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 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. (Specification selection article provided by Penny & Giles Controls Ltd. - www.pgcontrols.com)
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