(1) Using electronic test equipment primarily because it sounds important, and not because it is rationally justified. (2) Using specific transducers just because they are available, and not because they are mandatory. (3) Taking far more data than will ever be analysed. (4) Spending more time and money on development of new instrumentation techniques than on the problem it was meant to solve.
If the test instrumentation is made up of interchangeable modular components, it can be a general-purpose tool for many types of testing, and its cost can be spread out over years of vehicle development. Therefore it should be considered as a system, and ten specific questions should be resolved before assembling that system:
(1) What is the ultimate purpose: general driver or vehicle evaluation, or a specific problem to be solved, such as specific component durability, more aerodynamic downforce, or turbo lag reduction? (2) What parameters absolutely must be recorded, and what precision is required to distinguish meaningful results: 10 percent? 1 percent? 0.1 percent? (3) What is the maximum number of channels absolutely necessary at one time? Could you get by with eight?...sixteen? (4) What data sampling rate is most appropriate: ten per second, or as much as 500 per second? Does information change slowly, or rapidly and continuously, like shock travel,? (5) What is the most practical data logger, considering: channels, precision, sample rate, total capacity, cost, power, space, weight, and frequency response? (6) What is the best output data format? Is it enough to see plotted curves, or is a numerical printout needed for later automated analysis? (7) Is data transmission necessary -- either car to pits or station to station? Does an observer want constant trackside monitoring? (8) What transducers must be bought or rented? (See Table 9) (9) What data analysis procedures are necessary, such as damping, averaging, or scaling? Is raw data adequate, or a few simple calculations, or is computer reduction required. (10) Is it cost effective, or would common manual test equipment be adequate? Would you settle for 90 percent accuracy for only 10 percent of the cost?
Up to the last edition of this book, analog data recorders were a reasonable option, and still might be considered from a cost-saving standpoint if already available. An onboard strip-chart recorder is about as basic and easy to comprehend as anything, with immediate mechanical feedback of both calibration and performance. However, compared to modern computer systems, they have some major drawbacks, such as size and weight, sensitivity to g-loadings, and frequency response. Onboard tape recorders with download to a strip chart were once popular, but are as complicated and expensive as computers, which continue to become more powerful and less expensive.
Whether the data recording is analog or digital, most of the measurement sensors will be somewhat similar. The transducers are where the real engineering comes in. It is always necessary to convert whatever quantity is to be measured into electrical signals that are compatible with the recorder. The most common measurements desired in race car development are usually speeds, accelerations, displacements, forces, and temperatures, and occasionally pressures and stresses, although many more are possible.
Speed in miles per hour or rpm can be sensed by two methods, depending on the use of the data. Sometimes a tachometer generator may be used, which when mounted to a wheel or the engine, generates a precise DC voltage signal proportional to rpm. When it is also necessary to know the total distance or number of revolutions, it is more practical to use a digital sensor. A number of magnetic slugs can be glued evenly around a rotating circumference, and a magnetic pickup is mounted near enough to produce digital pulses. Speed is then proportional to the pulse frequency, and distance is proportional to the sum of the pulses. In addition, a math channel or electronic circuits can be designed to differentiate the velocity change to give a signal proportional to acceleration. A more precise digital pickup is the optical sensor, which can be bought off the shelf with over a thousand pulses per revolution.
For recording straight-line or slow cornering speeds to a greater precision, say for the first critical inches in a dragstrip launch, a fifth wheel may used, since it has minimal diameter change effects. But for most race car use, a front wheel is best, preferably the outside wheel, as it has the least effects from lifting or slipping.
For recording laptimes on the data, or recording a reference to the car's location on the track, infra-red triggers are commonly used. In fact, they are often used with a simple onboard timer and readout, without the need for an expensive multi-channel data logger. These receivers work with a trackside infra-red beacon transmitter, commonly mounted at the start-finish line. More sophisticated systems may use many coded beacons located strategically around the track to give segment times.
Track location recording will become more common and much more precise as better GPS systems are demilitarized. At the time of this writing, systems were being demonstrated that had astounding resolution of small fractions of an inch. This will make it possible to evaluate the different lines being used through corners, and if sensors are mounted at the front and rear of the chassis, even the pitch and yaw angles.
