Selecting the Right Rotating Torque Sensor

The two main reason for carefully selecting the right torque sensor are to ensure accuracy and avoid damage.

The torque sensor ensures accurate measurement and it is similar to mechanical fuse in design. In order to avoid mechanical failure its important to choose the right torque sensor as it is one of the weaker components of the driveline. At the same time, the uncertainty of torque data can influence the accuracy of the torque sensor. Taking into account all the electrical requirements of the torque sensor, it is possible to acquire all the required data to suit a specific application.

This article reviews various factors that help in choosing the appropriate torque sensor: types of torque sensors, application considerations, environmental considerations, dynamic considerations, physical requirements and costs or budget considerations.

Types of Torque Sensors

The following section describes different types of torque sensors along with their pros and cons.

Reaction Torque Sensors

Reaction torque sensors do not rotate and behave in a manner similar to that of a load cell and lever arm, without needing either. Figures 1a and 1b shows the schematic of reaction torque sensor.

Figure 1A. Schematic of Torque Sensor. Image credit: Hottinger Baldwin Messtechnik GmbH

Figure 1B. Mechanism of Torque sensor. Image credit: Hottinger Baldwin Messtechnik GmbH

The major advantages of the reaction torque sensors include:

• Less considerations including RPM considerations as the torque sensor does not rotate
• Inexpensive to use
• No changes required for the rotating shaft

However, the disadvantages of these sensors are as follows:

• As the mass of the dynamometer acts as a mechanical filter, the sensors have reduced dynamic response time
• They do not measure true torque in the shaft
• Dynamometer floats on the bearings help measure reaction torque
• Accuracy is not as perfect as an in-line torque sensor.

Circular Shaft - Slip Ring-Style Torque Sensor

Designed over 40 years ago, slip ring-style torque sensors feature graphite brushes that make contact with silver alloy slip rings. Figure 2 shows the schematic of circular shaft- slip ring-style torque sensor.

Figure 2. Schematic of circular shaft- slip ring-style torque sensor. Image credit: Hottinger Baldwin Messtechnik GmbH

The advantages of the slip ring-style torque sensor include:

• Low torque capacity at affordable price
• Low inertia
• High response times
• AC or DC excitation
• Mounting options

The disadvantages of the slip ring-style torque sensor are as follows:

• Electrical brush noise
• Brush maintenance and errors
• Bearing maintenance
• RPM limitations
• Stiffness
• Backlash or key imbalance

Circular Shaft - Rotary Transformers

Figure 3a and 3b shows the schematic and mechanism of circular shaft – rotary transformers, respectively. The mechanism of circular shaft – rotary transformers is similar to that of the slip ring sensor. However, they use rotary transformers rather than brushes and slip rings. One of the transformers excites the torque sensor, while the other takes back the data.

Figure 3A. Schematic of circular shaft- rotary transformers. Image credit: Hottinger Baldwin Messtechnik GmbH

Figure 3B. Mechanism of circular shaft- rotary transformers. Image credit: Hottinger Baldwin Messtechnik GmbH

The advantages of circular shaft – rotary transformers include:

• Higher RPM ratings then slip rings
• Mounting options
• Low torque capacity ranges at a reasonable cost
• Low inertia
• Non-contact data transmission

The disadvantages of circular shaft – rotary transformers include:

• Vibration sensitive
• Use of AC excitation source
• Stiffness
• Backlash and key imbalance
• Electrical noise
• Bearing maintenance and errors
• RPM limitations
• Low response time

Circular shaft - clamp-on style torque sensors

Clamp-on style torque sensors are employed when an in-line torque sensor needs to be installed without breaking the shaft. Figure 4 shows the schematic of clamp-on style torque sensors.

Figure 4. Schematic of clamp-on style torque sensors. Image credit: Hottinger Baldwin Messtechnik GmbH

The main advantages of the clamp-on style torque sensor include:

• Higher RPM ratings
• Low torque capacity ranges
• Low cost for high torque ranges

The disadvantages of the clamp-on style torque sensor include:

• Requires mathematical calculations
• Difficult to repeat
• Low accuracy or high uncertainty

Analog telemetry torque sensor

Analog telemetry is an accurate and inexpensive sensor which has been available since the early 1990s. Figure 5 shows the schematic of analog telemetry torque sensor.

