- What is a sensor and what are industrial sensors?
- Major Industrial Process which needs to be measured
- How to select industrial sensors?
- Types of sensors for industrial automation
- 1. Pressure sensor
- 2. Flow sensor
- 3. Level sensor
- 4. Temperature sensor
- 5. Gas sensor
- 6. Motion sensor
- 7. Accelerometer sensor
- 8. Density sensor
- 9. Humidity
- 10. Opacity sensor
- 11. Weigh Scale
- 12. Proximity Sensor
- 13. Conductivity Sensor
- 14. Displacement Measurement Sensor
- 15. pH Analyser
- 16. Vibration probes
- 17. Torque Sensors
What is a sensor and what are industrial sensors?
A sensor would measure a physical quantity and it would create a corresponding electrical signal according to the physical quantity. An industrial sensor is a device that would react to a physical quantity such as heat, light, sound, pressure, magnetism, motion, etc, and it would be converted into an electrical signal. The output signal of the sensor could be electrical, mechanical, magnetic, etc.
Major Industrial Process which needs to be measured
How to select industrial sensors?
Below mentioned are the major factors to be considered while selecting an Industrial sensor
- Response time
- Temperature range
- Humidity effects
- Power consumption
Types of sensors for industrial automation
- Pressure sensor
- Flow sensor
- Level sensor
- Temperature sensor
- Gas sensor
- Motion sensor
- Accelerometer sensor
- Weigh Scale
- Proximity Sensor
- Conductivity Sensor
- Displacement Measurement Sensor
- pH Analyser
- Vibration sensors
- Torque Sensors
1. Pressure sensor
The pressure measurement is really important in an industrial process, it is used to do the measurement of gasses and liquid pressure. These sensors would provide a corresponding electrical output to the pressure.
Classification of the pressure sensor
We can classify the pressure sensor according to the range of pressure they measure, the type of pressure measurement, and also based on the operating temperature range.
Absolute pressure sensor
This sensor would do the pressure measurement relative to a perfect vacuum and we can determine the atmospheric pressure by using this. Pressure barometers/ altimeters are examples of such pressure measurement.
Gauge pressure sensor
Pressure measurement would be done with respect to the ambient atmospheric pressure. This type of pressure sensor would have only one port, so the ambient air pressure would be transmitted through a hole to the sensing element.
Differential pressure sensor
This type of pressure sensor would do the measurement of pressure between two pressures. These sensors are also known as bidirectional differential pressure sensors.
Vacuum pressure sensor
This sensor would measure the pressure with respect to the vacuum and it is utilized to determine the pressure which is lower than the atmospheric pressure.
Sealed pressure sensor
This type of sensor would measure the pressure relative to fixed pressure and not to the atmospheric pressure, which could vary according to the location.
Different types of the pressure sensor
- Fiber optic pressure sensor
- Strain gauge
- Bourdon tube
Mercury and aneroid barometers are the two types of barometers and they can be used to determine the atmospheric pressure. In the case of a mercury barometer, it uses a long tube and this tube will be filled with mercury and this will be placed in a mercury container. The aneroid barometer uses a diaphragm and its size would change according to the air pressure.
The simplest type of manometer is the liquid-filled manometer and in this, the pressure which is to be measured and also the reference pressure will be provided to the tube’s open end. So in case if there is any pressure difference then the height of the liquid on both sides of the tube will be different.
Fiber optic pressure sensor
This is a non-contact type pressure sensor and it also has a high-frequency response. This type of pressure sensor has high accuracy and can be used in a harsh environment. In this sensor, the pressure measurement is done according to the variation in pressure through an effect on light. Light rays will be transmitted through an optical fiber to the diaphragm and thus the diaphragm movement can be determined. So when the light reflects back from the diaphragm we could determine the external pressure which is attached to the diaphragm.
This is a major type of industrial pressure sensor it would change its resistance when it is stretched. Basically, this pressure sensor is a wire which would change its resistance when it is stretched, so if the cross-sectional area of the wire is smaller then the resistance will be greater. If pressure is applied to the wire then the wire would stretch and vary its resistance.
The bourdon tube is a bent tube and it would vary its shape if it is subjected to internal and external pressure variations. So when the pressure is applied to the internal part of the tube then the tube will be straightened and it would regain its original position if the pressure is released.
