Hall-Effect Sensors and Their Operation
14-02 Distinguish the types of sensor used in ignition systems.
Another component of EI systems is the Hall-effect type sensor. Hall-effect sensors are commonly found in current ignition systems, including DI and EI applications, replacing the permanent magnet (PM) style due in part to its accuracy. A Hall-effect sensor is also called a switch due to the On/Off digital voltage signal they generate. The voltage amplitude of a Hall-effect sensor remains constant and is not affected by the rate of change of the magnetic field. Unlike PM sensors, which produce an AC signal that varies in amplitude and frequency with engine speed, a Hall-effect sensor generates a consistent digital voltage signal that changes rapidly from a maximum voltage to nearly zero volts and back again, repeatedly, regardless of engine rpm. Hall- effect sensors produce a square wave output signal that varies in frequency only. Hall-effect signals are used in ignition systems for cylinder identification and/or engine rpm, for ignition timing and control. An ICM or PCM quickly interprets the square wave signal.
The construction of a Hall-effect sensor includes a sensor, magnet, a three-wire connector, and a rotating shutter/interrupter blade (FIGURE 14-6). In later versions of this construction, DISs may use a trigger wheel that interrupts the magnetic field. In a distributor Hall-effect system, the magnet sits opposite the switch/sensor, and the rotating shutter blade spins between them. When an interrupter blade rotates between the sensor and the magnet, it interrupts the magnetic field, dropping the voltage at the sensor. When a window moves between the sensor and magnet, the voltage is high. The rotating blade creates a series of on/off pulses that an ICM or PCM can quickly interpret for engine rpm, crankshaft position (CKP), and cylinder identification.
FIGURE 14-6 Internal line drawing of a digital Hall-effect sensor. The amplifier is external from the Hall element and in series with the Schmitt trigger. The amplifier output cannot exceed the limits of the power supply. The digital output is internally regulated by the sensor.
In contrast to an inductive AC sensor, which uses a two-wire connector, a conventional Hall-effect sensor has three wires: one for reference voltage, one for ground, and one for signal wire input to the ICM/igniter or PCM. Unlike PM-style/inductive-style sensors, which generate their own voltage, a Hall-effect sensor requires an additionally supplied external reference voltage from the ICM/PCM (FIGURE 14-7). The reference voltage is commonly 5 or 12 volts. Occasionally, however, manufacturers may use 7, 8, or 10 volts. The sensor requires the supply voltage to create the switching effect that takes place internal to the sensor.
FIGURE 14-7 Cutaway view of a Hall-effect sensor. It has an output that is in one of two states: on or off. This sensor is coupled with an interrupter ring that makes it switch from on to off.
The name of the sensor and the knowledge of how it operates are attributed to Dr. Edwin Hall, who discovered that when an electrical current is applied to a piece of metal inserted between two magnets, it creates a secondary voltage in the metal perpendicular (at a right angle) to the supplied current and field. Hall-effect generators operate by using a potential difference, or voltage, created by exposing a current-carrying conductor to a magnetic field. A Hall-effect sensor produces a voltage signal controlled by the presence or absence of a magnetic field in an electronic circuit.
By design, Hall-effect sensors output a low-voltage AC signal, typically in millivolts. The low-amplitude signal requires signal-conditioning electronics integral to the sensor that amplify and condition the current based on temperature. A Schmitt trigger circuit then converts the enhanced analog signal into a digital output sensor before sending it to the module as an input for CKP/CMP (camshaft position) and engine speed.
A Schmitt trigger functions by comparing the output signal of the amplifier with a preset reference. If the amplifier output exceeds the reference, the Schmitt trigger will turn on. Conversely, when the output of the amplifier is below the reference point, the Schmitt trigger turns off, generating the digital On/Off signal.
