THE DC-LVDT DISPLACEMENT TRANSDUCER
The DC-LVDT is based upon two secondary coils, symmetrically
wound on to a primary coil.
Movement of the push rod displaces the position of the high
permeability armature which determines the voltage induced
from the primary to each secondary.
This voltage is essentially a linear function of displacement
and is conditioned by the hybrid circuit.
The coils are separated from the armature assembly by a stainless
steel bobbin which provides excellent isolation from the outside
environment such as fluids, dust,etc.
In ‘free armature’ unguided versions there is no physical contact
between, the armature and coils making it inherently a friction free
device providing infinite resolution with no hysteresis.
This means the LVDT can respond to the most minute movement
of the high permeability armature.
It also provides long life with excellent repeatability making
these devices a natural choice for closed loop control.
Devices are available for the majority of applications from
laboratory to sub sea which provides variations of the
stainless steel body tube and also the screened cable.
To find out how a LVDT Displacement Transducer works, click here...
This information is supplied by RDP Electronics Ltd.
Website: www.rdpe.com
Important factors for the specification of Linear Position Sensors
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 attachment 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