Applications of the pressure sensor
- Fluid and gas pressure measurement
- Measurement of altitude
- Oil and gas
- Subsea applications
- Oil processing
2. Flow sensor
This sensor has many applications in an industrial process, mostly the flow measurement is done by placing an obstruction in the flow stream. So because of this, there will be periodic turbulence behind it. So we can measure the flow by measuring the frequency of the turbulent vortices.
Functions of the flow sensor
- Determine the velocity of the flow in units of interval
- Determine the flow capacity in the pipe units
- Determine the total volume that has flowed like liters, gallons, etc
- Monitor the flow
In the above image, we can see the classification of flow meters, there are volumetric flow meters that would measure the flow volume directly and an example for this would be the positive displacement meter. Mass flow meter would measure the mass directly and an example for this would be the Coriolis flow meter. There are other types of flow meter such as velocity flow meter and it would determine the flow rate by multiplying the velocity with the area that it flows. The head-type flow meter would utilize an obstruction to measure the flow an example of this type of flow meter is the target flow meter.
Different types of the flow sensor
- Magnetic flow meter
- Coriolis mass flow meter
- Oval gear meter
- Orifice flow meter
- Target flow meter
- Paddlewheel flow sensor
Magnetic flow meter
In this type of flow meter, the liquid will flow through a magnetic field, and in order to do the flow measurement, the liquid should be a conductor. So when the liquid flows through the magnetic field there would be an induced voltage in the liquid. So in order to measure the flow, there will be two probes in the pipe, and when the liquid flow increases then the induced voltage will increase too.
Coriolis mass flow meter
In this type of flow meter, a vibrating tube is used to do the flow measurement. When the fluid flows through the tube, the tube would vibrate and this change in frequency or the amplitude can be used to do the measurement.
Oval gear meter
This type of flow meter is used to do the measurement of continuous and irregular flow. Mostly this flow meter is used with the positioner indicators and also the roller integration devices in a pipeline. We can use this meter for different fluids such as organic solution, salt, acid, alkali, etc.
Orifice flow meter
In this flow meter, it uses an orifice plate and in this plate, we can see a hole. This plate will be placed perpendicular to the flow and because of this the flow would take place through the plate hole and the flow path is narrowed. So while measuring the pressure differential across the narrow flow path we would get the flow rate.
Target flow meter
This type of flow meter would place an object in the flow path and it would measure the force which is applied by the fluid that passes through it. The object will be placed at the right angle to the fluid flow.
Paddlewheel flow sensor
This can be classified as a turbine flow meter. The flow would cause the paddle to rotate and we can determine the flow rate by this rotation. The flow and the revolution of the paddlewheel will be proportional. A magnet will be placed near the rotation axis of the paddle and from there it would extract pulses as signals.
How to select a flowmeter?
- Type of fluid (viscosity)
- Clean/dirty fluid
- Pipe diameter
Applications of the flow sensor
- Chemical factories
- Medical devices
- HVAC systems
- Leakage and blockage can be detected
- Pipeline bursts can be detected too
- Oil and gas industry
3. Level sensor
We can use these sensors to detect the level of bulk solids or liquids. These sensors can be used to measure the level of oils and powders. These sensors are inevitable in a fluid system, with the help of this sensor we would be able to determine the position or displacement of an object.
Different types of the level sensor
- Float level sensor
- Capacitance level sensor
- Ultrasonic level sensor
- Radar level sensor
- Optical level sensor
Point level sensor
As the name indicates this type of sensor would detect the specific level and the operator would be able to determine if the object is above or below that level. These sensors can be used as alarms.
Continuous level sensors
This type of level sensor would measure the liquid or dry material levels within a specified range and it would send an output that shows the level. So basically these sensors won’t measure a specified level like the point level sensor instead of that they would measure over a range of levels.
Point to point level measurement
In a certain process, we should determine if the media or object is present between two points. So these two points would be high and low level, so in order to do this type of process measurements, we can use the point to point level measurement method.
Float level sensor
In this type of sensor as the name indicates a float is used and it is used for alarming applications. So by using this level sensor we could determine if the level is low or high. There is a continuous float level sensor and it can be used to determine the exact amount, of media in a continuous manner.