Figure 5. Schematic of analog telemetry torque sensor. Image credit: Hottinger Baldwin Messtechnik GmbH

The advantages of the analog telemetry torque sensor include:

• Weight and length
• Low electrical noise
• High RPM ratings
• Low or virtually no backlash
• Stiffness
• Non-contact data transmission

The disadvantages of the analog telemetry torque sensor are:

• Susceptible to nearby metal objects
• High inertia
• Better but limited response times

Digital Telemetry Torque Sensor

Digital telemetry is the most suitable way of manufacturing torque sensors. Figure 6 shows the schematic of digital telemetry torque sensor.

Figure 6. Schematic of digital telemetry torque sensor. Image credit: Hottinger Baldwin Messtechnik GmbH

The advantages of the digital telemetry torque sensor include:

• High response times
• Set up software available
• May not be susceptible to nearby metal objects
• Low electrical noise
• High RPM ratings
• Low or virtually no backlash
• High stiffness
• Non-contact data transmission

The disadvantages of the digital telemetry torque sensor include:

• More data conversions
• Lower capacity ranges are expensive
• High inertia due to larger diameter

Dual-Range Torque Sensor

It is impossible to manufacture a true dual-range torque sensor. However, some digital torque sensors use software which allows them to operate as a dual-range torque sensor. Figure 7 shows the schematic of dual-range torque sensor.

Figure 7. Schematic of dual-range torque sensor. Image credit: Hottinger Baldwin Messtechnik GmbH

Application Considerations

A torque sensor datasheet can be used for comparing the sensors based on the applications. It denotes the performance of the torque sensor. The values present in the datasheet are recorded in laboratory test conditions, and hence they are static.

Accuracy Requirements

Measurement accuracy or uncertainty level is a key parameter to be considered when choosing a torque sensor. The level of uncertainty may vary for different applications. For example:

• Assembly equipment testing, fastener testing or torque-to-turn applications can manage high uncertainty level with a torque sensor including HBM T20 or HBM T22
• Endurance testing, end-of-production test and horsepower verification require moderate uncertainty levels, requiring a torque sensor such as an HBM T40B
• Efficiency testing, friction loss testing and drag torque require low uncertainty, using torque sensor such as an HBM T12

The uncertainty of the torque sensor consists of six components as follows:

• Off-axis loading (usually axial, lateral and bending movements)
• Repeatability
• Temperature effect at full scale
• Temperature effect at zero
• Linearity and hysteresis
• Sensor output and sensitivity

Figure 8 shows the measurement uncertainty formula to calculate an uncertainty number with the help of data from a torque sensor’s data sheet.

Figure 8. Measurement uncertainty formula. Image credit: Hottinger Baldwin Messtechnik GmbH

Capacity Range

It is important to take into account the peak torques and spikes while selecting torque sensors. When used outside the safe range, the spikes can damage the torque sensor. The damage cannot be detected if the time periods of these spikes are short and the response time of the torque sensor is low. Therefore, knowledge of torque range is critical while choosing the torque sensor to accommodate these spikes. Further, it is necessary to the sensor’s electrical outputs to measure the spikes.

Overload Ratings

The overload rating of the torque sensor is another prime factor to be considered. The industry standard for safe sensor overload is usually around 200% of full scale at which the sensor starts yielding. Catastrophic overload can occur at 400% of full scale when the sensor may fail by cracking or bending or the bolts may fatigue.

High overload rating indicates that the sensor is larger and heavier to withstand strong forces.

• Rotations-per-minute (RPM) rating- Most of the torque sensors are rated between 10,000 and 20,000RPM. While considering the sensor's RPM rating, critical speed, balance and weight are main factors to be evaluated.
• Weight - Owing to its weight and diameter, the torque sensor must spin slowly. High RPM rated torque sensors such as the HBM T11 have 30,000RPM. To reduce the weight of the sensor, the sensor is made of titanium.
• Balance - Before shipping, some torque sensors are balanced at the factory. Pre-balancing the torque sensor ensures a G 2.5 balancing rating.
• Critical speed - RPM is the critical speed at which the driveline remains unstable. The critical speed can be avoided by choosing the torque sensor with a foot mount and bearings.
• Output requirements - For reaction torque sensors - or older-model, slip-ring-type torque sensors, the outputs are provided in millivolt per volt. In general, the torque sensor’s output will be similar to that of certain instrumentation including data acquisition (DAQ) system or strain gauge conditioner.
• Frequency outputs are time base signals, which are less susceptible to noise when compared to amplitude outputs. High frequency output can reduce the propagation delay and increase the response time.
• Response times - To avoid aliasing, considering the response-time requirements along with the ratio between the sampling rate and response time is required. Certain torque sensors such as the HBM T12 have a 3:1, 6:1 or even a 12:1 ratio. High response times enable measurement of torsional vibrations. Figure 9 shows the graph illustrating response time of torque sensor.

