First, let's revisit what torque is and why it's
important. Torque is a measure of the forces that
cause an object to rotate. Reaction torque is the
force acting on the object that's not free to
rotate. An example is a screwdriver applying
torque to a rusted screw. With rotational torque,
the object is free to rotate. Examples include
industrial motor drives and gear reducers.
Torque and RPM determine horsepower, and
horsepower determines system efficiencies, so
being able to monitor and control torque can be
critical in optimizing overall system efficiency.
Torque measurement is paramount in engine and
transmission testing, turbine testing, pump testing
and testing of gear trains and power
measurement within propulsion systems.
Monitoring torque can be critical to the
performance of axles, drive trains, gear drives,
and electric and hydraulic motors. Other in-plant
applications include gas and steam turbines.
Really, torque is an important factor in anything
that rotates or spins on a shaft, spindle or axle. A
lumber mill might use a predetermined maximum
torque to initiate blade changes. This saves wear
on the drive system and increase product quality.
How torque-measuring technologies evolved
So, what's new and how did we get there? A
brief history of what has been used and how
torque measuring has changed may help
illustrate the benefits of the latest technologies.
Digital telemetry is the latest technology, and so
far, seems to have a number of benefits over
older systems. But before digital telemetry was
viable, there were several other methods,
starting in recent history with the slip ring signal
The slip ring
The slip ring is a simple means of getting a signal
transferred from the rotating element to the
stationary element. It has low speed limits. Ring
wear and dust generated by the brushes can
quickly impede signal transfer, so you must
routinely maintain the rings and the brushes to
ensure clean signal transfer. As testing
requirements became more demanding, so
evolved the rotary transformer.
The rotary transformer
Compared to the slip ring, the rotary transformer
method tolerates higher speed, is non-contact
and typically more accurate. However, it is less
tolerant to extraneous loading conditions like
bending moments and thrust loads. It also
requires more sophisticated signal conditioning
instrumentation using an AC carrier excitation.
So as testing demanded more and more so
evolved the total non-contact solution of using
Analog FM wireless telemetry systems use a
sensor with a built-in radio transmitter module, a
power supply and a receiver. Low voltage
signals from the strain gauges are amplified and
modulated to a radio frequency signal by the
transmitter. This radio signal is picked up by a
hoop antenna and decoded into analog voltage
by the receiver. This voltage is amplified and
scaled to show the torque value on an LED
display. This signal is routed to an external
connector for interfacing with a data acquisition
While this form of telemetry is an improvement
over other mechanical methods, it is bulky and
requires additional receivers for multiple
channels. The frequency response is limited to
about 1,000 Hz with an analog signal output.
The antenna, straps and battery-powered
systems can be difficult to install and tune. As
technology and electronics advanced, digital
telemetry became practical.
Early generation digital telemetry
The first generation of digital telemetry systems
frequently used a hoop antenna, a bulky
receiver and had limited data processing
capability. A coil acted as the rotary
transformer. The coil is excited by the radio
frequency that is transmitted via the antenna.
These early systems did allow two-way
communication flow, but because they had no
microprocessors, data processing was very
limited. All the discrete components added to the
cost and design limitations.
Next generation digital telemetry
As microprocessor and surface mount
technologies progressed, multiple chips were
added to the next generations of systems to
vastly improve data management and control.
A rotor electronics circuit board module is
embedded in the sensor and is potted and
sealed. Signal conditioning and digitizing is done
on the rotating sensor using this module. Antenna
and caliper-style coupling modules eliminate the
need for hoop antennas and are more immune
from vibration problems.
Antennas are limited to a few select sizes and are matched to the caliper
modules at the factory. This eliminates the need
to tune the system in the field. Outer protective
layers on the antenna and caliper module give
outstanding protection against moisture and oil.
A signal-processing module acts as the receiver
unit, handling the communications with the rotor
Resolution, stability and accuracy are all
improved over the early digital telemetry
systems. The new systems transfer digital data at
very high speeds, providing a frequency
response up to 3,000 Hz.
More than stand-alone sensors, these are
complete torque measurement systems, with
standard analog, frequency and digital outputs.
Fully software- driven, these digital telemetry
systems can be changed "on-the-fly" without
They can be designed with
clear upgrade paths for custom software,
developments and future requirements. It is also
where the "fit and forget" concept can be used
to lower maintenance costs of existing solutions.
If you are measuring torque "the old fashioned
way," you may be unaware of some inherent
inefficiencies. Here are a few headaches
digital telemetry can potentially eliminate.