Capacitance level sensor
This type of level measurement is done based on the change in capacitance, there will be an insulated electrode and it would act as a capacitor, and also there is a reference electrode that also acts as a capacitor. The capacitance would vary according to the tank level, if the water level is lower then, the capacitance would be less and if the tank level is high then the capacitance would be high too.
Ultrasonic level sensor
This type of level measurement is done by transmitting ultrasonic waves, this sensor would have a transistor, receiver, timing, and control, etc. So the ultrasonic waves would be transmitted to the tank that has a fluid and when the echo is received we could determine the level.
Radar level sensor
In this type of sensor, microwave pulses are transmitted to the process fluid which is to be measured. The microwave echo will be received as a result of the transmission, we can calculate the level by the received pulse signal.
This sensor uses an infrared LED and also a light receiver. Light will be emitted from the LED and this light would travel through a prism. So when there is no liquid then the light will be reflected within the prism.
Applications of the level sensor
- Food and beverage
- Material handling
- Chemical industries
- Water treatment industries
4. Temperature sensor
This is a type of sensor that would give temperature measurements in the form of an electrical signal. The output signal will be electrical and it represents the temperature level.
Types of temperature sensor
- Contact type temperature sensors
- Non -contact type temperature sensors
Different types of temperature sensor
- IR Temperature Sensor
- Optical Pyrometer
- Radiation thermometer
This device would measure the temperature by using two dissimilar metals, this device would convert the thermal energy into electrical energy. In this sensor, two dissimilar metal wires will be connected and it would form a junction in one end. This connected part will be heated and due to this, there will be a voltage across it. A meter will be connected to the opposite ends of the wire to measure the voltage.
Most RTD’s are constructed by using nickel or copper. It has a linear change in resistance over the temperature, so this type of temperature measurement is done by utilizing the electrical property of the metal. So the above-mentioned metal would change its resistivity according to the temperature.
This is another type of temperature sensing element, this device has a sintered semiconductor material. The major feature of this device is that if there is a small change in temperature then it would exhibit a large resistance change. This type of temperature sensor has great accuracy. The thermistor is really small and this is another important advantage we can use this device where space is very limited.
IR Temperature Sensor
The IR temperature sensor would be capable to detect moving objects, we can’t use a contact-based temperature sensor on a moving object so because of this, we can use an IR sensor. This sensor would be composed of a photocell and it would transmit and receive the infrared light. We can use this sensor to detect the motion.
This is another type of non-contact temperature measurement and it would do temperature measurement by brightness comparison. Basically, this device would utilize the human eye to match the brightness of the hot object, to the brightness of the calibrated lamp filament which is inside the instrument.
This is a non-contact type temperature sensor and it would do the temperature measurement by the amount of thermal electromagnetic radiation received from an object. The received radiation level will be then converted to a voltage. The major applications of these sensors are in industrial furnaces.
Applications of temperature sensor
- Motor – To measure the winding temperature
- Electric cables – To determine the internal temperature of the cable
- Mechanical engines – To determine the engine oil temperature
- Heating control systems
- HVAC systems
- Water incoming tubes – To determine the internal temperature of the water
- Rubber and plastic industry
- Biomedical industry
5. Gas sensor
We can use a gas sensor to detect the changes or presence of various gases. These sensors are inevitable for certain manufacturing plants and also in chemical industries. We could determine the presence of toxic or combustible gas by using this sensor. There are many types of gas sensors such as Oxygen, Carbon-dioxide, Nitrogen oxide sensor, etc.
Applications of gas sensor
- Chemical industries
- Oil and gas
- Petrochemical industries
6. Motion sensor
We can use this sensor to determine if an object has moved from its place with respect to its surrounding. We can use this sensor in security systems for the automatic triggering of lights and also for remote alarms. A motion sensor is capable to measure the motion indirectly by measuring the changes in a magnetic field or in pressure.
Types of motion sensor
This type of sensor would transmit the microwave pulses so when these pulses reflect from the moving object measurement can be done
This type of sensor would transmit the ultrasonic waves and measures the reflection of the moving object.
Applications of motion sensor
- Automated lighting system
- Smart floodlights
- Security system
7. Accelerometer sensor
So this device would convert the mechanical motion into an electrical output. We can use this device to know the physical acceleration of an object.
Types of accelerometers
Hall effect accelerometers
This type of sensor uses the hall principle to do the measurement, so basically it would measure the variation in the voltage which is caused by the variation in the magnetic field around them.