In an ignition system, the location of the Hall-effect sensor’s shutter blades/trigger wheel varies: It could be directly mounted to the distributor shaft, the rotor, crankshaft pulley, or cam gear. The rotating blade/wheel generates and then sends a trigger and/or position signal to the electronic control unit (ECU) to turn the primary circuit on and off. Hall-effect generators operate by using a potential difference, or voltage, created when exposing a current-carrying conductor to a magnetic field. By applying a magnetic field at right angles to the direction of current flow in a conductor, the lines of magnetic force permeate the conductor, deflecting the electrons flowing in the conductor to one side of the conductor. This deflection creates a potential difference or voltage across the conductor. The stronger the magnetic field, the higher the voltage. This dynamic is called the Hall-effect voltage, and by alternately exposing and shielding the magnetic field, the resulting current can be used to trigger a switching device. Hall devices consist of a semiconductor material (chip), which responds quickly and accurately when shielded from or exposed to a magnetic field.
In a distributor, the Hall-effect generator and its integrated circuit are on one leg of a U-shaped assembly, mounted on the distributor base plate (FIGURE 14-8). An integrated circuit is a semiconductor chip that contains miniature versions of various electrical components within one housing. A PM is on the other leg, and an air gap is formed between them. An interrupter ring (metal blade), which has the same number of blades and windows as engine cylinders, spins with the distributor shaft, moving the blades through the air gap. The purpose of an interrupter ring is to systematically block and expose the magnetic field. The ring is made of a ferrous metal shaped like a very shallow cup with slits, or windows, cut into it at evenly spaced intervals. Rotating the metal blade through the air gap between the magnetic field and silicon chip blocks the magnetic field, abruptly dropping the chip’s output voltage to zero. Conversely, with additional circuitry, the sensor can be made to operate in reverse by producing a voltage signal when the metal blade enters the air gap. Some systems use a distinctly sized blade to identify Cylinder 1.
FIGURE 14-8 Hall-effect setup in a distributor.
As previously stated, Hall-effect sensors may normally be on or off, depending on the manufacturer and circuit design. For example, GM uses a normally on CKP sensor that generates a fixed voltage when a window (no blade) is present between the magnet and sensor. When a shutter blade interrupts the magnetic field, blocking the window between the magnet and semiconductor (sensor), the voltage drops to near zero (pull-down) (FIGURE 14-9). Ford DISs, however, operate in reverse. An open window in the gap produces a low, near-zero (off) voltage. The sensor is on when a vane in the air gap blocks the magnetic field and thus the sensor’s internal electronics switch the sensors output voltage from near-zero voltage to its maximum voltage (pull-up) (FIGURE 14-10). Depending on the system, the trigger can occur off a rising edge (on, or high voltage), when the module’s transistor pulls the voltage high, which is a pull-up-design circuit, or off a falling edge (off, or low voltage), when the module’s transistor pulls the voltage low, a pull-down-design circuit. This is different from a PM sensor, which is not as precise, and it changes the amplitude based on the speed of the component (FIGURE 14-11).
FIGURE 14-9 Depending on the system design, the trigger from a Hall-effect sensor can be from a rising or falling signal.
FIGURE 14-10 Hall-effect sensor output compared to a PM generator signal. A Hall-effect sensor changes frequency only; the amplitude of the signal remains constant. A PM inductive sensor increases both amplitude and frequency with rpm.
FIGURE 14-11 Example images comparing a standard vane and a narrower segment on the interrupter vane, which identifies Cylinder 1 to the ICM or PCM. An oscilloscope trace shows the signature identifier as seen by the module. “PIP” is a Ford term for profile ignition pickup. It is the equivalent of a CKP sensor. Many manufacturers use a signature identifier in CKP sensors.
Magnetoresistive Sensors
Another sensor that generates a square wave signal is the magnetoresistive (MR) sensor. MR sensors are becoming more common in ignition systems. An MR sensor looks like a Hall-effect sensor, uses a three-wire connector, generates a digital signal (FIGURE 14-12). While both require a magnetic field to function, their operation, and therefore testing, is quite different. MR sensors can operate with a larger air gap between the sensor and the trigger wheel than a PM generator can.
