Position and Motion Detection Sensors
Many electronic systems require input data concerning position, motion, and speed. Most motion and speed sensors use a magnet as the sensing element or sensed target to detect rotational or linear speed. The types of magnetic speed sensors include magnetoresistive (MR), inductive, variable reluctance (VR), and Hall-effect. In addition, the potentiometer and commutator pulse counting can be used to detect position.
Some systems require the use of photoelectric sensors that use light sensitive elements to detect the movement of an object. In addition, solid state accelerometers, axis rotation sensors, yaw sensors, and roll sensors are becoming common components on many systems. This chapter will explore the operation of common position and motion detection sensors.
A common position sensor used to monitor linear or rotary motion is the potentiometer. A potentiometer is a voltage divider that provides a variable DC voltage reading to the computer. These sensors are typically used to determine the position of a valve, air conditioning unit door, seat track, and so on.
The potentiometer usually consists of a wire wound resistor with a moveable center wiper . A constant voltage value (usually 5 volts) is applied to terminal A. If the wiper (which is connected to the shaft or moveable component of the unit that is being monitored) is located close to this terminal, there will be low voltage drop represented by high voltage signal back to the computer through terminal B. As the wiper is moved toward the С terminal, the sensor signal voltage to terminal В decreases. The computer interprets the different voltage value into different shaft positions. The potentiometer can measure linear or rotary movement. As the wiper is moved across the resistor, the position of the unit can be tracked by the computer.
Since applied voltage must flow through the entire resistance, temperature and other factors do not create false or inaccurate sensor signals to the computer. A rheostat is not as accurate and its use is limited in computer systems.
Magnetic Pulse Generator
An example of the use of magnetic pulse generators is to determine vehicle and individual wheel speed. The signals from the speed sensors are used for computer-driven instrumentation, cruise control, antilock braking, speed sensitive steering, and automatic ride control systems. The magnetic pulse generator is also used to inform the computer of the position of a monitored component. This is common in engine controls where the computer needs to know the position of the crankshaft in relation to rotational degrees.
The components of the pulse generator are:
1 A timing disc that is attached to the rotating shaft or cable. The number of teeth on the timing disc is determined by the manufacturer and depends on application. The teeth will cause a voltage generation that is constant per revolution of the shaft. For example, a vehicle speed sensor may be designed to deliver 4,000 pulses per mile. The number of pulses per mile remains constant regardless of speed. The computer calculates how fast the vehicle is going based on the frequency of the signal.
2 A pickup coil consists of a permanent magnet that is wound around by fine wire.
An air gap is maintained between the timing disc and the pickup coil. As the timing disc rotates in front of the pickup coil, the generator sends an A/C signal. As a tooth on the timing disc aligns with the core of the pickup coil, it repels the magnetic field. The magnetic field is forced to flow through the coil and pickup core. Since the magnetic field is not expanding, a voltage of zero is induced in the pickup coil. As the tooth passes the core, the magnetic field is able to expand. The expanding magnetic field cuts across the windings of the pickup coil. This movement of the magnetic field induces a voltage in the windings. This action is repeated every time a tooth passes the core. The moving lines of magnetic force cut across the coil windings and induce a voltage signal.
When a tooth approaches the core, a positive current is produced as the magnetic field begins to concentrate around the coil. The voltage will continue to climb as long as the magnetic field is expanding. As the tooth approaches the magnet, the magnetic field gets smaller, causing the induced voltage to drop off. When the tooth and core align, there is no more expansion or contraction of the magnetic field (thus no movement) and the voltage drops to zero. When the tooth passes the core, the magnetic field expands and a negative current is produced. The resulting pulse signal is amplified, digitalized, and sent to the microprocessor.
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