This type of sensor would operate according to the piezoelectric principle and we can use it to measure the vibration and pressure.
Applications of accelerometer sensor
- Vibration detection
- Aviation industries
8. Density sensor
Density measurement is really important in gas and flow measurement, in the flow measurement the density measurement is really important for converting the volume flow into the mass flow.
Different types of the density sensor
- Ultrasonic densitometer
- Sonic liquid densitometer
- Gas densitometers
This is the most common type of density measurement this device is a calibrated glass float. This device is made up of glass and it has a cylindrical stem and a bulb weighing with mercury so that it would float up right. The fluid which should be tested must be added in a container or in a cylinder, and the hydrometer must be lowered to this tank or container till it would float freely. We must check for the liquid point which touches the hydrometer’s stem. This stem can measure the specific gravity of the liquid very easily.
We can use this type of density meter to measure the density of slurries or sludge. In order to do the density measurement, an ultrasonic pulse will be transmitted along the pipeline through which the slurry flows. The ultrasonic energy which is received back is the density of the slurry. We cannot use this type of measurement for highly dense materials.
Sonic liquid densitometer
This type of density meter would utilize sound to do the density measurement. This density meter has a device for m/m of sound velocity in liquid and also it would compensate for the pressure and temperature changes. The major disadvantage of this density meter is that it can only be used with clear liquids and also if there is a variation in bulk modulus then there would be an error in the measurement. The major application of this meter is to determine the interference between two dissimilar hydrocarbon products.
This density meter operates by the principle that a buoyant force that is exerted upon a body suspended in a gas will be proportional to the gas density. This meter has a buoyant float attached to it.
We can define humidity as the presence of water vapor in the air, many industrial processes could be affected if the humidity level increases. Certain processes such as chemical, physical and biological would be affected by the water vapor present. So the determination of humidity in industries is really important.
Absolute humidity – Ratio of mass(vapor) to volume
Relative humidity – Ratio of mass(vapor) to the percentage of mass (saturated mass)
Specific humidity – Ratio of mass (vapor) to total mass percent
Different types of the humidity sensor
- Capacitive humidity sensor
- Thermal humidity sensor
- Resistive humidity sensor
Capacitive humidity sensor
In this type of humidity sensor, hygroscopic dielectric material will be placed between two electrodes like a capacitor. The dielectric constant of the water vapor is really high than that of the sensor. The sensor’s capacitance would increase if it absorbs water vapor.
Thermal humidity sensor
This device has two chambers and both of them would have a thermal conductivity sensor. One of the chambers is closed and filled with reference gas while the other would receive the gas sample. The difference in thermal conductivity of the two gases would be compared.
Resistive humidity sensor
This type of humidity sensor utilizes the conductivity of the non-metallic conductors. This sensor is composed of a polymer-metal oxide thin film placed between two conductive electrodes. This thin film has movable ions and the conductivity of this film would increase if the moisture content increases.
Applications of the humidity sensor
- Chemical industries
- Automated systems
10. Opacity sensor
This type of sensor is used to measure the amount of light that is transmitted, mostly we can use an opacity sensor to determine the smoke. We can use an opacity sensor for the measurement of opacity in industrial applications. We could determine the quality of air by using this sensor. The major opacity measurement methods are single-pass transmissometers and double-pass transmissometers.
Different types of opacity sensor
The major components in this device are the light source, beam splitter, control detector, and measurement detector. A light source would transmit the light through the gas or air and it would reach the measurement detector and according to the variation in the light, source opacity would be determined.
Double pass transmissometer
This type of opacity measurement would use the reflective property of the light to do the opacity measurement. The major component of this device would be the light source, reflector mirror, detector, and auto collimating reflector. The light will be transmitted from the light source through the air or gas to the auto collimating reflector and then it would be reflected back to the transmitter section and it would reach the detector.
Applications of opacity sensor
- Iron and steel
- Thermal power plant
- Iron and steel
- Oil refinery
11. Weigh Scale
Weight sensing is an important part of the industrial processes like in a conveyor unit we would require a weighing sensor. Mostly the weight measurement is done with the comparison with the known weights. It is also done by the deflection of a body and also by measuring the hydraulic and pneumatic pressure.
