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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Do not underestimate the value of asking suppliers about their experience.
Specification selection article provided by
Penny & Giles Controls Ltd. - www.pgcontrols.com
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