Sensors PLC

Continuous sensors convert physical phenomena to measurable signals, typically voltages or currents. Consider a simple temperature measuring device, there will be increase in output voltage proportional to temperature rise. A computer could measure the voltage, and convert it to a temperature. The basic physical phenomena typically measured with sensors include;

-          angular or linear position

-          acceleration

-          temperature

-          pressure or flow rates

-          stress strain or force

-          light intensity

-          sound

Sensors are also called transducers. This is because they convert input phenomena to an output in different form. This transformation relies upon manufactured device with limitation and imperfection. As a result sensor limitations are characterized with below major facts:

1. Accuracy: This is the maximum difference between the indicated and actual reading. For example, if a sensor reads a force of 100N with a + - 1% accuracy, then the force could be anywhere from 99N to 101N.
2. Resolution: Used for systems that step through readings. This is the smallest increment that the sensor can detect; this may also be incorporated into the accuracy value. For example, If sensor measures up to 10 inches of linear displacement, and it outputs a number between 0 and 100, then the resolution of device is 0.1 inches.
3. Repeatability: When a single sensor condition is made and repeated, there will be a small variation for that particular reading. If we take a statistical range for repeated readings ( eg : + - 3 deviations) this will be the repeatability. For example, If a flow rate sensor has repeatability of 0.5 cfm, reading for an actual flow of 100cfm should rarely be outside 99.5cfm to 100.5 cfm.
4. Linearity: In linear sensor the input phenomena has linear relationship with the output signal. In most sensors this is desirable feature. When the relationship is not linear, the conversion from the sensor output (eg. Voltage) to a calculated quantity (eg. Force) become more complex.
5. Precision: This considers accuracy, resolution, repeatability or one device relative to another. In other way, we can say a data is precise when the repeatability of that data is more than others at different operations.
6. Range: Natural limit for sensor. For example, a sensor for reading angular rotation may only rotate 200 degrees.
7. Dynamic response: The frequency range for regular operation of sensor. Typically sensors will have an upper operation frequency, occasionally there is lower frequency limits. For example, our ears hear best between 10 HZ and 16KHZ.
8. Environmental:  Sensors all have some limitations over factors such as temperature, humidity, dirt/oil, corrosives and pressures. For example many sensors will work in relative humidity (RH) from 10% to 80%.
9. Calibration: When manufactured or installed, many sensors will need some calibration to determine or set the relationship between the input phenomena, and output. For example, a temperature reading sensor may need to be zeroed or adjusted so that the measured temperature matches the actual temperature. This may require special equipment, and need to be performed frequently.
10. Cost: Generally more precision costs more. Some sensors are very inexpensive but the signal conditioning equipment costs are significant.