High-Precision Torque Sensor for Flexible Measuring

Table of Contents

Introduction
Two Approaches for Dual-Range Sensors
Multiple Spring Elements Allow for Multiple Measuring Ranges
Electrical Dual-Range Sensor Simulates Small Measuring Range
     Signal Noise
     Temperature Response of the Zero Point TC0
     Hysteresis (Relative Reversibility Error)
     Parasitic Loads
FlexRange Functionality
Conclusion

Introduction

Torque measurements in automotive applications pose a unique challenge. In certain applications there is a preference for covering numerous measuring ranges with just one torque sensor. There are several methods for a solution to this challenge. For example, the sensor’s measuring range can be extended mechanically or electrically. In the first case, however, accuracy is compromised as factors contributing to measurement ambiguity such as signal noise, hysteresis and the temperature response of the zero point are reinforced with the spread of the measuring range. In the other case the mechanical properties weaken because of the very complex structure of the measuring body. HBM has thus opted for a different approach, creating the 12HP digital torque flange with FlexRange. This sensor covers the whole measurement range with just one spring element, with an extremely high level of accuracy.

Requirements for automotive and motors components – better energy efficiency, longer ranges and lower consumption– are progressively rising. This also means higher demands for accuracy in development and research – and consequently for testing equipment as well. Torque measurements are an important factor for many testing applications in the automotive sector. This is particularly challenging if measuring ranges of different sizes have to be covered during a measuring process, as for instance in engine tests. Then a torque sensor has to obtain both high and low torques, based on the test – with even accuracy over the whole measuring range. The key challenge is to accomplish a balance between error tolerance and measurement accuracy.

In several applications, for instance brake tests, the peak torques that happen are extremely high in comparison to the average measured torques. The nominal (rated) measuring range of the sensor is dimensioned suitably to guarantee the sensor is not damaged, overloaded or even destroyed by peak torques. Peak torques represent the highest torque in the application. If a sensor is adapted to the maximum torque, however, it may perhaps be over dimensioned for measuring the other torques that take place during the test. Over-dimensioned sensors have a drawback: vital data sheet information, which must be used to assess errors, refers to the nominal (rated) measuring range, not to the average measured torque.

Dual-range sensor

Figure 1. Dual-range sensor with two different measuring bodies in which the small measuring body must also receive the nominal (rated) torque of the larger measuring range.

Electrical dual-range sensor

Figure 2. Electrical dual-range sensors have one spring element and simulate a second smaller measuring range electrically.

The relevant error evaluation may therefore create an unfavorable result. This is because crucial parameters named in the data sheet such as the non-linearity, temperature response TC0, and the hysteresis as well as effects because of parasitic loads, basically refer to the nominal (rated) measuring range of the sensor.

Two Approaches for Dual-Range Sensors

The ideal solution for recording numerous measuring ranges during a single test process would be constant adaptation of the sensor measuring range to the corresponding maximum torque. Since this is not practical for technical reasons, however, many different variants of dual-range sensors have been produced that are able to cover both a large and a small measuring range. Two different principles act as the foundation for dual-range sensor: sensors with two spring elements and electrical dual range sensors with just one continuous spring element and two separate measurement channels.

Multiple Spring Elements Allow for Multiple Measuring Ranges

Dual-range torque sensors measure in two torque ranges that vary in size. To accomplish this they have two spring elements or measuring bodies which vary in size and have different nominal (rated) measuring ranges and are connected in series or parallel. Each of these measuring bodies hold a specially coordinated strain gauge bridge connected to a data acquisition system. This enables determination of the material strain of the measuring body, from which the torque can then be derived. Torque transducers of this type are called "true" dual range transducers. The drawback of the variant with the spring elements connected in series is that it is only appropriate for static or quasi-static torque measurements.

In dynamic applications, the overload protection of the smaller spring element would result in signal overlapping. Since the smaller spring element also records the high torques of the larger spring element, it is fitted with mechanical overload protection that eliminates the connection when torques are very high, shifting the torque to the larger sensor. Without this overload protection there is a danger that the smaller element will be deformed. If the overload protection does engage, however, the result is a vague signal.

That can result in inaccurate interpretations later when the measurement results are assessed. The second, small measuring range frequently also has a very "soft" design in this arrangement, to be able to produce a sufficiently high characteristic value at low torques. Due to this, the small measuring range reacts very sensitively to parasitic loads such as axial forces, which result in crosstalk against the torque and may even destroy or damage the sensor in extreme cases.

