Strain gages measure automotive forces—the complete interview

The complete version of our interview with Ralph Shoeberg, founder and president of RS Technologies; a shorter version of this interview appeared in the February 2006 issue of our Automotive & Aerospace Test Report.

Greg Reed, Contributing Technical Editor -- Test & Measurement World, 2/1/2006 8:39:00 AM

Click here to read the print version.

In 1856, Lord Kelvin observed that metallic conductors subjected to mechanical strain exhibit a change in their electrical resistance. Since that time engineers have attempted to measure and quantify strain by converting mechanical motion into an electrical signal. In automotive applications, Ralph Shoberg, founder and president of RS Technologies, has gained a reputation for extracting precise strain measurements from a multitude of moving parts. In a recent telephone conversation, I spoke with him about strain gages and ways to apply them.

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. When the body to which a strain gage is attached is placed under load, the resistance of the strain gage changes, producing a proportional change in its output signal.

The gages are usually wired together into a Wheatstone bridge circuit that typically uses strain-gage elements in multiples of four. Instrumentation is required to excite the bridge and amplify and condition the strain-gage output signal into meaningful engineering units. In the case of a component that moves or rotates, there are telemetry systems that carry the output signal from the test part to the instrumentation. Modern computerized data-acquisition systems can be configured to interface with virtually hundreds of data inputs on a completely instrumented test vehicle.

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 practical tips or guidelines for applying strain gages, sensors, and related technologies for automotive applications?

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. There are hundreds of gage designs available for torque and force measurement applications. 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 an optimum spot where the stress being transmitted through the component can be measured. Correct decisions made on gage type and placement will largely determine the accuracy and success of the measurement.

In most applications, some surface preparation is required to mount the strain gages securely. This may involve removal of plating or coatings to machining of the surface of the part to ensure proper bonding of the strain gages.

Another complicating factor can be environmental in the form of water, oil, or other fluids. This requires the use of protective coatings that range from simple water-resistant materials to hermetically sealed covers that protect the strain-gage circuit from damage.

Because of the low output level of strain gages, strain measurements are prone to interference from other sources of electrical energy. Capacitive and magnetic coupling to long cable runs, electrical leakage from the part through the gage backing, and differences in grounding potential are a few of the possible sources of difficulty. The results of this type of electrical interference can range from a negligible reduction in accuracy to rendering the data invalid. This requires some scheme of shielding the measurement leads on the component to intercept any error-producing currents and keep them out of the measurement loop.

Some guarding of the measuring equipment may also be necessary to isolate the measuring elements. This may include placing a guard lead between the test specimen and the negative terminal of the power supply. This will force the floating power supply and all the measuring equipment, including the strain gages, to the same electrical potential as the test specimen. Because of minute differences in resistance of the lead wires within the Wheatstone bridge, some sort of temperature compensation should be done to ensure that any change in the output is due to the strain on the component and not due to changes in temperature.

The bridge excitation voltage level affects both the output sensitivity and the gage self-heating. From the measurement standpoint, a high excitation level is desirable but a lower level reduces gage self-heating. The electrical power in the gage is dissipated as heat that must be transferred from the gage to the surroundings. In order for this heat transfer to occur, the gage temperature must rise above that of the specimen and the air. The gage temperature is therefore a function of the ambient temperature and the temperature rise due to power dissipation.

Excessive gage temperature can cause various problems. If the temperature becomes too high, the carrier and adhesive materials are no longer able to faithfully transmit the strain from the specimen to the grid. This adversely affects hysteresis and creep and may show up as instability under load. High gage temperatures also affect zero balance or unstrained stability. Thus, proper adjustment of excitation voltage is necessary to avoid these pitfalls.

Proper calibration is necessary to ensure correct scaling of the instrumentation. This requires placing the component under a known load and measuring the output. By placing five or 10 loads at various levels leading up to the full-scale capacity, an estimation of the linearity of the circuit can be made. The shunt calibration method can be used whereby a simulated load is placed on the circuit electrically to scale the instrumentation.

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.

Another challenge is getting the strain output signal from the component to the instrumentation. Often times, the component is in a location that is difficult to access. Getting the output signal to the instrument while avoiding the pitfalls discussed above can be especially difficult when the component is periodically or constantly in motion. The benefits of a good experimental stress-analysis program require that innovative solutions be devised to obtain the necessary data.

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

A There are several approaches to data acquisition. The type of equipment employed depends in large part on exactly what kind of data the customer requires. 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.