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Soft sensor : the challenge of mechatronics
What is a soft sensor?
A sensor is a device that converts the state of an observed physical quantity into a usable quantity. For instance, a sensor converts the speed measurement of a car into a display on the dashboard for the driver. There are two kinds of sensors:
physical sensor
A device to measure a physical quantity, for instance :
- pressure sensor: barometer
- speed sensor: tachymeter
- temperature sensor: thermometer
soft sensor
Sometimes called «observer», it estimates a quantity indirectly using :
- measurable quantities for a low cost
- a dynamic model of the observed system
- observer theory (Kalman filter …)
To sum up, the soft sensor is a software program which estimates a physical quantity using information from other sensors instead of measuring it directly.
How does it work ?
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Physical system : car, wheel, motor
Known input : speed, temperature
Measurements : speed (km/h), temperature (°C)
Disturbances : environment
Quantity to estimate : tire pressure
Mathematical model : mathematical representation of the system
Calibration : adjusting the mathematical model with real results and calculated ones to take disturbances into account
Quantity to estimate : pressure (bar)
Concrete examples :
Estimation of efforts between tire and road:
- What for: trajectory control (ESP, ABS …)
- Physical sensor: it exists but is expensive (a dynamometric wheel costs more than €100k)
- Soft sensor: uses other measurements easier to access (tire deformation, speed, vibrations …)
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Tire pressure estimation:
- What for: detect flat tire, under-inflation …
- Physical sensor: classical pressure sensor (one on each tire)
- Soft sensor: use the tire deformation measure (flattening) or speed differentials between wheels
Reasons why this technology is developing are mainly economic. Indeed, soft sensor is a computer program, hence, there is no cost to duplicate or manufacture it. Its cost is due to IT development, a one-time process. Therefore it is a very interesting way to avoid physical sensor utilization.
Practical application
Anti-pinch : from physical to soft sensor
Nowadays, more and more motorised windows are integrated on vehicles : window winder, sunroof, sliding car door… In order to insure people security , anti-pinch laws are created and mandatory.
A tool for analysing and measuring tire wear
On quarry vehicles, tyres represent an important investment. It is crucial to be able to follow their wear evolution. A tool being able to estimate in real time tyres states allows a better control of investment costs.