Accelerations can be measured in many ways. Forward or coasting accelerations are usually so small that taking the slope of the speed curve (differentiation) is most accurate. For transient lateral acceleration (non-skidpad) or braking, however, the most direct method is the electronic potentiometer accelerometer. These can occasionally be found through aerospace surplus stores in the desired 2.0-g range and are easily wired in with an accurate fixed reference voltage. A bubble level is also necessary to insure a perfect zero free from gravity effects.
Displacement sensors are most commonly used to record suspension travel, or steering or throttle movements. In most cases a linear potentiometer may be available with enough travel to fit the total displacement. But a more flexible device, in both mounting and total travel, is a rotary potentiometer attached to a spring-loaded cable reel. This can easily be attached to any fixed surface, with the wire cable connected to the moving component. Ordinarily, the output inches of travel can be equal to inches of vehicle component movement. However, aerodynamic downforce has become so critical, and so sensitive to ground clearance, that it must be measured more accurately than spring deflection, to account for tire deflection. To deal with the moving ground problem, very precise laser displacement sensors have been developed for racing applications, although they are about the most expensive sensors available. Four are often used, although two at one end and one at the other will give both pitch and roll angles, as well as average ground clearance.
Forces are difficult to measure if no movement can be tolerated, in which case strain gauges are necessary. Otherwise, the easiest technique is to allow movement against a known-rate spring and again record the displacement. Wheel loads or vertical aerodynamic forces are most often measured in this way, by recording the average suspension deflection at a specific speed. Strain gauges have the capability of measuring forces very precisely with no displacement, but they are inconvenient for all but the most precise stress work. They require professional attachment and careful calibration, and they tend to record every little unimportant force or vibration. Forces from fluid pressures can also be measured through the use of aerospace pressure transducers, to record fuel, oil, brake, or air pressures.
Torques at the driveshaft or axles are obviously very important information, and just a further application of strain gauges. However, they require either slip rings (which are impractical under race track conditions) or a short-range data transmitter from the rotating shaft. Although not uncommon to passenger car development, only recently have they become available as a part of racing data acquisition systems.
It may also be desirable to record temperatures in the moving race car, other than by the fluid temperature gauges available to the driver. Thermocouples can be mounted on the brakes, exhaust, or air inlets, and wired into the recorder for a continuous record. However, in most cases the peak temperature is all that is wanted. If so, it is far easier, faster, and cheaper to use a simple heat-sensitive positive indicator. The most familiar device is called Temp-Plate, which consists of a row of colored dots on an adhesive strip. The dots blacken successively with precise increases in temperature and are available in increments of 10 to 50 degrees, anywhere from 100 to 1100 degrees Fahrenheit.
Tire temperatures are critical to setup, as explained in Chapter 1, especially real-time on-track data, as opposed to waiting to get pitlane readings with a contact pyrometer. Non-contact infra-red temperature sensors have been available for decades, and so inexpensive that it's surprising they aren't used more often. Some teams go so far as to have 3 sensors reading each of all four tires during development (of such variables as camber angle), then during the race, just monitor one average reading per tire (when allowed by the rules).
Tire pressure recording gets back to the torque problem of transmitting data from a rotating object. However, since this has become relatively common in passenger car applications, it should become less expensive and more practical in racing, especially from a safety justification.
Aerodynamic surface pressures are commonly recorded in the development of high downforce bodies. However, the demand for more data points, and the recognition of slow data sampling requirements, has led to multipexing. Scanivalve technology, where many pressure taps are fed into a single electronic sensor, has been adapted from wind tunnel applications, and as many as 64 separate surface pressures may be recorded on a single analog channel.
When anything is measured indirectly, as with transducers and a recorder, accurate calibration is critical. Forces can be calibrated by loading with known weights, and displacements calibrated by measuring with a tape measure. With accelerometers, 1.0 g can be checked by using the earth's gravitation, while anything greater will require a centrifugal acceleration wheel. Time can be checked with an electronic stopwatch, or a constant accurate time reference can be provided with a standard frequency oscillator wired to the recorder. It can also be useful to have an accurate digital voltmeter when setting up a recorder and potentiometers. Finally, it helps to have all calibration gear easily accessible and to have the recorder controls within easy reach of the driver. It is often necessary to record just one small segment in a series of laps or for the driver to stop and check calibration out on the track.