A guide to selecting non-contact position sensors
Nowhere is the trend
towards non-contact sensing more evident than in position measurement.
With such a bewildering array of options, it can be difficult
to select the most appropriate instrument for a specific application.
Step 1 in selecting the right
sensor is to be clear about the requirements, particularly resolution,
repeatability and linearity. Over-specifying will cause unnecessary
expense.
Step 2 is to have a basic
understanding of the sensor's operating principles, as these
dictate the inherent strengths and weaknesses of any instrument.
The following provides a description of the most common principles
and their relative merits.
Potentiometers -
the benchmark
Despite the trend towards non-contact sensors, potentiometers
('pots') remain very common and provide a useful benchmark. Pots
measure a voltage drop as a contact(s) slides along a resistive
track. These sensors are widely available, compact and light.
A simple device will cost less than £1, whereas a precision
instrument may cost more than £100. Pots operate well in
applications with modest duty cycles and benign environments.
Unfortunately, pots are susceptible to wear and foreign matter.
Higher quality devices quote long life, but this ignores vibration
cycles.
| Strengths: Low cost; simple; compact; lightweight.
Can be made accurate. |
| Weaknesses: Susceptible to wear;
vibration; foreign matter; extreme temperatures. |
Optical
Optical sensors are also common, ranging from simple devices
that cost less than £10 through to precision units costing
in excess of £1,000. Their basic principles are similar:
light is shone through or onto a grating and the resulting signals
used to calculate position. Packaged, rotary encoders are widely
available with 100-5,000 counts per revolution. Failures occur
if the lens or grating become obscured by foreign matter.
It is important
to note that if the encoder specifies 1,000 counts per revolution,
this does not mean that it is accurate to 1/1000th of a revolution.
Data sheets need to be carefully analysed, particularly for unpackaged
encoders, which require accurate installation. Most units offer
limited shock resistance.
| Strengths: High resolution;
good accuracy if mounted precisely; wide availability. |
| Weaknesses: foreign matter; catastrophic
failure; shock; extreme temperatures. |
Magnetic
Magnetic sensors all use a similar principle: as a magnet moves
relative to a detector, the field changes. Magnetic sensors overcome
many of the drawbacks of optical devices, as they are tolerant
to foreign matter, but they are rarely used for precision applications
due to hysteresis. Data sheets need careful analysis with respect
to temperature coefficients and the effects of nearby materials
or electrical sources.
| Strengths: Fairly robust; most
liquids have no effect. |
| Weaknesses: Temperature; hysteresis; precision
mechanical installation; nearby steel/DC sources and poor impact/shock
performance. |
Magnetostrictive
These sensors use a phenomenon called 'magnetostriction'. When
a magnet approaches certain materials it causes energy passing
along the material to reflect. Position can be measured from
the time a pulse of energy takes to move through a strip of magnetostrictive
material. The delicate strip must be carefully held in a wave
guide and so shock or vibration is often problematic. Each sensor
needs calibrating by the manufacturer, making magnetostrictive
sensors relatively expensive.
The technique
is sensitive to other influences - most notably temperature.
Magnetostrictive data sheets often quote accuracy at constant
temperature.
| Strengths: Robust; well suited
to high pressures; % accuracy increases with length. |
| Weaknesses: Relatively expensive;
temp. effects; inaccurate over short distances (<100mm). |
Capacitive
A capacitor is a device that accumulates charge. Typically, it
has two conductive plates separated by an insulator. The stored
charge varies with the size of the plates, their overlap, separation
and permeability of the material between the plates. In its simplest
form, a capacitive sensor measures plate separation. Displacements
are usually <1mm for load, strain and pressure measurement.
Another form uses plates that are cut or etched along the measurement
axis. As another plate moves across them, capacitance varies,
indicating position.
Capacitance
also varies with temperature, humidity, nearby materials and
foreign matter, which is why capacitive devices are seldom used
in safety-related applications.
| Strengths: Compact; low power |
| Weaknesses: Large temperature
and humidity coefficients; sensitive to foreign matter. |
Traditional
Inductive
Linear inductive sensors are referred to as variable reluctance
or linearly variable differential transformers (LVDTs). Rotary
forms are known as synchros or resolvers. These work on inductive
or transformer principles. As a magnetically permeable element
moves, it varies inductance or transformer coupling. In LVDTs
and resolvers, the ratio of the induced signals indicates position
and this ratiometric technique is the key to their performance
and solid reputation.
Whereas optical
or magnetic sensors require electronics adjacent to the sensing
point, inductive sensors can use remote electronics in benign
condition with the sensor located in harsh environments.
| Strengths: High accuracy; reliable;
robust; extreme environments; widely available. |
| Weaknesses: Expensive; bulky;
heavy. |
New
Generation Inductive
New generation inductive sensors - such as those made by Zettlex
- use the same principles as traditional inductive sensors, offering
good, non-contact performance in harsh environments. Rather than
bulky transformer constructions, these sensors use printed circuits.
The transition to printed windings brings other advantages:
* Reduction in cost, size and weight
* Greater flexibility in form factor
* Eradication of inaccuracy from the winding process
* Complex measurement geometries such as curvilinear, 2D &
3D sensing
* Multiple sensors can be co-located using multi-layer circuit
boards (e.g. redundant sensors in safety applications).
EMC performance
is generally as good as resolvers or LVDTs.
| Strengths: High accuracy; reliable; robust;
multiple geometries; compact; lightweight; resilient in harsh
environments. |
| Weaknesses: More expensive than
potentiometers. |
From a press release
issued by Zettlex
(UK) Ltd.
March 2013
