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Guide for Selecting a Rotary Torque Sensor: Brush vs. Brushless Models



The accurate measurement of torque applied to rotating drives and fastening applications is an important criterion for evaluating production efficiency and quality assurance in manufacturing and assembly. There are many process monitoring applications that require rotary style torque sensor be used to capture and record traceable measurement results.

A rotary torque sensor is a finely tuned instrument designed for testing and monitoring torque applications. Designed for torque evaluation and verification, the rotary torque sensor is a laboratory grade instrument that is commonly used for quality control, R&D and calibration applications. The torque sensor connects to a torque tester or torque meter.

This special class of torque sensors require the ability to rotate but be able to measure the torque applied to a joint. Historically rotary torque sensors provided this feature by using brushes, to contact the shaft measuring torque, in a similar manner to the operation of electric motors.

The typical brush type rotary torque sensor has a few disadvantages. These include:

1. Some additional torque is required to overcome the friction of the brushes.
2. There may be problems associated with "brush bounce" in applications of pulse tools with significant vibration.
3. More maintenance is required, especially in high usage applications, because of brush wear.
4. Additionally, high RPM operation will decrease the force on the brushes due to centrifugal force precluding operation at very high RPM.

The recent innovation with the rotary torque sensors being "brushless" strengthens the weakness of the sensor by alleviating the disadvantages as stated above. The common "brush bounce" that plagues the accuracy testing of pulse tools is cured with the innovative "brushless" rotary torque sensor. The brush-less rotary torque sensor is a sound investment serving as part of the equipment infrastructure of any calibration lab. Instead of a rotary torque sensor that needs yearly maintenance or the need to replace every few years, the brushless rotary torque sensor is built to last and be maintenance free.

Brushless Rotary Torque Sensor Description
With a brushless rotary torque sensor, some means must be provided to transmit the torque that is applied to the rotating shaft to the outer body, which is fixed, without any mechanical contact. There are two major approaches to this technology.

One approach is to use an emerging technology known as Magna-lastic. This technology uses a changing magnetic field to indicate torsional stress. This torsional stress must be measured for variation with the use of a magnetometer. A magnetometer is typically used to measure variations in the Earth's magnetic field. Such sensors may not provide the accuracy of traditional strain gauge transducers and may be more costly to manufacture due to the cost of the magnetometer circuitry.

The other approach is to use standard strain gauge technology and provide electronics to implement this technology. Mountz, Inc. pursued this innovative approach. Two functions are required, providing a voltage to the strain gauge, and receiving torque information from the center shaft. Both are accomplished with the use of single set of coils and this is a significant technological innovation as well as a more cost effective approach. Other rotary style torque sensors require two sets of coils, one set to excite and power the rotating electronics and a second set to retrieve the torque information from the rotating shaft.

Mountz, Brushless Rotary Torque Sensor Technology
A DC voltage that can be anywhere between 15V and 24V powers the transducer. This voltage is converted to +/- 12 VDC by a DC to DC converter to power the stationary electronics. Due to the voltage conversion and regulation, the source does not need to be accurate, and does not affect the output of the sensor as the excitation voltage does in a brush type sensor. Because this is a switching regulator there is a frequency available from the switching regulator. This frequency is extracted from the circuitry and gets transmitted, across the coil described next, to power the electronics on the rotating shaft.

The sensor employs one set of coils. One coil is located on the external, non-rotating body, and the other coil is located on the internal rotating shaft. The same coil is used to provide both the voltages to run (excite) the electronics on the rotating shaft, and to receive the torque information back from the stain gauges. This is the major innovation of the product. It reduces the cost and simplifies the design. This is accomplished by splitting the bandwidth used into transmit band and receive band. A frequency, for the supply voltage, is sent across the coil in one band to be converted to a DC voltage, by a bridge rectifier circuit, to power the rotating electronics. Conversely the differential strain gauge voltage is amplified, converted from voltage to frequency, modulated, and send across the coils in the receive band. The stationary electronics converts the modulated torque signal using a frequency to voltage converter, filters it with a low pass filter, and amplifies it to provide the single ended 0V to +/- 5V signal for torque indication.

There is one other component to all this. You can provide a voltage (+5V) to the torque sensor to force it to output a full-scale CW torque reading. The 5V gets delivered, through an optical coupler, which switches in a circuit that provides a special signal to the rotating electronic to enable this function. This signal is sent across the coils and a low pass filter allows the lower frequency to be detected on the rotating shaft. When this happens, a calibration switch puts a calibration resistor across one leg of the bridge to force a full-scale output. This output, in turn, gets transmitted back across the coil to allow calibration of the sensor.

Obviously, the technology, required for a brushless rotary torque sensor, is far more complicated from a technological point of view than for a brush type rotary sensor. A full understanding is not required to take advantage of the benefits offered by this technology. The "brushless" rotary torque sensor is the ideal tool, coupled with a torque tester, where applications require low torque readings as well as eliminating the nagging "brush bounce" that occurs in pulse tool readings. The "brushless" design is simply the next evolutionary step with the rotary style sensor design.

The Mountz brushless rotary torque sensors an ideal torque-auditing tool for testing the actual torque being applied on the assembly application. Controlling torque is essential for companies to ensure their product's quality, safety and reliability isn't compromised. The failure of a three-cent fastener that isn't properly tightened can lead to catastrophic or latent failures. Fasteners that are insufficiently torqued can vibrate loose and excessive torque can strip threaded fasteners. Using a quality torque sensor has become increasingly important for many companies to ensure that proper torque is being applied and maintains gauge requirements associated with the ISO 9001 Quality Standard.

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