Solenoid Actuators
The basic operation of solenoid actuators is very simple. The term ‘solenoid’ means: ‘many coils of wire wound onto a hollow tube’. However, the term is often misused, but has become so entrenched that terms like ‘starter solenoid’ – when really it is starter relay – are in common use. A good example of a solenoid actuator is
a fuel injector. When the windings are energized the armature is attracted due to magnetism and compresses the spring. In the case of a fuel injector, the movement is restricted to about 0.1mm. The period that an injector remains open is very small – under various operating conditions, between 1.5 and 10ms is typical. The time it takes an injector to open and close is also critical for accurate fuel metering.
The reaction time for a solenoid-operated device, such as a fuel injector, depends very much on the inductance of the winding. The resistance of commonly used injectors is about 16 . Some systems use ballast resistors in series with the fuel injectors. This allows lower inductance and resistance operating windings to be used, thus speeding up reaction time. Other types of solenoid actuators, for example door lock actuators, have less critical reaction times. However, the basic principle remains the same.
Motorized Actuators
Permanent magnet electric motors are used in many applications and are very versatile. The output of a motor is, of course, rotation, but this can be used in many ways. If the motor drives a rotating ‘nut’ through which a plunger is fitted, and on which there is a screw thread, the rotary action can easily be converted to linear movement. In most vehicle applications the output of the motor has to be geared down, this is to reduce speed and increase torque. Permanent magnet motors are almost universally used now in place of older and less practical motors with field windings.
Some typical examples of where these motors are used are:
- windscreen wipers
- windscreen washers
- headlight lift
- electric windows
- electric sun roof
- electric aerial operation
- seat adjustment
- mirror adjustment
- headlight washers
- headlight wipers
- fuel pumps
- ventilation fans
One disadvantage of simple motor actuators is that no direct feedback of position is possible. This is not required in many applications; however, in some cases, such as seat adjustment when a ‘memory’of the position may be needed, a variable resistor type sensor can be fitted to provide feedback. This device is used to control idle speed by control- ling air bypass. There are two basic types in common use. These are single winding types, which have two terminals, and double winding types, which have three terminals. Under ECU control, the motor is caused to open and close a shutter, thus controlling air bypass. These actuators only rotate about 90° to open and close the valve. As these are permanent magnet motors, the term ‘single or double windings’ refers to the armature.
The single winding type is fed with a square wave signal causing it to open against a spring and then close again, under spring tension. The on/off ratio or duty cycle of the square wave will determine the average valve open time and hence idle speed. With the double winding type the same square wave signal is sent to one winding but the inverse signal is sent to the other. As the windings are wound in opposition to each other if the duty cycle is 50% then no movement will take place. Altering the ratio will now cause the shutter to move in one direction or the other.
