The Basics of Torque Measurement using
sensors, instrumentation and telemetry

Torques can be divided into two major categories, either static
or dynamic. The methods used to measure torque can be
further divided into two more categories, either 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
start with an understanding of the difference between a static
and dynamic force. To put it simply, a dynamic force involves
acceleration, were 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
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, it 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 cars drive axle as it cruises
down the highway (at a constant speed) would be an example
of a rotating static torque, because even though there is rotation,
at a constant speed there is no acceleration. The torque
produced by the cars 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 because
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. (figure 1)


figure 1

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 and avoid 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.

From the previous example above, the dynamic torque produced
by an engine would be measured by placing an inline torque sensor
between the crankshaft and the flywheel, avoiding the rotational
inertia of the flywheel and any losses from the transmission. To
measure the nearly static, steady state 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:
'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, commonly called
the reaction torque. (figure 2)



figure 2

Measuring the reaction torque avoids the obvious problem
of making the electrical connection to the sensor in a
rotating application (discussed below), but does come with
its own set of drawbacks. A reaction torque sensor is often
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 sensors response to loads other than
those that are intended to be measured), and sometimes 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.

Slip Ring
The most commonly used method to make this connection between
rotating sensors and stationary electronics is the slipring. 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. (figure 3)



figure 3 - Slip rings and brushes

Sliprings are an economical solution that performs well in a wide
variety of applications. They are a relatively straightforward,
time proven solution with only minor drawbacks in most 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 moderate speeds the electrical connection between the
rings and brushes are relatively noise free, however at higher
speeds 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 will 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, the brush ring interface is a source of drag torque that can
be a problem, especially for very low capacity measurements or
applications were the driving torque will have trouble overcoming
the brush drag.

Rotary transformer
In an effort to overcome some of the shortcomings of the slip ring,
the rotary transformer system was devised. It uses a rotary
transformer coupling to transmit power to the rotating sensor. An
external instrument provides an AC excitation voltage to the strain
gauge bridge via the excitation transformer. The sensors strain gauge
bridge then drives a second rotary transformer coil in order to get
the torque signal off the rotating sensor. (figure 4)



figure 4

By eliminating the brushes and rings of the slip ring, the issue of wear
is gone, making the rotary transformer system suitable for long term
testing applications. The parasitic drag torque caused by the brushes
in a slip ring assembly is also eliminated. However, the need for
bearings and the fragility of the transformer cores still limits the
maximum rpm to levels only slightly better than the slip ring.

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

Infrared (IR)
Like the rotary transformer, the infrared (IR) torque sensor utilizes
a contactless 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.
However, instead of being used to directly excite the strain gauge
bridge, it is used to power a circuit on the rotating sensor. The
circuit provides excitation voltage to the sensor's strain gauge 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. (figure 5)



figure 5

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 more expensive than a simple slip ring, it offers several benefits.
When configured without bearings, as a true non-contact measurement
system, the wear items are eliminated, making it ideally suited for
long term testing rigs. Most importantly, with the elimination of the
bearings, operating speeds (rpm's) go up dramatically, to 25,000 rpm
and higher, even for high capacity units. For high speed applications
this is often 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
transmitters are used to remotely connect any sensor, whether force
or torque, to its remote data acquisition system by converting the
sensor's signal to a digital form and transmitting it to an FM receiver
were it is converted back to an analog voltage.

For torque applications they 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 could be a drive shaft or half shaft
from a vehicle for example. The transmitter offers the benefits of
being easy to install on the component as it is typically just clamped
to the gaged shaft, and it is re-usable for multiple custom sensors.
It does have the drawback of needing a source of power on the
rotating sensor, typically a 9V battery, which makes it impractical
for long term testing. (figure 6)



figure 6

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

In applications that require the measurement of dynamic torque,
special care must be taken to measure the torque in the proper
location, and to not effect 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.

Sliprings are an economical solution, but have their limitations. More
technically advanced solutions are available for more demanding
applications, but will generally be more expensive. By thinking through
the requirements and conditions of a particular application, the
proper torque measurement system can be chosen the first time.

Article kindly provided by Ken Winczner of Sensor Developments

 
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