Load Cells Interfacing
Load Cells Interfacing
In practical measurements these transducers need to be mounted on a support structure which transforms the mechanical stress in to the required measurable strain. These structures are called load cells whose shape and general design depend on the application. An example of a load cell used in conjunction with strain gauges to measure the bending and torsion stresses acting on a robot wrist is shown in Figure 1 (a) and (b).
Figure 1 Example of a load cell using strain gauges to measure wrist forces and torques (after McCloy and Harris, courtesy of Open University press, 1986)
This load cell and its connector are machined from a single block for stability and are available from several sources, such as the Lords Corporation in the USA and Meta Machines in the UK.
Interfacing of Force Transducers With load Cells
The output from the Wheatstone bridge is typically of the order of microvolts and is, of course, an analogue signal. To interface it to a robot computer controller it needs AID conversion which, in turn, requires signal amplification since the typical input range for an AID converter i.c. is 0-2 V. The front end of the interface circuit therefore needs to provide voltage amplification of the order of 60-100 dB with very low drift and a high input impedance.
Dedicated ‘strain gauge amplifier’ integrated circuit, such as the SGA100, are available nowadays to meet these specifications and offer an effective solution to the problem; however they are expensive and can become unstable and so require a little experience (or strict adherence to the data sheets) if a reliable result is to be obtained.
Figure 2 Typical low-cost computer interface circuit for strain gauge bridge
A cheaper alternative can be implemented using low-drift, general purpose, instrumentation amplifiers such as the LM725. An example of a strain gauge interface circuit based on such a device and providing 60 dB of front-end amplification is shown in Figure 2.
