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Soft sensor : the challenge of mechatronics

More and more industries are looking for a way to measure physical quantities (pressure, temperature …) without using sensors. The solution is with soft sensors. The challenge is: how to obtain the value of a physical quantity without measuring it?

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 motorized 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 analyzing 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.