Strain gages measure automotive forces

Ralph Shoberg, founder and president of RS Technologies. Click here to read the complete interview.

Q What components are in a typical automotive strain-gage system?

A The strain-gage system begins with the strain gages themselves. We use what are technically termed "bonded metallic foil grid resistance strain gages." These are extremely thin foil grids with an equally thin nonconductive substrate that are bonded to the component under test. The gages are usually wired together into a Wheatstone bridge circuit that typically uses strain-gage elements in multiples of four. Instrumentation excites the bridge and conditions the strain-gage output signal into meaningful engineering units. Computerized data-acquisition systems can be configured to interface with virtually hundreds of data inputs.

Q As a custom strain-gage engineering provider, what types of automotive applications does RS Technologies service?

A We have serviced a variety of applications, the majority of them involving some sort of torque or force measurement. The most common application is for driveline torque measurement, either on a rotating drive shaft or half-shaft. But essentially any component that is placed under some sort of stress during vehicle operation can be gaged and the resulting forces measured.

Q Can you provide some tips for applying strain gages?

A The most important step in engineering a strain-gage application involves a pair of decisions. The first is the selection of the proper size and type of strain gage. Included in the strain-gage selection process is selection of the correct gage factor, which is the resistance change of the gage when placed under strain. A general rule of thumb is the more deflection of the part, the lower the gage factor. And conversely, the less the deflection, the higher the gage factor.

The second important decision is the proper placement of the gages on a spot where the stress being transmitted through the component can be measured. Correct decisions about gage type and placement will largely determine the accuracy and success of the measurement.

Q What are some specific challenges in mounting automotive strain-gage equipment?

A The greatest challenge is probably gaging the components without affecting their usual operation. If the strain-gage system or instrumentation impacts the weight and performance of the component, the effectiveness of the data will be limited. Thus, it is important to keep the weight of the components low and minimize or eliminate interference with the operation of the component and the vehicle.

Q How does one go about acquiring data from a strain-gage system?

A There are several approaches to data acquisition. In some cases, all that's required is a peak measurement. In other cases, a dynamic trace is required to observe the strain over a variety of loads or operating conditions. Data-acquisition systems range from simple readout instruments to high-speed, high-accuracy computerized systems. At the least, they will have the proper power supply and signal-conditioning circuitry so that the data measurement can be observed or recorded.

Click here to read the complete interview.