Sensors


1. Introduction
As mechanical systems become more sophisticated there is increasing need to provide efficient control. This normally requires a combination of:

General areas of application include:
These different uses have differing requirements, in (i) failure will probably not be catastrophic, it might be in (ii) and (iii). In (ii) the system may be idle for long periods of time, but reliable operation is vital when needed, eg automobile airbag actuating systems.

Automotive sensors are assigned to one of 3 reliability classes according to the application:

The following sections briefly examines the main types of sensor:

2. Position Sensors
The are commonly based on the resistance of a length of film or coiled wire. These may be straight for linear measurement or circular for angular measurement. The variation in resistance between one end of the wire or track and a moving wiper allows the position of the wiper to be accurately determined.

3. Velocity and RPM sensors
These are often based on electromagnetic effects. A common type of inductive sensor uses a bar magnet with a soft iron pole pin which has a coil around it. When the teeth on a ferromagnetic ring gear move past the sensor, a fluctuating voltage is generated in the coil proportional to the rate of change of magnetic flux, this is proportional to the speed that the teeth are moving past the device. The frequency of the fluctuation is equal to the frequency at which the teeth are moving past the device.
This type of sensor has often been used in conjunction with the starter ring on car engine fly wheels to provide engine speed indication and a second device is normally used to provide ignition timing information.

4. Acceleration Sensors
The majority of these are based on piezo electric devices. When a piezo electric crystal is put under fluctuating stress (direct or shear) a voltage is generated between opposite faces. In an accelerometer one or more piezo crystals are used to support a small inertial mass within the casing. When the casing is subject to acceleration, the inertial mass tends to remain stationary and as it moves with the casing, the accelerating force is translated through the piezo crystal generating a voltage across the crystal faces. This usually needs amplifying. This type of device only responds to fluctuating accelerations.

These types of device are widely used for:
Application Range
Knock control - providing a signal for the engine management computer to adjust the ignition timing 1 - 10g
Airbag deployment, seat belt tensioner - passenger safety 50g
Suspension control - passenger comfort and handling 1 - 10g

5. Pressure Sensors
Pressure measurement can be by diaphragm deflection or force sensor.

6. Flow Measurement
In spark ignition engines air flow is the primary control parameter. For small ratios of maximum to minimum flow rates (eg 10:1) the pressure drop across a fixed orifice may be used. However in car engines this the maximum to minimum flow rates is much greater than 10:1. For these types of application a variable flap device can be used. Air being drawn past the flap causes it to open, the rotation of the flap being monitored by a suitable potentiometer. The voltage drop across the potentiometer can then be used to determine the air flow. This type of device has the disadvantage of partially obstructing the airflow and lowering the engine efficiency slightly and not being able to respond to very rapid fluctuations.
A mass flow meter that offers virtually no resistance to the flow and has no moving parts is the hot wire or hot film device. A closed loop control device maintains a constant temperature differential between the hot fine platinum wire or thin film resistor and the passing air. The engine management system converts the signals into mass flow. This system has a fast response rate, in the millisecond range, however it can not recognise flow direction, so strong pulsations in the inlet manifold can cause substantial measuring errors.

7. Torque Sensors
Torque is measured either by measuring the angle of twist of a length of shaft or by measuring the stress in a portion of shaft.

8. Temperature Sensors
In motor vehicles temperature sensing (with contact thermometers) is normally done by utilising the sensitivity to temperature variation of electrical resistance materials. These may have a positive (PTC) or negative (NTC) temperature coefficient. The following types of devices are in use:

9. Other Sensors
Optical sensors are used to detect raindrops on car wind screens and then to automatically operate wipers. Optical devices are also used to trigger washing of head lamp glass when dirt buildup warrants.

Developments in miniature solid state devices means that the range of sensors available is increasing and as their use increases, prices drop, making them more attractive.
The developments in thick and thin film technologies have made it possible to manufacture a range of largely silicon based devices. Micro machining has made it possible to produce beams and diaphragms that are required for strain gauges, accelerometers and pressure gauges, at the chip level level

Reference
'Mechatronics', D A Bradley, D Dawson, N C Burd and A J Loader, Chapman and Hall, 1991, ISBN: 0-412-34200-6.

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David J Grieve, 6th August 2003.