FIGURE 14-12 A square wave signal shown on the left. The amplitude of this signal remains constant regardless of rpm. As speed increases, the frequency of the signal increases. The supply voltage varies by manufacturer and application. The most common is 5 volts. The signal on the right is an AC signal from a magnetic pickup or VR sensor. Both the amplitude and frequency of this signal increase and decrease with rpm.
MR sensors use a PM located inside the sensor at the end that is nearest the reluctor wheel. The magnet field changes strength as the reluctor rotates near the magnet, aligns with the magnet, and then moves away. The MR sensor uses an alloy material that changes its resistance under the effect of magnetic fields, altering the current flow through the sensor.
When used as crankshaft position (CKP) sensors, MR sensors supply trigger information to the ICM or PCM for ignition timing. In a distributor-based ignition system, MR sensors function as camshaft position (CMP) sensors located on the distributor base. For example, GM uses these sensors in the distributor systems for fuel injector control and onboard misfire diagnostics.
A conventional CKP MR sensor consists of two MR sensors: MR1 and MR2. The sensors are phased slightly apart from each other, spaced on either side of the magnet located at the end of the sensor near the crankshaft reluctor wheel. When the reluctor wheel passes the sensor, it changes the magnetic field of the two pickups (FIGURE 14-13). Both sensors produce identical voltage signals in response to the changing magnetic field. However, due to MR2’s location, its signal output is slightly behind MR1’s, since each tooth of the reluctor wheel passes MR1 first and MR2 second. Due to the difference in phasing, the MR sensor can measure rpm more accurately (down to 0 rpm) than a Hall-effect sensor or PM generator, making it much more useful for timing control and misfire diagnostics.
FIGURE 14-13 MR CKP sensor operation showing identical but delayed voltage output until it is modified internally by the sensor’s condition electronics. MR sensors, like Hall-effect sensors, produce a square wave signal due to a Schmitt trigger. However, unlike a Hall-effect, the square wave signal generally does not pull down to zero volts. Rather, as resistance changes, the voltage output switches from high to low, near zero volts—typically 0.3 to 0.6 volts.
Internal sensor electronics subtract the difference of the two sensors, generating a signal differential output. Like a Hall-effect sensor, the MR1 and MR2 sensors produce an analog voltage that must be conditioned into a square wave output. When using a Schmitt trigger, when the differential signal crosses the high threshold of the preset reference, the digital signal output is high. Conversely, when using a Schmitt trigger, when the differential signal crosses the low preset threshold, the digital signal output is low.
TECHNICIAN TIPA common mistake is thinking that all two-wire sensors are analog AC (passive) sensors. Not all MR sensors are three-wire sensors. Although most MR sensors use a three-wire connector, that is changing: Current vehicles use a two-wire MR sensor. The use of two-wire sensors is common in modern antilock brake systems (ABSs) and in stability-assist systems. MR sensors provide greater accuracy than passive sensors. They also can detect the direction of rotation and zero speed, both critical inputs for stability-assist and hill-start-assist systems. Some two-wire sensors have a power supply and a signal connector pin. The ground is a part of the mounting for the sensor. Other sensors use a power wire and a ground pin. As the trigger wheel moves past the sensor, the magnetic field changes inside the sensor. The changing magnetic field causes a slight current output internal to the sensor that changes the voltage output. The signal then rides on either the power wire or ground wire back to the module. On MR sensors, true 0 volts won’t appear on the low part of the signal. The voltage will drop to near zero, typically around 0.3 to 0.6 volts.
