Page 116 - Mechanical Engineers' Handbook (Volume 2)

7 Resistance Bridge Transducer Measurement System Considerations 105

measure strain, pressure, torque, force, and so on on the rotating machine component, signals
from bridge transducers must be coupled from this component to a stationary instrumentation
system. Instrumentation slip rings accomplish this function.
In their simplest form, slip rings consist of a metal ring on the rotating machine com-
ponent against which a brush attached to the stationary machine portion is spring loaded to
make ohmic contact. Precious metals are generally used for mating surfaces to minimize
contact resistance.
Slip rings came into existence in the 1940s with initial application in the aircraft in-
dustry. In the 1950s, mercury slip rings came into existence. These latter rings, which first
found application at Rolls Royce in England, use mercury as the signal transfer medium.
The mercury is entrapped between the rotor and the stator of the ring assembly. Today, slip
rings are capable of operating from very low RPM to tens of thousands of RPM.
Noise induced in slip rings is of the ohmic contact type, that is, it is roughly proportional
to current. A high brush pressure reduces noise at the expense of increased brush wear.
Brush wear is a function of the brush pressure, material, finish (usually microinch), and
flatness. One technique for lowering contact noise is to mount several brushes in parallel on
the same ring.
Because ohmic changes in the slip rings can be of the same order of magnitude as
resistance changes in the bridge transducer, full bridges are almost always used on the
rotating part to avoid inserting slip rings within the bridge itself. Slip rings in the output
circuits of bridge transducers using voltage monitoring do not create significant problems
because any small resistance changes in the rings are in series with the large input impedance
of the voltage-measuring device and are effectively ignored. Slip rings in the input circuits
of bridge transducers operating from a constant-voltage source can create problems if they
cause fluctuating voltage drops in series with the transducer. For this reason, constant-current
sources are preferred when using slip rings.
Other techniques for extracting data from rotating machinery have evolved over the
years. These include rotary transformers, light modulation, and radio frequency (RF) telem-
etry. Of these schemes, RF telemetry has displayed the most promise with commercially
available low-power transmitters capable of operating up to 30,000g.

7.4 Noise Considerations
Many other sources besides slip rings can induce unwanted spurious signals in these trans-
ducers. Since the unamplified output from bridge transducers is typically ones or tens of
millivolts and never more than a few hundred, they are easily influenced by noise sources.
The following discussion defines noise, documents how to verify its existence (or hopefully
nonexistence), and provides some hints as to how to suppress noise in bridge transducer
measuring systems. Reference 12 provides a basis for this discussion.
The output of measuring system components represents combinations of responses to
environments. These environments can be divided into two categories: desired and all others
(undesired). For example, consider a bridge pressure transducer in a hostile explosive envi-
ronment. Its desired environment is pressure. Other undesired environments it encounters are
temperature, acceleration, ionized gas, and so on. Ideally, the transducer would respond to
pressure alone. In practice, an additional response is elicited from the transducer due to the
other environments; usually, but not always, the response is small compared to the pressure
response.
Two response types exist for a bridge transducer: self-generating and non-self-
generating. Non-self-generating responses are due to changes in the material properties or
geometries within a transducer. Power has to be applied to the transducer to elicit a non-