Evaluation Engineering - 23

fractions of the units of the actual sensor
measurement, or, in digitally augmented
measurements, in bits, which is just a
fractional measure expressed in powers of two, as found in computers. Thus,
10-bit resolution is one part in 1024 (210);
12-bit is one part in 4096 (212); etc.
Response denotes a sensor-based
measuring system's performance under
dynamic input conditions, that is, when
the system's mechanical input is changing rapidly. It is particularly important to
recognize that response is a measuring
system parameter, not merely a sensor
parameter or specification.
In practice, there are several ways to
characterize response, typically based
on whether the system is a first order
or second order system. Traditional
analog systems have used Bode plots to
show frequency response and phase lag
for repetitive inputs. For step function response, three times the system time constant is a typical measure of dynamic performance. In digital sampling systems,
the update rate for a specified number
of bits is one of the preferred measures of
response. Regardless of the choice of how
Figure 1: Sensor
repeatability testing
apparatus.

to specify response, the ultimate purpose
is to understand how well the measuring
system can respond to a changing input
before the system's output becomes inaccurate, unusable, or unstable.

Alliance Sensor Group's
MHP-7 series linear
inductive in-cylinder
position sensor

Interactions
From the foregoing definitions, it is easy
to see that a system's repeatability could
easily be affected by its resolution. If the
measuring system's resolution is inadequate, it would likely be a significant
limiting factor to excellent measurement
system repeatability. In practice, sensor
repeatability may be excellent, but measuring system repeatability cannot be any
better than that permitted by the system's
resolution.
While the interaction of repeatability
with resolution in a measuring system
is pretty easily understood, the interactions of resolution and response are not
so straightforward. When the system's
mechanical input is changing rapidly, the
effects of resolution on system output are
usually masked by the larger effects of
decreased system output due to limitations imposed by the system's dynamic

response. But if the mechanical input to a position measuring system changes slowly or
intermittently, especially in a jerky
way, then the effects of stiction (static
friction) come into play.
Typically, the effects of stiction in position measuring systems can be nonlinear
and are often not very repeatable, so determining or characterizing system resolution can be much more complicated, if
even possible. And because the system
resolution interacts with system repeatability, as was noted above, measurement
errors can increase substantially, particularly if the system is providing position feedback for closed-loop control. Of
course, any effects caused by stiction will
also appear as nonlinearity in the sensor's
output. But because stiction effects are
not very repeatable, digital linearization
techniques to offset the nonlinear effects
will not be practical.
For this reason, efforts to reduce stiction are usually necessary to minimize
any measurement errors caused by stiction in very slow-moving or intermittentmotion positioning systems. These efforts
can involve applying techniques such as
"dither," a low-amplitude signal of high
frequency that is input into the system
to supplant stiction with much reduced
dynamic friction, or by decreasing friction
on the moving surfaces of the sensor by
improving their surface finishes and by
coating them with a lubricant.
Edward E. Herceg is currently
vice president and chief technology officer of Alliance
Sensors Group, a division of
H. G. Schaevitz LLC. With a
distinguished career in the sensors industry
spanning more than half a century, he is highly
regarded both for his applications engineering
expertise and for his technical innovations.

NOVEMBER 2019 EVALUATIONENGINEERING.COM

http://www.EVALUATIONENGINEERING.COM

Evaluation Engineering

Table of Contents for the Digital Edition of Evaluation Engineering