In another variant of "true" dual-range sensors, the spring elements of different sizes are connected in parallel. This type can manage without overload protection, thus also avoiding the interference intrinsic in signal overlapping. However, in this case as well the smaller spring, elements must also record large torques. There is therefore a danger that the smaller spring element will be overloaded, causing plastic deformation.

To avoid this, the smaller spring element is engineered so that along with the larger measuring body it is able to support the highest torque. However, this results in a very low characteristic value for the second strain gauge bridge. The result: Inadequate resolution along with a high level of inaccuracy, including realization of the temperature response.

T12HP Digital Torque Sensor

Figure 3. T12HP Digital Torque Sensor.

Electrical Dual-Range Sensor Simulates Small Measuring Range

The T12HP torque sensor also varies from "non-true" dual-range sensors that have just one spring element or measuring body and mimic a second spring element electrically.

It contains an extra measuring amplifier connected to the sensor which is adapted to the smaller measuring range. This second measuring amplifier amplifies the output signal, typically by a factor of 5 or 10. Therefore a second useful signal is available which also signifies smaller torque loads. The drawback of this principle: The second measuring range only seems to boost accuracy. The critical parameters for measurement uncertainty refer to the nominal (rated) measuring range and therefore not to the amplified useful signal. Since the second signal in a "non-true" dual-range sensor just spreads the signal electrically, these impacting factors are also amplified if no additional values are explicitly specified in the data sheet for the second range, which increases the measurement uncertainty.

Vital factors are:

  1. Signal noise
  2. Temperature response of the zero point TC0
  3. Hysteresis (relative reversibility error)
  4. Parasitic loads

Signal Noise

Every electronic signal includes background noise that is also signified in the measurement. The signal for the smaller measuring range in a dual-range sensor is lower in quality by its very nature, because this signal noise also increases with the amplification. A comparison of the zero signal noise in the large measuring range (1:1) and in the small one (for instance 1:5) reveals that the electrical amplification also increases the noise by a factor of about 5.

Therefore tolerances in the measurement signal are also amplified, for example those owing to temperature effects. The signal noise is low with the T12HP torque sensor because the second, smaller measuring range is not produced by electronic amplification. The high standard accuracy paired with the high resolution of the sensor – FlexRange functionality – covers the whole measuring range. Thus the signal noise stays low even when the signal strength is low in the lower range.

Temperature Response of the Zero Point TC0

The temperature impacts the measurement accuracy of a sensor. If the measurement signal is amplified for an electrical dual range sensor, the temperature reaction of the zero point TC0 also increases. The SG measuring bridge is attuned to the nominal measuring range with the signal intensity factor 1:1. A signal spread with the factor 1:5 also raises the accuracy by a factor of 5 if nothing to the contrary is indicated in the data sheet.

If the temperature response of the larger measuring range is stated as 0.1%/10 K, a subrange full scale value of 0.5%/10 K is accordingly obtained for the second, smaller measuring range. Note also in this case whether a separate value is stated in the data sheet for the temperature response of the second range. If not, the spread in the measurement signal will not result in any corresponding improvement in accuracy. On account of HBM's FlexRange technology, the T12HP is capable of covering the whole measuring range with just one amplification. With a very low value of only 0.005%/10 K, very high accuracy is achieved even in the subrange.

Hysteresis (Relative Reversibility Error)

If the measurement signal’s characteristic curve is recorded first with uninterruptedly increasing torque and then with identical continuously declining torque, the output signals will not match exactly. They will each deviate from the characteristic curve. The highest deviation between the falling and rising load is referred to as the hysteresis or the relative reversibility error. It relies on the design of the measuring body and the elastic properties of the spring element material.

The quantity of the hysteresis relies on the stress and the strain resulting from it in the measuring body, thus on the maximum torque. If there is an alteration to the smaller measuring range during a torque measurement, for instance a brake test with the brake open and closed, the hysteresis remains "saved" in the spring element because of the high initial load or the strain in the spring element. When the measuring range alters, however, so does the deviation from the characteristic curve – from the large distance to the small one.

Due to this, there is a gap in the recorded measurement signal curve where the change happens called the zero point offset or the point of discontinuity. This error is amplified similarly to the gain factor of the measurement signal. For instance, if the relative reversibility error for an electrical dual-range sensor is 0.05% of the nominal torque in the large measuring range (1:1), after switching directly to the small measuring range (1:5), an offset error of 0.25% of the nominal torque may take place.

The T12HP torque transducer covers the whole measuring range – and there is no change of measuring range. The FlexRange technology therefore allows for one uninterrupted measurement signal and eliminates the point of discontinuity in the application and in the accuracy evaluation.

