The Fundamentals: The Basics Of Torque Measurement

Brushing up on the available methods and tools for measuring torque will help you improve your accuracy, as well as protect your wallet.

Torques can be divided into two major categories: static or dynamic. The methods used to measure torque can be further divided into two more categories: reaction or in-line. Understanding the type of torque to be measured, as well as the different types of torque sensors that are available, will have a profound impact on the accuracy of the resulting data, as well as the cost of the measurement.

Static vs. dynamic
In a discussion of static vs. dynamic torque, it is often easiest to start with an understanding of the difference between a static and a dynamic force. To put it simply, a dynamic force involves acceleration, while a static force does not.

The relationship between dynamic force and acceleration is described by Newton's second law; F=ma (force equals mass times acceleration). The force required to stop your car with its substantial mass would be a dynamic force, as the car must be decelerated. The force exerted by the brake caliper in order to stop that car would be a static force, because there is no acceleration of the brake pads involved.

Torque is just a rotational force–or a force through a distance. From the previous discussion, torque is considered static if it has no angular acceleration. The torque exerted by a clock spring would be a static torque, since there is no rotation and, hence, no angular acceleration.

The torque transmitted through a car's drive axle as it cruises down the highway (at a constant speed) would be an example of a rotating static torque. In such a case, even though there is rotation, at a constant speed there is no acceleration. The torque produced by the car's engine will be both static and dynamic, depending on where it is measured. If the torque is measured in the crankshaft, there will be large dynamic torque fluctuations as each cylinder fires and its piston rotates the crankshaft. If the torque is measured in the drive shaft, it will be nearly static since the rotational inertia of the flywheel and transmission will dampen the dynamic torque produced by the engine.

The torque required to crank up the windows in a car (remember those?) would be an example of a static torque, even though there is a rotational acceleration involved, because both the acceleration and rotational inertia of the crank are very small and the resulting dynamic torque (Torque = rotational inertia x rotational acceleration) will be negligible when compared to the frictional forces involved in the window movement. This last example illustrates the fact that for most measurement applications, both static and dynamic torques will be involved to some degree. If dynamic torque is a major component of the overall torque or is the torque of interest, special considerations must be made when determining how best to measure it.

Reaction vs. inline
Inline torque measurements are made by inserting a torque sensor between torque carrying components, much like inserting an extension between a socket and a socket wrench. The torque required to turn the socket will be carried directly by the socket extension. This method allows the torque sensor to be placed as close as possible to the torque of interest, preventing possible errors in the measurement such as parasitic torques (bearings, etc.), extraneous loads and components that have large rotational inertias that would dampen any dynamic torques.

According to the above example, the dynamic torque produced by an engine would be measured by placing an inline torque sensor between the crankshaft and the flywheel, thus avoiding the rotational inertia of the flywheel and any losses from the transmission. To measure the nearly static, steadystate torque that drives the wheels, an inline torque sensor could be placed between the rim and the hub of the vehicle, or in the drive shaft. Because of the rotational inertia of a typical torque drive line and other related components, inline measurements are often the only way to properly measure dynamic torque.

A reaction torque sensor takes advantage of Newton's third law that "for every action there is an equal and opposite reaction." To measure the torque produced by a motor, we could measure it inline, as described above, or we could measure how much torque is required to prevent the motor from turning,which is commonly called the reaction torque.Measuring the reaction torque helps us avoid the obvious problem of making the electrical connection to the sensor in a rotating application (discussed later). This method, however, comes with its own set of drawbacks.

A reaction torque sensor often is required to carry significant extraneous loads, such as the weight of a motor or, at least, some of the drive line. These loads can lead to crosstalk errors (a sensor's response to loads other than those that are intended to be measured), and sometimes to reduced sensitivity, as the sensor has to be oversized to carry the extraneous loads. Both of these methods–inline and reaction–will yield identical results for static torque measurements.

Making inline measurements in a rotating application will nearly always present the user with the challenge of connecting the sensor from the rotating world to the stationary world. There are a number of options available to accomplish this, each with its own advantages and disadvantages. They are:

Slip ring…
The most commonly used method to make the connection between rotating sensors and stationary electronics is the slip ring. It consists of a set of conductive rings that rotate with
the sensor and a series of brushes that contact the rings and transmit the sensors' signals.

Slip rings are straightforward and economical solutions that perform well (with only a few minor drawbacks) in a wide variety of applications. The brushes, and to a lesser extent the rings, are wear items with limited lives that don't lend themselves to long-term tests or to applications that are not easy to service on a regular basis. At low- to moderatespeeds, the electrical connection between the rings and brushes are relatively noise-free. At higher speeds, though, noise will severely degrade their performance.

