|
Important factors for the specification of Linear Position Sensors LVDT |
Determine
the displacement
The length of displacement that needs to be measured will most likely
determine the type or range of sensors available (rod, slide or cable
operated).
Consider the mounting of the sensor
Can the sensor be mounted close to the movement, integrated within the
equipment, or will it need to be situated away from the moving part?
Consider the aftachment method
The attachment between the sensor and the moving part can either be a
fixed mechanical interface or a spring biased probe that follows the
moving surface.
Vibration conditions
Careful consideration needs to be given to the impact of vibration on
the sensor, and whether this can be detrimental to operation and life.
This factor may determine the type of sensing element to select -
contacting or non-contact.
Shock conditions
High levels of shock can seriously affect the operation of a sensor,
either permanently damaging the device or degrading the output, so
careful selection of a device that can withstand this treatment is
important.
Temperature variation or extremes
Extremes of temperature (hot or cold) need to be considered, and whether
the sensor will be required to operate within its specification at these
extremes or just survive under storage conditions. Some sensor
technologies are particularly susceptible to changes in temperature,
resulting in drifting output signals, which could be mistaken by a
control system as a valid movement of a machine part.
Resistance to ingress of particles and liquids
Environmental protection of the sensor may be required where it is
operating in harsh conditions, to stop the ingress of harmful particles
or liquids that may damage the sensor. Protection to lP68 can be
specified in some specialist designs, but IP66 is normally readily
available as an option on standard models.
Corrosion resistance
Protection from the effects of corrosive materials may be required. A
sensor that has been manufactured using corrosion-resistant materials
(such as stainless steels or engineering polymers) will be necessary in
these applications.
Hazardous areas
If the application is in an area where explosive gases are present, then
consideration must be given to selecting a sensor that has been
specially designed, tested and approved to be safe to operate in this
environment.
Sensor life
The duty cycle of the application being measured is important when
selecting the type of sensor to use. A typical benchmark for linear
potentiometers is 200 million operations, but a really heavy-duty cycle
may be better suited to a sensor that uses technology operating on a
non-contacting principle, although this can have an impact on cost.
Accuracy
The accuracy of the sensor is determined by a combination of the output
signal conformity ('linearity' or 'non-linearity') and the temperature
coefficient of the sensor. Overall system accuracy should be considered
over the operating temperature range of the equipment.
Sensor resolution
The resolution of a sensor is the smallest measurable change in the
output signal. Most linear position sensors now use technologies that
provide virtually infinite resolution; this is normally stated in sensor
manufacturers' technical data.
Repeatability
The ability of the sensor to provide repeatable signals is of paramount
importance. Sensor manufacturers will quote a figure for the deviation
in indicated position when a point along a stroke length is approached
repeatedly from the same direction. This factor is often confused with
the sensor resolution.
Hysteresis
This is the difference in indicated position for the same point when
reached from opposing directions. This may be an important factor to
consider but most linear position sensors have minimal or negligible
values.
Power supply available
An important factor to consider is the supply requirement to the sensor.
Most operate on values within the range of 5VDC to 3OVDC.
Output signal required
The output from the sensor can vary, but can be DCV, ACV, DCmA or a
range of digital signals (such as TTL, R5232 or CAN). The control
interface to the sensor will usually determine the type of signal
required to be specified.
EMC/EMI
The ability of a sensor to withstand operation in electrically noisy
environments has become more important since the introduction of
European regulations on EMC/EMI. CE marks ensure testing and compliance
with regulations.
Cost of ownership
A factor often overlooked when selecting a position sensor is the cost
of ownership over a period of time. Selecting a sensor on price alone
may compromise the reliability of a system, particularly if constant
failure involves service costs, downtime and lost production.
Product availability
Sensors that are readily available from stock or manufactured within
days of ordering can provide a considerable advantage to project
development times. Additionally, holding spare parts to support
after-sales is virtually eliminated.
Supplier experience
Do not underestimate the value of asking suppliers about their
experience.
|