Optical-Type Sensors
Another type of EI system uses a light-emitting diode (LED) and a phototransistor to create an optical sensor (FIGURE 14-14). These systems may also be called photoelectric. High- and low-resolution optical sensors in the distributor base sense the CKP sending an appropriate digital voltage signal to the ICM or PCM. A signal rotor plate interrupter, which is attached to the distributor shaft (FIGURE 14-15). Although various designs are used, a typical rotor plate has 360 slits at 1-degree intervals on its outer edge. Inboard of these slits is another row of fewer notches producing the low-resolution signal. This row typically includes four slots on a four-cylinder engine, six slits on a six-cylinder engine, and so on. One slot is larger than the others, to identify the position of Cylinder 1 to the PCM. Still other systems use differently sized slots for each cylinder. This allows the control unit to know the exact location of the crankshaft. By combining the two signals, the module/PCM can determine CKP and CMP
FIGURE 14-14 An optical sensor is used to determine the position of the distributor, which allows the ICM to fire the correct cylinder at the correct time. These sensors are very sensitive to external light, which means if the distributor cap is not installed correctly, that mistake may cause a misfire.
FIGURE 14-15 GM Optispark interrupter disc showing both high- and low-resolution slots. The low-resolution slots are on the inner portion of the disc and are arranged in the firing order. Differently sized slots indicate each cylinder in the firing order.
TECHNICIAN TIPDuring service, exercise extreme caution when handling the reluctor wheel or MR sensor. Any dents, cracks, or other imperfections can alter the sensor’s output signal, causing an irregular pattern or excessive noise. A damaged reluctor or sensor can affect ignition, fuel, or cam timing, as well as onboard misfire diagnostics.
As the rotor plate turns, it passes between an LED above the rotor plate and a phototransistor below the plate. When provided with a suitable voltage, LEDs transmit a fine beam of light. Phototransistors receive this light and use it to make a voltage output signal. When a slot is in alignment, the light beam passes through it and a signal is transmitted to the control unit. When the slit is out of alignment, the light beam is interrupted and the signal falls to zero.
TECHNICIAN TIPDual reference signal systems generally use the low reference signal to provide engine RPM and CKP information. The high reference signal fine-tunes the igni-tion timing, particularly at high engine rpm. The vehicle will not run without a low-resolution signal. Most systems will allow starting without the high-resolution signal. However, a long crank time may be noticed, along with reduced high-rpm performance.
The control unit uses the signals it receives at every 1 degree to gauge engine rpm and crank angle position in 1-degree increments. It uses the signals from the inner slits to gauge the piston position, the signature slit identifying Piston 1. The signals from both sets of diodes are converted to on/off pulses in the sensor’s internal circuitry. The optical sensor allows the ECU to calculate the exact position of the crankshaft and which stroke the cylinder is on.
The ECU monitors the number of pulses that occur during each camshaft revolution. After computing data from the various input sensors against the optimum settings for each operating condition recorded in its memory, the ECU determines ignition timing and dwell. It then switches the primary circuit on and off by controlling the operation of a power transistor often mounted close to the ignition coil. The power transistor has its emitter connected to ground. The collector is connected to the coil’s negative terminal, and the base is connected to the ECU. A voltage applied from the module sends a small current through the base/emitter portion of the transistor to switch it on. Current can then flow from the ignition switch and through the coil primary winding and the collector/emitter to complete the primary circuit.
TECHNICIAN TIPThe optical sensor is very precise due to the small slits that the light passes through. However, because they are so small, the slits are easily blocked by dirt, debris, oil, and on some models, even coolant. Make sure the slits in the rotor plate stay clean when working around it. Repair any obvi-ous leaks that can clog the slits.
Once the appropriate dwell period has elapsed, the module switches off the voltage applied to the base of the transistor, and the transistor switches off the primary circuit. The high secondary voltage then pushes current to the center of the rotor arm and to the appropriate cylinder. This cycle is repeated for each cylinder in turn. The ECU determines engine timing and dwell under all operating conditions, including spark advance and retard.
TECHNICIAN TIPFor proper operation, the distributor cap must fit securely to keep ambient light from altering the optical sensor output. Additionally, some optical sensor applications use a vent system comprising a vented distributor cap, to help prevent the formation of ozone caused by the arcing between the rotor tip and cap terminals. A vent system generally includes a vacuum hose, a filter, and a one-way check valve that uses engine vacuum to remove the ozone.