Figure 9. Graph illustrating response time of torque sensor. Image credit: Hottinger Baldwin Messtechnik GmbH

Environmental Conditions

Environmental conditions including electromagnetic impulse (EMI), corrosion, oil, dirt and temperature also affect the choice of torque sensor.

Temperature - Torque sensors are usually temperature-sensitive, and hence it is required to determine the gradient or air soak temperature change in environment.

The design of the torque sensor is determined in terms of protection against dirt, oil and corrosion. Certain sensors are more hermetically sealed when compared to others.

EMI - Torque sensors are antennas with coils and wires inside, which ensure proper shielding and grounding. AC strain gauge conditioning is more silent when compared to DC strain gauge conditioning. It reduces the errors caused by thermal or 1F noise sources. Figure 10 shows the graph illustrating electromagnetic impulse of torque sensor.

Figure 10. Graph illustrating electromagnetic impulse of torque sensor. Image credit: Hottinger Baldwin Messtechnik GmbH

Dynamic Considerations

Modern torque sensors are torsionally stiff and have high frequency response times. This ensures measurement of dynamic torque. Figure 11 illustrates a diesel-engine torque measured on the dynamometer with an in-line torque sensor and a lever arm and load cell. The lever arm tends to act as a mechanical low-pass filter with 20Hz frequency, if the frequency of the oscillating torque is higher than the natural frequency of the dynamometer.

Figure 11. Schematic of a diesel-engine torque measured on a dynamometer. Image credit: Hottinger Baldwin Messtechnik GmbH

Rotational Effects - The output of the torque sensors is mainly affected by the rotation effects. Windage is one type of rotational effect which is the effect on the torque sensor spinning in air. Actual forces acting on the spinning sensor are another type of rotational effect. The effect on the torque sensor becomes greater at high RPMs.

Critical speed - Critical speeds of the rotating shaft can be eliminated by choosing the ideal torque sensor with infinite stiffness and no length and no weight. Obviously this is not possible. Hence the vendor should strive at manufacturing a torque sensor with high stiffness, very light and very short. The shaft tends to become unstable at a specific RPM, and acts as a sine wave or vibrates at critical conditions. Therefore, it is appropriate to determine the torsional analysis of the rotating shaft before using the test stand.

HBM suggests to use simple test stand with fewer parts to minimize errors. The weight of the torque sensors should be maintained as close to a bearing block. Figure 12 shows the torque sensor and coupling that are close to the bearing block.

Figure 12. A torque sensor and coupling are close to a bearing block. Image credit: Hottinger Baldwin Messtechnik GmbH

Parasitic loading - These are bending moment forces, lateral limit forces and axial forces that occur during the spinning of shaft. These off axis forces can introduce large errors to a rotating torque sensor. Therefore, it is necessary to eliminate these forces to ensure improved accuracy and reduced uncertainty of test stand. Figure 13 shows the schematic of parasitic loading of test stand

Figure 13. Schematic of parasitic loading. Image credit: Hottinger Baldwin Messtechnik GmbH

Physical Requirements

The physical limitations of the applications should be taken into account while choosing the torque sensor. Flange-to-flange torque sensors are short, rigid and light in weight. By contrast, circular-shaft torque sensors are long, heavy and less rigid.

While considering the space requirements, flange-to-flange sensors have larger diameter. Circular, keyed shafts have small diameter, enabling pedestal-mount or foot-mount of the sensor. A circular torque sensor can act as a bearing block, changing the critical speed of the driveline. The type of coupling to be used is determined by the torque sensor mounting configuration. Flange-to-flange torque sensors are usually floating torque sensors which require one dual-flex type coupling, and are not foot-mounted. Circular, keyed shaft torque sensors without foot mount also employ one dual-flex coupling. Two dual-flex type couplings are used along with the foot mount.

Cost or budget considerations

The cost of the rotating torque sensors usually starts at $2,000. The square drive-type torque sensors with slip rings are less expensive. However, torque sensors of over 10000Nm are the expensive sensors.

The following factors influence the cost of the torque sensor:

• Custom configurations
• More outputs
• Non-contact
• A higher RPM rating
• Better accuracy (less uncertainty)
• Higher capacity range

This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.

For more information on this source, please visit HBM, Inc.