A point of discontinuity always happens if for example there are varying accuracy levels in a dual-range sensor which always depend in their application or interpretation on preloading and thus on the hysteresis.

a change in the characteristic curve due to the hysteresis

Figure 4. When changing from the larger to the smaller measuring range during a torque measurement, the characteristic curve changes due to the hysteresis.

General realization of the point of discontinuity

Figure 5. General realization of the point of discontinuity in the accuracy evaluation when changing measuring ranges of electrical dual-range sensors.

Parasitic Loads

Axial offsets happen in virtually all drivetrain applications depending on design and assembly. This is due partially to tolerances in the dimensional accuracy of the components that are used and alignment issues and partially to other influences such as temperature. The remaining offset can be particularly compensated for by the use of compensating couplings. However, the crosstalk resulting from parasitic loads cannot be compensated for without additional elaborate measurement technology measures.

This effect is reduced to a minimum in the T12HP because of the advanced geometry of the measuring body and due to the very high accuracy and quality of the strain gauge application. The parasitic loads are zero point-relative and spreading the measurement signal in an electrical dual range sensor increases the effect of the loads by the gain factor. These sensors create large measurement errors in the small measuring range. With a single range torque sensor like the T12HP, the parasitic loads are manageable.

FlexRange Functionality

HBM realized the physical limits of strain gauge technology in the development of the T12HP. Furthermore, the use of the HBM carrier frequency technology ensures improved signal quality. The result is a torque measuring system with very high basic accuracy and stability as well as high effective resolution of the measured value. This unique combination makes it possible to consider certain measuring ranges with a "magnifying glass", with sufficiently high accuracy and resolution.

It is this philosophy that renders FlexRange functionality possible: superior stability, accuracy and resolution for every measured value with just one measuring body and one signal path. Consequently, no second measuring range is required to meet the need for satisfactory accuracy and resolution, even in the subrange.

FlexRange Functionality

Figure 6. Thanks to high-quality strain gauge and carrier frequency technology, the FlexRange functionality of the T12HP torque sensor from HBM offers the highest resolution with maximum accuracy.

Conclusion

There are numerous possibilities in torque measurements for measuring torques of different quantities in two measuring ranges. There are losses in accuracy in most variants because of the design, particularly in the smaller measuring range. True dual range sensors with multiple measuring bodies are not truly suitable for dynamic applications because of the overload stop which is needed.

Since the second range is often designed to be sensitive so that it will produce a large useful signal, the permitted limit loads are consistently low and lead to more crosstalk against the torque signal with a relatively large error fraction.

In electrical dual-range sensors with just one measuring body, spreading the measurement signal amplifies interfering properties such as hysteresis, signal noise, temperature response of the zero point TC0 and parasitic loads. Unless otherwise specified in the data sheets, this does not increase the measurement accuracy of the second range.

The temperature response of the zero point TC0 as well as the signal noise are magnified and at the change from the large measuring range to the small one a point of discontinuity may take place. The principle of the electrical dual range sensor also amplifies parasitic loads based on the application.

These interfering effects are reduced in the T12HP torque sensor with FlexRange functionality. The transducer integrates the filtering and scaling flexibility of digital signal editing with very high standard accuracy and resolution, thereby offering the benefits that would be anticipated from a dual-range sensor – but without any of the drawbacks of that solution.

Due to the patented measuring body with a very high basic accuracy and resolution, integrated with carrier frequency technology, ensured error limits of just 0.007% or 0.005%/10 K can be realized for technical data such as hysteresis and linearity, temperature effect on zero signal TC0 and more. The T12HP with its FlexRange functionality is therefore obviously superior to the dual-range sensor in many aspects. The philosophy of the T12HP, to use the total potential of strain gauge technology up to its physical limits, thus considerably increasing the standard accuracy of the sensor, greatly simplifies the technology for users.

Therefore HBM's T12HP measurement flange does not require a second amplifier in the subrange. With its superior mechanical properties it offers very high accuracy over the whole measurement range. This makes the T12HP torque transducer ideal for high-precision efficiency measurements and also for very dynamic torque measurements with varying measurement ranges, for instance in brake tests with active and released brakes, in running or towed engine tests, or in transmission and tire tests.

The T12HP digital torque flange with FlexRange technology

Figure 7. The T12HP digital torque flange with FlexRange technology and only one spring element offers significant advantages compared to conventional dual-range sensors.

HBM, Inc

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

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

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