According to the American National Standards Institute, a sensor is a device that provides a useful output in response to a specified measurand. It acquires a physical quantity and converts it into a signal suitable for processing (e.g., optical, electrical, mechanical).
Commonly, sensors convert measurement of physical phenomena (such as biological, chemical, electric, electromagnetic, temperature, magnetic, mechanical motion, optical and radioactive) into an electrical signal.
When the input is a physical quantity and output electrical, the device is called a sensor. When input is electrical, and the output is a physical quantity, we call it an actuator. All sensors are pervasive, embedded in automobiles, airplanes, cellular telephones, radios, chemical plants, industrial plants, and countless other applications. Without them, industrial automation is impossible.
Therefore, choosing the right sensor is a critical part of the industrial automation design cycle. It requires an understanding of the type of motion, precision of movement, magnitude of motion, operating conditions, cost, availability, lifetime, power consumption, etc.
Industrial automation sensors are classified into four categories –
- Proximity
- Mechanical
- Optical
- Inductive/Capacitive
- Position/Velocity
- Potentiometer
- LVDT
- Encoders
- Tachogenerator
- Force/Pressure
- Vibration/acceleration
Proximity Sensors
Proximity Sensors in industrial automation are widely used in conveyor lines (counting, jam detection, etc.) and machine tools (safety interlock, sequencing) with digital (on/off) sensors to detect the presence or absence of an object. They consist of a sensor head (optical, inductive, capacitive), detector circuit, amplifier, and an output circuit (TTL, solid-state relay). Proximity sensors are used to identify the physical contact and position of contact in operation-critical or safety-critical situations.
Optical Proximity sensors comprise a light source (LED), a light detector (phototransistor), a modulation of the signal to minimize ambient lighting conditions. Typically used in stack height control, box-counting, fluid level control (filling and clarity), breakage and jam detection, these sensors work in three operational modes: beam mode with a range of 20m, retro-reflective with a range of 1-3 meters, and diffuse-reflective mode with a range of 12-300 millimeter. The good things about an optical proximity sensor are that they are non-contact, small, insensitive to vibration and shock and have no moving parts. However, they have some downsides. They always require alignment and can be blinded by ambient light conditions such as welding and requires clean, dust, and water-free environment.
Ultrasonic proximity sensors use sound pulses with a frequency range of 40KHz-2MHz to measure amplitude and time of flight. They are often used to detect the level of solids and liquids and approach warning before collisions. The examples of famous use cases include car wash applications, paper roll thickness monitor, wastewater flow management. Inductive and capacitive proximity sensors, often regarded as very robust and reliable, use change in the local magnetic field to detect the presence of metal targets.
Position and Velocity Sensors
The position and velocity measurement is often required in feedback loops for positioning and velocity control. Potentiometers are analog sensors, working as a voltage divider. The common types of potentiometers include wire-wound (wiper slides along with coil of Ni-chrome wire), cermet (wiper slides on conductive ceramic track), and plastic film. The good thing about the potentiometer is that it requires an analog signal for control and is low cost. The common downside is that it is affected by temperature and cannot be used in dusty or wet environments.
Linear Variable Differential Transformer (LVDT) contains a moving magnetic core in a cylinder. The cylinder sleeve has a primary coil, driven by an oscillating voltage. The sleeve also includes two secondary coils that detect this oscillating voltage with a magnitude equal to displacement. The automatic nulling, achieved using two coils, makes LVDTs very accurate in submillimetre. LVDT Signal Conditioning uses AC modulation, demodulation, and phase comparison. LVDT is of high accuracy and can work in a harsh environment.
Optical Encoders are digital sensors commonly used to provide position feedback for actuators. They consist of a glass or plastic disc that rotates between a light source (LED) and a pair of photo-detectors. A disk is encoded with alternate light and dark sectors, so pulses are produced as the disk rotates.
In incremental encoders, pulses from LEDs are counted to provide a rotary position. Two detectors are used to determine the direction, and an index pulse is used to denote start point. Meanwhile, absolute encoders have a unique code that can be detected for every angular position, often in the form of a “grey code,” a binary code of minimal change. They are much more complex and expensive than incremental encoders. All encoders are compact and reasonably rugged. They always require accurate position information and a digital feedback loop.
Primarily, a motor running in reverse, tachometers measure the rotary speed using a DC generator. They are much less common now and were attached to motors to enable direct analog feedback.
Force and Pressure
There are many techniques to measure forces and pressures. Very often, the force is converted into a change in length or height of a spring. The difference in dimensions is subsequently measured using an LVDT, strain gauges (metal that changes resistance when stressed), and piezoelectric materials that generate a current when deformed. Force sensors are complex since force is not converted directly into an electric signal. The LVDT sensor produces a voltage proportional to the applied force within the linear range of the spring.
Force sensors are divided into two classes: Quantitative sensor, which measures the force and represents its value in terms of an electrical signal (strain gauges and load cells, for example), and Qualitative sensor, which indicates whether a sufficiently strong force is applied or not.
Pressure sensors can be classified into five categories, based on the pressure ranges they measure, temperature ranges of operation, and the type of pressure they measure. The absolute pressure sensor measures the pressure relative to the perfect vacuum pressure (0 PSI or no pressure). Atmospheric pressure is an absolute pressure, and it is about 100kPa (14.7 PSI) at sea level.
Gauge pressure sensor measures the pressure of a given atmosphere in a given location. A tire pressure gauge is an excellent example of gauge pressure. As the tire pressure gauge reads 0 PSI, it implies 14.7 PSI (atmospheric pressure) in the tire. A vacuum pressure sensor is used to measure pressure less than the atmospheric pressure at a given location.
Differential pressure sensor calculates the difference between two or more pressures introduced as inputs to the sensing unit. It is also used to measure flow or level in pressurized vessels. A sealed pressure sensor works like a Gauge pressure sensor, although it is pre-calibrated by manufacturers to calculate the pressure relative to sea level.
Vibration and Acceleration
Acceleration sensors detect vibration, acceleration, and shock levels in machines and facilities, measuring the dynamic acceleration of a physical device as a voltage. Accelerometers are full-contact transducers usually mounted on high-frequency devices.
Accelerometers make use of the piezoelectric effect, which happens when a voltage is generated across certain types of crystals when they are stressed. The acceleration of a test object is transmitted to a seismic mass inside the accelerometer, creating a proportional force on the piezoelectric crystal. This external stress causes a high-impedance electrical charge, which is proportional to the applied force as well as acceleration. It requires an external amplifier or charge converter to amplify the generated charge and minimize susceptibility to external noise sources.
Used in gas turbines, axial compressors, small and mid-size pumps, vibration and acceleration sensors detect high-frequency vibration signals, related to bearing supports, casing and foundation resonances, vibration in turbine/compressor vanes, defective roller or ball bearings, noise in gears, etc. They detect static shaft displacements, unbalance response, misalignment, shaft bending, excessive loads in bearings, dynamic instabilities, etc. Vibration monitoring sensors help to detect machine damage in good time and prevent costly consequential damage. Vibration is often measured using a ceramic piezoelectric sensor or accelerometer.
