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.
FIGURE. A potentiometer sensor circuit measures the amount of voltage drop to determine position.
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.
FIGURE. Components of the magnetic pulse generator. A strong magnetic field is produced in the pick-up coil as the teeth align with the core.
The components of the pulse generator are:
- 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.
- A pickup coil consists of a permanent magnet that is wound around by fine wire.
FIGURE. Pulse signal sine wave.
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.
FIGURE. The magnetic field expands as the teeth pass the core.
FIGURE. A positive voltage swing is produced as the tooth approaches the core. When the tooth aligns with the core, there is no magnetic movement and no voltage. A negative wave form is created as the tooth passes the core.
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.
NOTE! The magnetic pulse (PM) generator operates 011 basic magnetic principles. Remember that a voltage can only be induced when a magnetic field is moved across a conductor. The magnetic field is provided by the pickup unit, and the rotating timing disc provides the movement of the magnetic field needed to induce voltage.
Magnetoresistive (MR) sensors consist of the magnetoresistive sensor element, a permanent magnet, and an integrated signal conditioning circuit to make use of the magnetoresistive effect. This effect defines that if a current carrying magnetic material is exposed to an external magnetic field its resistance characteristics will change. This results in the resistance of the sensing element being a function of the direction and intensity of an applied magnetic field. Magnetoresistive sensors cannot generate a signal voltage on their own, and must have an external power source. The magnetoresistive bridge changes resistance due to the relationship of the tone wheel and magnetic field surrounding the sensor.
NOTE! The magnetoresistive principle provides rotational speed measurements down to zero. For this reason they are sometimes called “zero speed sensors.”
Magnetically Coupled Linear Sensors
Linear sensors are used for such functions such as fuel level sending units. The most common type of fuel level sensor is a rheostat style with wire wound resistor and a movable wiper. The wiper is in constant contact with the winding and may eventually rub through the wire. Many manufacturers are now using magnetically coupled linear sensors that are not prone to the wear.
FIGURE. Magnetically coupled linear sensor used to measure fuel level.
Magnetically coupled linear sensors used for fuel level sensing have a magnet attached to the end of the float arm. Also a resistor card and a magnetically sensitive comb are located next to the magnet. When the magnetic field passes the comb, the fingers are pulled against the resistor card contacting resistors that represent the various levels of fuel.
FIGURE. Comb design magnetic sensor pulls the fingers against the resistor card.
When the tank is full, the float is on top along with the magnet. As the fuel level falls, the float drops and the position of the magnet changes. The magnet is so close to the sensor that it attracts the closest metal fingers. The fingers contact a metal strip. Different contact sites on the strip produce different resistances that are used to determine the fuel level.
FIGURE. Hall-effect principles of voltage induction.
FIGURE. Typical circuit of a Hall-effect switch.
FIGURE. The magnetic field causes the electrons from the supply current to gather at the Hall layer negative terminal. This creates a voltage potential.
Based on the principle that if a current is allowed to flow through thin conducting material that is exposed to a magnetic field, another voltage is produced. The switch contains a permanent magnet, a thin semiconductor layer made of gallium arsenate crystal (Hall layer), and a shutter wheel. The Hall layer has a negative and a positive terminal connected to it. Two additional terminals located on either side of the Hall layer are used for the output circuit. The shutter wheel consists of a series of alternating windows and vanes. It creates a magnetic shunt that changes the strength of the magnetic field from the permanent magnet.
The permanent magnet is located directly across from the Hall layer so that its lines of flux will bisect at right angles to the current flow. The permanent magnet is mounted so that a small air gap is between it and the Hall layer.
A steady current is applied to the crystal of the Hall layer. This produces a signal voltage that is perpendicular to the direction of current flow and magnetic flux. The signal voltage produced is a result of the effect the magnetic field has on the electrons. When the magnetic field bisects the supply current flow, the electrons are deflected toward the Hall layer negative terminal. This results in a weak voltage potential being produced in the Hall switch.
A shutter wheel is attached to a rotational component. As the wheel rotates, the shutters (vanes) will pass in this air gap. When a shutter vane enters the gap, it intercepts the magnetic field and shields the Hall layer from its lines of force. The electrons in the supply current are no longer disrupted and return to a normal state. This results in low voltage potential in the signal circuit of the Hall switch.