Different types of weighing sensors
- Strain gauge
- Load cell
- Mass balance weighing instrument
This device has elastic property and this device consists of an electric resistive film and it would change its shape when weight is applied to it. So during the deformation, there would be a change in resistance which can be used to determine the weight.
A load cell can convert the force into an electrical signal, this device is really accurate. A load cell is composed of many strain gauges. Basically, it would calculate the strain, and then the force will be converted into electrical energy. There are hydraulic and pneumatic load cells that can also be used to do the measurement of weight.
Mass balance weighing instrument
This type of weight measurement would compare the gravitational force on the measured mass to the gravitational force of another body of known mass. The major types of these instruments are weighed beam, pendulum scale, electromagnetic balance, and beam balance.
12. Proximity Sensor
This device can detect an object without making any physical contact. So if this sensor comes in near to an object then it would detect it. So this sensor would transmit an electromagnetic beam and according to the received signal, we could determine the presence of an object.
Different types of the proximity sensor
- Inductive proximity sensor
- Optical proximity sensor
- Capacitive proximity sensor
- Magnetic proximity sensor
- Ultrasonic proximity sensor
Inductive proximity sensor
This type of proximity sensor can be used to detect ferrous and non-ferrous material. So it uses the inductance to determine the object, so an EMF force will be transmitted to the target object. So if there is a metal object near this field then the metal object would reduce the amplitude of the EMF.
Optical proximity sensor
This type of sensor would use infrared light to determine the object. We can determine almost any object by using this sensor. So there would be a transmitter to emit the light and also a receiver to absorb the reflected light. There are three types of optical proximity sensors and they are through-beam, retro-reflective, and diffuse sensors.
Capacitive proximity sensor
We can use this sensor to detect most of the substance. This sensor can determine the substance that has a high dielectric constant. This sensor would react to any change in the dielectric medium that surrounds the active part. This sensor would be able to detect objects through glasses and plastics.
Magnetic proximity sensor
A magnetic proximity sensor is used to detect the magnetic objects, so it could determine the objects by their magnetic field. This sensor consists of reed contacts and when it comes close to a magnetic object, then the magnetic field would make the contact electrical.
Ultrasonic proximity sensor
This type of proximity sensor utilizes ultrasonic sound waves to detect the object. So it would transmit the ultrasonic waves and while receiving the echo it can determine the object.
Applications of the proximity sensor
- Position measurement
- Dynamic motion detection
- Assembly testing
- Aviation safety
- Differential system
- Conveyor system
13. Conductivity Sensor
Conductivity sensors are required for certain process control, like product monitoring, water monitoring, or leakage detection. Mostly these sensors are used to determine the conductance of a solution.
Types of the conductivity sensor
Contact type conductivity sensor
This type of sensor has two metal electrodes and this will be in contact with the solution. The voltage will be applied to the electrodes and the electric field makes the ions in the solution, move back and forth. The ionic current is dependent upon the concentration and also the area of the solution. There would be an analyzer in the sensor which would determine the conductance of the solution.
Inductance conductivity sensor
This sensor is composed of two wire-wound metal toroids, one of the toroids is the drive coil and the other one is the receive coil. The sensor is in contact with the solution the analyzer would apply voltage to the drive coil and this would create voltage in the solution. Due to the voltage, there will be ionic current flow and it will be proportional to the conductance of the solution.
Applications of the conductivity sensor
- Water and waste-water
- Food and beverage
- Power plants
- Oil and gas
14. Displacement Measurement Sensor
This type of sensor would detect the change in the position of the body. The measurement of displacement and speed is really important in many, position and speed control processes. So, proper measurement of these parameters is required to acquire better control performance.
Different types of the displacement measurement sensor
- Potentiometric displacement sensor
- Inductive displacement sensor
- Capacitive displacement sensor
- Eddy current sensor
- Ultrasonic displacement sensor
- Optical displacement sensor
- Magnetostrictive displacement sensor
Potentiometric displacement sensor
We can use this type of displacement sensor for linear and angular displacement measurement. So this device would convert the rotary and linear displacement to an electrical signal. The resistance of this device would change according to the rotation of the shaft.
Inductive displacement sensor
The inductive displacement sensor is composed of a magnetic circuit and a coil would be wound on it. if there is a motion between the conductor and the magnetic field, then a voltage will be induced in the conductor. LVDT is an inductive displacement sensor, it is an electromechanical device that would create an electrical signal. This signal would be proportional to the displacement of the transformer and the armature.
