and run
program Scale on Windows desktop.
b) Perform the two-point calibration by means of the respective buttons. First the calibration
must be done for the zero mass on the lever, then for a 1 kg.
c) Measure the mass of the available weights using measurement button.
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d) Evaluate the real resolution (noise) of measurement by repeated measurements of one
weigth.
e) The display shows not only the measured mass (according to calibration) but also the
“raw” strain gauge output in mV/V – i.e. bridge output voltage relative to supply voltage.
What is the offset (signal for idle bridge) and what is the sensitivity (change per 1 kg)?
f) Estimate the level of the strain gauge bridge output voltage if the supply voltage is 2.5 V.
Measurement of the torque and twist angle
a) Disconnect the strain gauge scale from the module and connect the device for the torque
measurement. The torque Mk is caused by the weight of the mass Z hanging at the lever of
the length R, which is attached to the torsion rod (see fig. 3). Run the program Torque.
b) Calibrate the module - similar to the previous case.
c) Set the value of the used weight before each measurement. The theoretical torque is
computed from this value. The theoretical moment serves for the calculation of the
measurement error. (There is a difference between the values computed from the twist
angle (measured by optoelectrical encoder) and from the train measured by the strain
gauge bridge).
Strain gauge
bridge
(fish-bone)
Optical
encoder
Fig. 3
[1] FieldPoint Operating Instructions Materials of National Instruments:
http://www.ni.com/compactfieldpoint/
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