The signal voltage leaves the Hall layer as a weak analog signal. To be used by the computer, the signal must be conditioned. It is first amplified because it is too weak to produce a desirable result. The signal is also inverted so that a low input signal is converted into a high output signal. It is then sent through a Schmitt trigger where it is digitized and conditioned into a clean square wave signal. The signal is finally sent to a switching transistor. The computer senses the turning on and off of the switching transistor to determine the frequency of the signals and calculates speed.
The Hall-effect just discussed describes its usage as a switch. It can also be designed as an analog (or linear) sensor that produces an output voltage that is proportional to the applied magnetic field. This makes them useful for determining to position of a component instead of just rotation. For example, this type of sensor can be used to monitor fuel level or to track seat positions in memory seat systems.
FIGURE. Hall-effect sensor used for fuel level indication.
A fuel level indication can be accomplished with a Hall-effect sensor by attaching a magnet to the float assembly. As the float moves up and down with the fuel level, the gap between the magnet and the Hall element will change. The gap changes the Hall-effect and thus the output voltage.
FIGURE. TWo-wire linear Hall-effect sensor.
As discussed, typical Hall-effect sensors and switches use three wires. However, linear Hall-effect sensors can also be constructed using two wire circuits. This is common on systems that use a DC motor drive. The reference voltage to the sensor is supplied through a pull-up resistor. Typically this reference voltage will be 12 volts. Whenever the motor is operated the reference voltage will be applied. After the motor is turned off, this reference voltage will remain for a short time.
Internal to the motor assembly is a typical three terminal Hall sensor. The reference voltage is supplied to terminal 1 of the Hall sensor. A pull-up resistor also connects the reference voltage to terminal 3 of the Hall sensor. This becomes the signal circuit. The two pull-up resistors will be of equal value. Terminal 2 of the Hall sensor is connected to the sensor return circuit. A magnet is attached to the motor armature to provide a changing magnetic field once per motor revolution.
When the Hall sensor is off the voltage supplied to the Hall sensor will be near that of the source voltage. Since this is an open circuit condition in the Hall sensor at terminal 3, the voltage drop over the signal circuit pull-up resistor will be 0.
When the motor rotates and the influence of the magnetic field turns on the Hall sensor, the signal terminal 3 is connected to ground within the sensor. This pulls the signal voltage low and results in the formation of a series circuit from the reference supply to terminal 3. Since each of the pull-up resistors are equal the voltage drop will be split between the two. Approximately half the voltage will be dropped across the pull-up resistor in the computer and the other half over the pull-up resistor in the motor assembly. The Hall-effect sensor will remain powered since the reference voltage to terminal 1 is connected between the two resistors and the 6 volts on the circuit is sufficient to operate the sensor.
Acceleroineters are sensors designed to measure the rate of acceleration or deceleration. Common sensors include mass-type, roller-type, and solid state accelerometers. The first extensive use of the accelerometer was in the airbag system. The use of accelerometers has expanded greatly in today’s vehicles. They are now used on vehicle stability systems, roll over mitigation, hill hold control, electronic steering, and navigational systems. These sensors may perform specific functions other than forward acceleration and deceleration forces. For example they will operate as a gyro to determine direction change and rotation.
FIGURE. Sensing mass held by a magnet will break loose if deceleration forces are severe enough.
Accelerometers react to the amount of G force associated with the rate of acceleration or deceleration. In airbag systems they are used to determine deceleration forces that indicate the vehicle has been involved in a collision that requires the airbag to be deployed. Early accelerometers used in airbag systems were electromechanical designs. The masstype sensor contains a normally open set of gold-plated switch contacts and a gold-plated ball that acts as a sensing mass. The gold-plated ball is mounted in a cylinder coated with stainless steel. A magnet holds the ball away from the contacts. When the vehicle is involved in a frontal collision of sufficient force, the sensing mass (ball) moves forward in the sensor and closes the switch contacts.
In many air bag systems, solid-state accelerometers are used to sense deceleration forces. The piezoelectric accelerometer generates an analog voltage proportional to a G force. The accelerometer contains a piezoelectric element that is distorted during a high G force condition and generates an analog voltage in relation to the force. The analog voltage from the piezoelectric element is sent to a collision-judging circuit in the airbag computer. If the collision impact is great enough, the computer deploys the air bag.
Accelerometers can also be designed as piezoresistive sensors that use a silicon mass that is suspended from four deflection beams. The deflection beams are the strain sensing elements. The four strain elements are in a Wheatstone bridge circuit. The strain elements on the beam generate a signal that is proportional to the G forces. The resistance changes over the bridge are interpreted by an internal chip that then communicates the status to the control module using a frequency modulated digital pulse.