Capacitive displacement sensor
We can use this sensor to measure the rotational or translational motion. This sensor would detect a lot of things such as motion, chemical composition, electrical field. Capacitance would change in case if there is a change in dielectric constant, gap between the plates, and also area of the capacitor plates.
Eddy current sensor
Eddy current is the current which is created in a conductive material by the magnetic field. There will be two coils in this sensor and one will be the active coil and the other is the balanced coil. The active coil would produce the magnetic field, so the active coil would detect the nearby objects and the balanced coil would do the balancing of the output bridge circuit and also the temperature compensation. So while creating the eddy current by applying a magnetic field to the object or conductor, the induced current would apply a magnetic field to the active coil and this would reduce the coil inductance.
Ultrasonic displacement sensor
Mostly these sensors are used for underwater applications, this device consists of a piezoelectric element and this would emit pulses of acoustic energy to the target. So when the signal is reflected from the target, the time required to transmit and receive the signal can be measured to determine the target distance.
Optical displacement sensor
This type of displacement sensor is similar to the ultrasonic displacement sensor. Instead of acoustic energy, this device would transmit light source, the intensity of the light would decrease according to the distance. So when the sensor transmits light source to the object the amount of light that reflects back from the object can be measured to determine the distance.
15. pH Analyser
PH measurement is really important for many chemical industries, the PH measurement can be described as the measurement of the amount of hydrogen or hydroxide ions in an aqueous solution. Basically, it is the measurement of hydrogen ion activity. PH control is required for certain industrial processes and by doing this we could improve the quality of the product. By doing the PH measurement we could determine if the solution is basic or acidic.
The major components of a PH analyzer are the reference electrode, the glass measuring electrode, and there is also a sensing element to do the automatic temperature compensation. The glass measuring electrode is PH sensitive and it would be in contact with the solution and this electrode would create a voltage that would be proportional to the PH of the solution. The reference electrode would give a known potential and the difference between the voltage of the glass electrode and the reference electrode will be proportional to the PH of the solution.
Applications of PH analyzer
- Chemical industries
- Water & wastewater
- Food & beverage
- Biochemical reaction monitoring
- Oil & gas
16. Vibration probes
Vibration can be defined as machine imbalance, it is the oscillation that could happen in the machinery due to the centrifugal force. The effect of the vibration in the machinery is dependent upon the magnitude, frequency, and duration of the vibration. We can see vibrations in the rotating machines and the major reason for this is the unequal mass distribution in a rotating part.
Different types of vibration probes
- Proximity probe
- Velocity probe
- Accelerometer probe
- Eddy current probe
This sensor would measure the shaft movement mostly it will be placed in the bearing housing.
This sensor would detect the total vibration of the point at which this device is located. This probe can be used to measure the velocity of the vibrating body.
Mostly these sensors are placed in the bearing housing of a running machine.
This device would measure the acceleration of the vibrating body. These sensors can be used with low to high frequencies. The frequency of this device will be higher than that of the vibrating body. It would measure the vibrations after recording the acceleration.
Eddy current probe
This device is used to detect the vibration in rotating machines. This device can monitor the shaft axial position, and it could also measure the relative expansion between the rotor and the casing of the machine. This is a noncontact type sensor and it is used to measure the displacement and it would show the shaft radial vibration, shaft rotor position.
Applications of vibration sensor
- Power industry
- Food and beverage
17. Torque Sensors
The torque measurement is needed to determine the required mechanical power, it will be useful to determine the load information which is required for stress and deflection analysis.
Different types of torque sensor
- Magnetostrictive torque sensor
- Piezoelectric torque sensor
- Strain gauge torque sensor
Magnetostrictive torque sensor
This type of sensor would detect the permeability change by measuring the variation in its own magnetic field.
Piezoelectric torque sensor
A piezoelectric material will be compressed and a charge will be generated and it will be measured.
Strain gauge torque sensor
This device will be mounted on pairs in a shaft and one of them would measure the increase in length and the other one will measure the decreased section in the opposite direction.
This device can be used to determine the brake torque and power, mostly this device would do this by various methods. Dynamometers are classified into absorption dynamometers, transmission dynamometers, and driving dynamometers.