Sensors: The Sense Organs of Mechatronic Systems

A sensor is the observation layer of a mechatronic control loop: it converts a physical or chemical quantity into a signal that the controller can process. Resolution alone does not define usefulness; bandwidth, noise, sensitivity, hysteresis, drift, calibration, and timing determine whether the signal represents the state that the controller assumes it represents.

• A component that detects a change in the physical environment is called a sensor.
• A component that converts the detected quantity into electrical energy is called a transducer.
• In mechatronic systems, mechanical, magnetic, thermal, optical and chemical changes are converted into electrical voltage or current by sensors.
• Sensors are used to measure displacement, velocity, acceleration, distance, force, strain, temperature, pressure and current.
• Actuators control the environment according to data received from sensors.
• Microprocessors process the data obtained from sensors.
• Processing a Sensor Signal

The signal generated by a sensor in response to a physical quantity is often not directly usable.

A weak signal must be amplified.

The signal may require filtering.

A nonlinear signal may require linearization.

Depending on the application, analog signals may need to be converted to digital form or digital signals to analog form.

Resistance or voltage variation may need to be converted into current variation.

After such operations, the signal becomes suitable for use and recording.

• Displacement Sensors

Linear displacement sensors are used to check material thickness, part dimensions and whether a component is present at a location.

Angular displacement sensors monitor angular movement of shafts.

Displacement sensors may be contact or non-contact devices.

Potentiometers, strain gauges, optical and absolute encoders, pneumatic sensors and differential transformers can be used.

An LVDT consists of windings and a movable core.

• A voltage is applied to one winding and the output is obtained from the other windings.
• Movement of the core changes the output voltage, enabling displacement measurement.
• Speed Sensors

When position and angular displacement from an incremental encoder are evaluated over time, speed can be obtained.

A tachogenerator measures the rotational speed of a shaft.

Tachogenerators contain a permanent-magnet and coil structure. Rotational speed is determined from the terminal voltage.

The speed of linearly moving mechanisms such as slides can be measured with a tachogenerator when the motion is converted to rotation.

• Acceleration and Vibration Sensors

Measurement can be performed with an accelerometer, flexible beam and strain gauge.

• Force and Strain Sensors

Changes in many physical quantities can be expressed proportionally through deformation and strain.

Operating Principles of Force Sensors

• Apply a balancing force opposite to the load and measure that force with a transducer, as in scales.
• Measure the deformation caused by the applied load with a transducer, as in a circular-section rod.
• Temperature Sensors

1. Thermocouple

• Constructed by joining the ends of two metallic conductors.
• The system includes a measured temperature T1 and a reference temperature T2.
• The potential difference between the ends indicates the temperature difference between T1 and T2.

1. Thermistor

• A material whose conductivity or resistance changes with temperature.
• The electrical resistance of a semiconductor is generally inversely related to temperature.
• The temperature-dependent resistance change of a semiconductor is much greater than that of ordinary metals.
• Thermistors are produced in forms such as cylinders, beads and disks.

1. RTD: Resistance Temperature Detector

• Operation is based on the proportional increase of electrical resistance with temperature in certain materials.
• Platinum, copper, nickel and tungsten are commonly used.
• Platinum provides strong performance in temperature range, linearity, stability and oxidation resistance.
• The sensor generally consists of wire wound as a coil around a cylindrical former.
• The structure is commonly protected by a tube.
• Resistance can be measured with a Wheatstone bridge.

1. Pyrometer

• Measures thermal radiation emitted by an object.
• Radiation energy is directed onto a cooler surface in the measuring instrument.
• Temperature is determined from the resulting change in that surface.
• A pyrometer can measure moving objects.
• Its structure permits non-contact temperature measurement.
• Pressure Sensors

Use elastic elements such as diaphragms, bellows, capsules and tubes that deform under pressure.

A displacement transducer measures the deformation and therefore the pressure.

Strain gauges or LVDTs can also be used.

• Flow Sensors

Flow meters and velocity probes are commonly used.

Flow meters operate by measuring mass flow or volumetric flow.

Turbine Flow Meter

• Consists of a multi-bladed rotor centered and positioned axially inside a pipe.
• The rotor turns under the effect of the flowing fluid.
• The flow rate or average velocity is directly proportional to the turbine’s angular velocity.
• A permanent magnet in the device generates voltage pulses in an electrical winding as the blades rotate.

I retain the original component-by-component notes below because they cover signal conditioning and a wide set of sensor families. In a production system I would bind every sample to more than its numeric value: timestamp, unit, coordinate frame where relevant, calibration version, saturation/error state, and compensation metadata are part of the measurement contract.

References

• **[1]** W. Bolton. (2011). Mechatronics: Electronic Control Systems in Mechanical and Electrical Engineering, Fifth Edition. Pearson.
• **[2]** Rolf Isermann. (2005). Mechatronic Systems: Fundamentals. Springer. doi:10.1007/1-84628-259-4