The maximum rotational speed (rpm) for a slip ring is determined by the surface speed at the brush/ring interface. As a result, the maximum operating speed will be lower for larger, typically higher torque-capacity sensors by virtue of the fact that the slip rings will have to be larger in diameter, and therefore have a higher surface speed at a given rpm. Typical max speeds will be in the 5,000 rpm range for a medium-capacity torque sensor.

Finally, be aware that the brush ring's interface can be a source of drag torque. This can be a problem, especially for very low-capacity measurements or applications where the driving torque will have trouble overcoming the brush drag.

Rotary transformer…
Rotary transformer systems were devised in an effort to overcome some shortcomings of the slip ring. They use a rotary transformer coupling to transmit power to a rotating sensor. An external instrument provides an AC excitation voltage to the strain gage bridge via the excitation transformer. The sensor's strain gage bridge then drives a second rotary transformer coil to get the torque signal off the rotating sensor. By eliminating the slip ring's brushes and rings,wear is gone, making the rotary transformer system suitable for long-term testing applications. Parasitic drag torque from brushes in a slip ring assembly also is eliminated. But, the need for bearings and the fragility of transformer cores still limit maximum rpm to levels only slightly better than the slip ring.

This system also is susceptible to noise and errors induced by the alignment of the transformer primary-to-secondary coils. Because of the special requirements imposed by rotary transformers, specialized signal conditioning also is required in order to produce a signal acceptable for most data acquisition systems, further adding to the system's cost–which is already higher than a typical slip ring assembly.

Infrared (IR.)…
Like the rotary transformer, the infrared (IR) torque sensor utilizes a contact-less method of getting the torque signal from a rotating sensor back to the stationary world. Similarly using a rotary transformer coupling, power is transmitted to the rotating sensor. Instead of being used to directly excite the strain gage bridge, this is used to power a circuit on the rotating sensor. The circuit provides excitation voltage to the sensor's strain gage bridge and digitizes the sensor's output signal. This digital output signal is then transmitted, via infrared light, to stationary receiver diodes, where another circuit checks the digital signal for errors and converts it back to an analog voltage. Since the sensor's output signal is digital, it is much less susceptible to noise from such sources as electric motors and magnetic fields.Unlike the rotary transformer system, an infrared transducer can be configured either with or without bearings, for a true maintenance-free, no-wear, no-drag sensor.

While it is more expensive than a simple slip ring, the infrared torque sensor offers several benefits.When configured without bearings, as a true non-contact measurement system, the wear items are eliminated, making this sensor ideally suited for long-term testing rigs.More importantly, with the elimination of the bearings, operating speed (rpm) goes up dramatically–to 25,000 rpm and higher, even for highcapacity units. For high-speed applications, this often is the best solution for a rotating torque transmission method.

FM transmitter…
Another approach to making the connection between a rotating sensor and the stationary world utilizes an FM transmitter. These devices are used to connect any sensor, whether force or torque, to a remote data-acquisition system, by converting the sensor's signal to a digital form and transmitting it to an FM receiver–where it is converted back to an analog voltage. For torque applications these receivers are typically used for specialty, one-of-a-kind sensors–such as when strain gages are applied directly to a component in a drive line. This application, for example, could be a drive shaft or half shaft from a vehicle.

The FM transmitter can be easily installed on the component, as it is typically just clamped to the gaged shaft. It also is re-usable for multiple custom sensors. Drawbacks include needing a source of power on the rotating sensor, typically a 9V battery, which makes it impractical for longterm testing.

Conclusion
Understanding the nature of the torque to be measured, as well as the factors that can alter this torque in the effort to measure it, will have a signficant impact on the reliability of the data collected.

In applications that require the measurement of dynamic torque, special care must be taken to measure in the correct location–and to not affect the torque by dampening it with the measurement system.

Knowing the options available to make the connection to the rotating torque sensor can greatly affect the price of the sensor package. Slip rings are an economical solution, but have their limitations.More technically advanced solutions are available for more demanding applications, but they usually will be more expensive.

By thinking through the requirements and conditions of a particular application, you can choose the right torque measurement system for the application the first time- and every time. TF

David Schrand is engineering manager with Sensor Developments, Inc. (SDI), of Orion, MI. SDI was established in 1976 as an engineering consulting firm specializing in the science of force measurement and sensor design. Today, the company produces solutions for many industries and applications, including automotive, aerospace, OEM, medical, nuclear and textile operations. For more information on the technologies referenced in this article, log on to www.sendev.com