Sensors and Transducers
A sensor detects a physical quantity. A transducer converts energy or information from one form to another. In mechatronics, sensors close the loop between mechanical motion and electronic control. A controller can only correct what the sensor can observe accurately, quickly, and repeatably.
Learning Objectives
By the end of this lesson, you should be able to:
- classify common mechatronic sensors by measured quantity and output type;
- distinguish accuracy, precision, resolution, repeatability, hysteresis, and latency;
- choose analog, digital, incremental, or absolute feedback;
- plan calibration, filtering, shielding, and fault detection;
- avoid sensor placement mistakes caused by backlash, compliance, and noise.
Common Sensor Types
| Quantity | Sensor examples | Typical output | Notes |
|---|---|---|---|
| Position | potentiometer, encoder, LVDT, magnetostrictive sensor | voltage, pulses, serial data | load-side position is best when backlash matters |
| Speed | tachometer, encoder derivative, Hall sensor | voltage, frequency, pulses | filtering changes control-loop delay |
| Force or weight | strain gauge, load cell, force-sensitive resistor | bridge mV, resistance | needs stable excitation and amplification |
| Proximity | inductive, capacitive, optical, ultrasonic | switch, analog, IO-Link | environment strongly affects reliability |
| Temperature | thermistor, RTD, thermocouple, IC sensor | resistance, voltage, digital | thermal coupling and self-heating matter |
| Current | shunt, Hall sensor, current transformer | voltage, digital | bandwidth and isolation are key choices |
Sensor Specifications
Resolution is the smallest detectable step. Accuracy is closeness to the true value. Precision is consistency of repeated readings. Repeatability is the ability to return to the same reading under the same condition. Hysteresis is different output for the same input depending on direction of travel. Latency is delay between physical change and reported value.
A control loop often cares more about repeatability, noise, bandwidth, and latency than absolute accuracy.
Signal Chain
For an analog sensor, use the same scale-and-offset idea from calibration:
$$
y = (x - x_0)k
$$
For an ADC:
$$
V_\text{in} = \frac{\text{code}}{2^N - 1} V_\text{ref}
$$
where N is ADC resolution in bits and Vref is the reference voltage. If Vref moves, the measurement moves.
Placement Matters
A motor encoder measures motor shaft motion. If there is gearbox backlash, belt stretch, coupler slip, or leadscrew windup, the load may not be where the motor says it is. A load-side encoder improves accuracy but may need better mechanical protection, cable routing, shielding, and homing strategy.
Calibration and Fault Detection
Practical systems need known references. Examples include a home switch, calibration weight, precision resistor, gauge block, known temperature point, or hard mechanical datum.
Fault checks should detect:
- open circuit, short circuit, or out-of-range value;
- stuck reading while motion is commanded;
- impossible rate of change;
- disagreement between redundant sensors;
- loss of encoder counts or invalid digital checksum;
- sensor value that conflicts with limit-switch state.
Noise, Filtering, and Delay
Filtering reduces noise but adds delay. In a slow monitoring system, heavy averaging may be acceptable. In a position or force control loop, delay can cause overshoot or oscillation.
Useful checks include measuring raw values, estimating noise in engineering units, comparing filtered and unfiltered signals, and confirming that the controller still meets response requirements.
Common Mistakes
- Choosing high resolution while ignoring noise and reference stability.
- Placing the sensor before backlash when load position matters.
- Forgetting cable shielding, grounding, and strain relief.
- Filtering so heavily that the control loop becomes unstable.
- Using an analog sensor without checking ADC input range.
- Calibrating before final mechanical mounting.
Summary
Sensors make mechanical state visible to electronics. Choose them by measured quantity, range, accuracy, repeatability, latency, environment, signal chain, placement, calibration method, and fault behavior.
Further Reading
- Texas Instruments sensor signal-conditioning application notes.
- Honeywell and TE Connectivity sensor application guides.
- National Instruments sensor fundamentals and measurement tutorials.