Analog inductive sensors can be advantageous in the
design of machine control systems. These sensors are especially useful for
applications requiring precision position sensing and measurement, as well as
for use in component error-proofing. In your final selection of a specific
sensor, however, it is important to examine certain performance characteristics
to ensure the unit you choose is optimized to best match the requirements of
your machine.
Inductive Sensors in Machine Control
The main reason industrial position sensors with analog
outputs are useful to system designers is because of their ability to negate
the need for discrete output switching. Instead of switching a discrete output
when a target reaches a specific distance from the sensor face, an analog
sensor provides a variable output with an output voltage or current level
proportional to the distance of the target from the sensor face.
This performance makes an analog inductive sensor well-suited for
applications that demand precision position sensing and measurement. This type
of capability is very useful for an end-user engineer who needs to measure the
angular position of a rotating shaft, for example. By designing an eccentric
cam onto the shaft and using the analog inductive sensor's proportional output
to detect the distance to the cam, the designer can calculate the shaft
position.
In another application example, analog inductive sensors can
effectively monitor deflection of a critical component. For example, these
sensors can measure axial deflection of a rotating saw blade; the control
system can then be directed to reduce speed based on the sensor output, thereby
preventing significant damage to the machine from a distorted blade.
Analog sensors can also be used for component error-proofing,
signaling a divert cylinder to reject a too-thick part, for example. In another
typical quality inspection application, the specific insertion depth of a
fastener or secondary part in an assembly is measured and a range of control
system actions are taken based on the results. In more advanced systems, these sensors can be used to provide statistical
process control data (e.g., changes in measured part thickness over
time).
Given that many major inductive sensor manufacturers make analog
sensors in this category and they all appear similar, how much do the specific
performance characteristics of a particular analog sensor matter? Or are all
analog inductive sensors the same?
Linearity Matters
To answer this question, it is important to understand what
is meant by the term "linearity" as it applies to analog position sensor
outputs. This is depicted by the solid line in Table 1.
Simply put, an analog inductive sensor that is "linear" has the
same range per division scale across the full sensing range of the device. This
means a 1 mm change in the position of the object results in the same change in
output current or voltage, no matter where the target object is within the
range envelope of the sensor. In this case, the solid line represents the output
of a "linear" analog sensor.
The dashed line shows a sensor with a nonlinear response. Using a
device with this output characteristic, a 1 mm change in range to the target
when that target is in the middle of the sensor range will result in a change
in output current or voltage that is quite different from the amount of change
seen with the target at one end of the range band or the other.
How does this linearity
characteristic impact the design of your control system? Consider an
application where a cutting tool is located against a rotating metal part, with
the position of the cutting tool based on the measured thickness of the
material. As the material thickness increases, the tool is retracted and, as
the thickness drops, the tool position is moved inward. A non-linear analog
inductive sensor with a maximum range of 10 mm positioned to measure the
thickness of the part yields the 0-10V analog voltage measurements shown in
Table 2 based on varying thicknesses of the part.
As the table indicates, analog position sensors with non-linear
outputs used in industrial applications should be applied with caution, as the
non-linearity can complicate the design of the control system and place
restrictions on the physical application of the sensor to be used on OEM equipment.
Looking at this table, it is clear some signal processing compensation will be
required to deal with the evident non-linearity. Since the non-linearity in
this case is also non-uniform at the two ends of the sensing band, a lookup
table is likely easier to implement than a compensation equation. Either way,
the designer's PLC code is made more complex and the response time of equipment
potentially reduced as workarounds are created to deal with the non-linear
nature of this sensor output.
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In the event the performance shown in the table is not
repeatable, then the range of operation of the sensor may need to be restricted
to only the central portion of the range band in an attempt to use the most
"linear" portion of the performance curve shown in Table 1. This approach has
downsides as well, as the sensors in question often have very limited sensing
ranges. If the decision were made to use only the most "linear" portion of the
non-linear sensor shown in Table 2, the designer would only have a 3 mm range
window centered on 5.5 mm to work with. Managing a small range window of that
size may prove impossible and force the designer into a different sensing
technology entirely.
Reduce Complexity
So what's a control system designer to do? Consider the
same example described above, but using a sensor with a linear output (see
Table 3).
In this case, the controls design task is much easier. No lookup
table. No compensation equation. Simplified PLC logic. And, since the full
operating range of the sensor can be used, it can be pulled further out of
harm's way. For the OEM's customers this means greater machine uptime and
higher throughput and overall equipment effectiveness.
These examples make it clear that linearity is an important
characteristic of analog position sensors. Fortunately, new approaches to
analog position sensor design have yielded dramatic improvements in linearity.
In addition, technological breakthroughs have allowed sensors manufacturers to
offer greatly extended sensing ranges in analog devices. Taken together, these
design improvements allow the control system designer much greater flexibility
in the application of analog sensors on a machine, and serve to reduce the
complexity of any control system.
Matt Simms is the product line manager of Eaton's line of industrial sensors. For more information, go to http://www.eaton.com/Eaton/index.htm.
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