Rotation Sensor

Capacity range
When considering rotation sensors, it's important to accommodate all of the peak torques and spikes. These spikes, if outside the safe range of use, can damage a torque sensor. And if the time periods of these spikes are short enough and the response time of the torque sensor is low enough, you may not even see them in your data until the damage is done. So, it's important to know your torque range and choose a torque sensor that can accommodate these spikes. Moreover, you must decide whether you want to measure these torque spikes or just survive them. If you want to measure the spikes, you have to make sure that your sensor's electrical outputs can measure the spikes that you want to read. However, if you're only concerned with measuring the working (average) torque, electrical filtering or using torsionally soft members in the driveline can remove or dampen the spikes.

Overload ratings
It's also important to know the overload ratings of a rotation sensor. Typically, the industry standard for safe sensor overload is around 200 percent of full scale - the point at which the sensor will start to yield. It would take many overloads at this rating for the rotation sensor to start failing, usually seen in a zero shift. Catastrophic overload typically can occur around 400 percent of full scale - the point at which bolts may fatigue or the sensor may fail by cracking or bending.If you prefer a more generous overload rating, some vendors offer a 400 percent safe overload and an 800 percent catastrophic overload. However, raising the overload rating increases the stiffness of the rotation sensor and decreases the resolution, or signal-to-noise ratio, at the low end. It also typically means that the sensor is larger and heavier by design to accommodate stronger forces. You need to decide whether to sacrifice a higher overload rating for size, weight and signal-to-noise ratio.

Rotations-per-minute (RPM) rating
Most rotation sensors in the marketplace are rated between 10,000 and 20,000 RPMs. Weight, balance and critical speed are factors to consider when choosing a sensor's RPM rating. Weight - Typically, when you go up in capacity range, the rotation sensor must spin more slowly - mostly due to its weight and diameter. There are some rotation sensors rated to higher RPMs, such as the HBM T11, which is rated to 30,000 RPMs. This sensor is made of titanium to cut down on the weight. Balance - Some rotation sensors are balanced at the factory before being shipped. If a rotation sensor is pre-balanced, it's typically to a G 2.5 balancing rating. Check with the sensor's manufacturer to find out if it will arrive balanced. Critical speed - The critical speed is the RPM at which the driveline wants to become unstable. Usually the rotation sensor will be the first thing to fail in this type of a situation. To help avoid a critical speed, you can choose a rotation sensor with bearings and a foot mount. This will shorten the unsupported shaft as the rotation sensor will act as a bearing block.

Output requirements
Your rotation sensor's output will typically go to some sort of instrumentation, such as a strain gauge conditioner or data acquisition (DAQ) system. On reaction rotation sensors - or older-model, slip-ring-type rotation sensors - the outputs are usually millivolt per volt, which is a strain gauge output. When using such an output, typically an AC or DC strain gauge conditioner is used to power the sensor and then condition the output to a usable signal, usually analog or digital. Plus or minus 10 volts DC amplitude signals are often more susceptible to noise than a frequency or digital output. Frequency outputs are a time base signal and are less susceptible to noise than amplitude outputs.

Response times
When evaluating rotation sensors, it's important to consider your application's response-time requirements as well as the ratio between the sampling rate and response time to avoid aliasing. Some rotation sensors have a 3:1, 6:1 or even a 12:1 ratio, such as the HBM T12. These high response times will allow you to measure torsional vibrations.

Temperature
Like all strain-gauge-based sensors, rotation sensors are sensitive to temperature, so it's important to know whether you are dealing with a gradient or air soak temperature change in your environment. A gradient change affects one side of the rotation sensor first, when making its way down the metal shaft. Effectively, one side of the rotation sensor can be warmer than the other side, possibly causing the sensor to twist. An air soak is a more uniform temperature change, similar to placing the sensor in an oven. An air soak temperature change is compensated for by the manufacturer and is listed on the sensor's data sheet. A rotation sensor's manufacturer can often supply information on how the sensor performs while under temperature changes. Usually, rotation sensors are put in ovens wiup and down to gather data on how well the rotation sensor performs under a changing-temperature environment. It's important to note that the more strain gauges a sensor uses in its construction, the better it can perform during temperature changes - as long as they're laid on the metal sensor properly. Also, if a steel sensor is manufactured properly, the effects of temperature change are minimized.

Dirt, oil and corrosion
Consider a rotation sensor's construction in terms of protection against dirt, oil and corrosion - particularly the more sensitive parts. Also, some sensors are more hermetically sealed than others, which is important in a harsh environment. You also need to know how the rotation sensor data is transferred and whether the transfer method is susceptible to oils and dirt. For example, in slip-ring sensors, dirt and oil are more likely to affect the signal transmission back to the stationary world. These sensors are more susceptible to oil and dirt than digital telemetry systems.

Electromagnetic impulses (EMIs)
Rotation sensors are basically antennas - they have wires and coils inside, so proper cabling, shielding and grounding is critical. Find out what kind of signal conditioning is being used on the rotation sensor. Is it AC or DC excitation across the Wheatstone bridge? At HBM, we generally recommend AC strain gauge conditioning because it's more noise-immune than DC strain gauge conditioning. It minimizes the errors that DC can cause due to thermal or 1F noise sources.

Rotational effects
Rotational effects can influence the rotation sensors output. One type of rotational effect is windage - or the effect on the rotation sensor just spinning in air. Another type includes the actual forces on the spinning sensor - like how spinning affects the strain gauges, wires and electronics. At higher RPMs, the effect on the rotation sensor becomes greater. These effects should be small but can be seen on a rotation sensors output signal. Unfortunately rotational effects aren't listed on most datasheets. This is because the rotational effects vary depending upon RPM and can change from application to application, making it difficult to quantify. However, some companies do compensate for rotational effects. It's important to ask the vendor about the rotational effects of its rotation sensor and how they are mitigated.