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When deciding which sensor technology is suitable for your measurement application, the first consideration is whether you need to measure linear or rotational movement (sliding or rotating sensor operating shaft)? The second consideration is the output signal required, absolute or incremental (analogue or encoder)? The third question is whether the sensor can be contacting (potentiometer) or does the application need a contactless solution (LVDT, RVDT or Hall Effect)?
If you require a signal that specifies a unique measurement position then you will require a sensor with an absolute output. Absolute output usually means that the unique measurement position is held electro-mechanically by the sensors operating shaft. Active Sensors manufacture position sensors with an absolute output signal, as the majority of high performance control and measurement systems require signals of this type. Incremental devices typically need to be re-calibrated when powered up and this procedure can be impractical in most applications.
The operating environment of the application will have an impact on the measurement technology choice (environmental testing). Consideration should be given to possible temperature changes and the presence of any shock loads and vibration. The presence of any moisture or fluid will also dictate the level of sealing required for the sensor housing (IP rating). Any moisture ingress will have an adverse affect on the sensors operational life. The application may have other unique environment aspects such as high pressures or the presence of corrosive chemicals. Contacting and contactless sensors each have the ability to operate in these environments, but there is always a trade-off in performance i.e. service life and sensor unit cost.
The measurement range of a position sensor varies from millimetres to metres, or 1° to 360° of rotation. Accuracy, which typically has the components of non-linearity, repeatability, resolution, hysteresis and thermal drift are usually stated on the manufacturers data sheet. For many applications repeatability and thermal drift are the most important components. All sensors when operated at elevated temperatures will exhibit some unwanted increase in the output signal caused by temperature. The drift is caused by material property changes and the expansion of the sensors electrical and mechanical components. Potentiometers typically experience lower levels of thermal drift compared to contactless sensors with integral electronics. Checks should be made to establish if the accuracies stated are at one temperature (usually room temperature) or over the temperature range of the sensor. The accuracy of a sensor is normally specified in absolute units, i.e. millimetres or microns, or in relative units such as percent of full-scale measurement.
For dynamic measurement applications, the maximum velocity to be monitored will influence the choice of sensor technology. You will need to ensure the sensor, and also the control or data-acquisition system has adequate sampling rates to record the resulting data. Also, as the sensor is going to be a part of a measurement or control system, determine the preferred electrical input and output signal requirements. The usual choices are analogue, typically 5V or 6-30V (AC or DC), current (4-20 mA) or digital (PWM, CAN and RS485).
More often than not, the cost of a sensor is not an important factor when first designing a measurement and control system, but may become an issue as the project progresses. Ensure you consider the initial purchase cost as well additional accessories that maybe required, for example, special signal-conditioning electronics, electrical connectors or mounting fixtures.
Choosing the correct measurement technology should become self evident after determining your requirements from the above information. If you require additional information regarding technology selection, please do not hesitate to contact our technical team: contact us
