Electronic Switching Systems


Electronic Switching Systems

Electronic ignition systems control the primary circuit, using an NPN transistor instead of breaker contact points. The transistor's emitter is connected to ground and takes the place of the fixed contact point. The collector is connected to the negative (-) terminal of the coil, taking the place of the movable contact point. When the triggering device supplies a small amount of current to the base of the switching transistor, the collector and emitter act as if they are closed contact points (a conductor), allowing current to build up in the coil primary circuit. When the current to the base is interrupted by the switching device, the collector and emitter act as an open contact (an insulator), interrupting the coil primary current.

Engine Position Sensors

The time when the primary circuit must be opened and closed is related to the position of the pistons and the crankshaft. Therefore, the position of the crankshaft is used to control the flow of current to the base of the switching transistor.

A number of different types of sensors are used to monitor the position of the crankshaft and control the flow of current to the base of the transistor. These engine position sensors and generators serve as triggering devices and include magnetic pulse generators, metal detection sensors, Hall-effect sensors, and photoelectric sensors.

The mounting location of these sensors depends on the design of the ignition system. All four types of sensors can be mounted in the distributor, which is turned by the camshaft.

Magnetic pulse generators and Hall-effect sensors can also be located on the crankshaft. These sensors are also commonly used on DIS ignition systems. Both Hall-effect sensors and magnetic pulse generators can also be used as camshaft reference sensors to identify which cylinder is the next one to fire.

Magnetic Pulse Generator

Basically, a magnetic pulse generator consists of two parts: a timing disc and a pick-up coil. The timing disc may also be called a regulator; trigger wheel, pulse ring, armature, or timing core. The pick-up coil, which consists of a length of wire wound around a weak permanent magnet, may also be called a stator, sensor, or pole piece. Depending on the type of ignition system used, the timing disc may be mounted on the distributor shaft, at the rear of the crankshaft, or on the crankshaft vibration damper.

The magnetic pulse or PM generator operates on basic electromagnetic principles. Remember that a voltage can only be induced when a conductor moves through a magnetic field. The magnetic field is provided by the pick-up unit and the rotating timing disc provides the movement through the magnetic field needed to induce voltage.

As the disc teeth approach the pick-up coil, they repel the magnetic field, forcing it to concentrate around the pick-up coil. Once the tooth passes by the pick-up coil, the magnetic field is free to expand or unconcentrate, until the next tooth on the disc approaches. Approaching teeth concentrate the magnetic line of force, while passing teeth allow them to expand. This pulsation of the magnetic field causes the lines of magnetic force to cut across the winding in the pick-up coil, inducing a small amount of AC voltage that is sent to the switching device in the primary circuit.

When a disc tooth is directly in line with the pick-up coil, the magnetic field is not expanding or contracting. Since there is no movement or change in the field, voltage at this precise moment drops to zero. At this point, the switching device inside the ignition module reacts to the zero voltage signal by turning the ignition's primary circuit current off. As explained earlier, this forces the magnetic field in the primary coil to collapse, discharging a secondary voltage to the distributor or directly to the spark plug.

As soon as the tooth rotates past the pick-up coil, the magnetic field expands again and another voltage signal is induced. The only difference is that the polarity of the charge is reversed. Negative becomes positive or positive becomes negative. Upon sensing this change in voltage, the switching device turns the primary circuit back on and the process begins all over.

The slotted disc is mounted on the crankshaft, vibration damper, or distributor shaft in a very precise manner. When the disc teeth align with the pick-up coil, this corresponds to the exact time certain pistons are nearing TDC. This means the zero voltage signal needed to trigger the secondary circuit occurs at precisely the correct time.

The pick-up coil might have only one pole. Other magnetic pulse generators have pick-up coils with two or more poles.

Metal Detection Sensors

Metal detection sensors are found on many early electronic ignition systems. They work much like a magnetic pulse generator with one major difference.

A trigger wheel is pressed over the distributor shaft and a pick-up coil detects the passing of the trigger teeth as the distributor shaft rotates. However, unlike a magnetic pulse generator, the pick-up coil of a metal detection sensor does not have a permanent magnet. Instead, the pick-up coil is an electromagnet. A low level of current is supplied to the coil by an electronic control unit, inducing a weak magnetic field around the coil. As the reluctor on the distributor shaft rotates, the trigger teeth pass very close to the coil. As the teeth pass in and out of the coil's magnetic field, the magnetic field builds and collapses, producing a corresponding change in the coil's voltage. The voltage changes are monitored by the control unit to determine crankshaft position.

Hall-Effect Sensor

Introduced in early 1982, the Hall-effect sensor or switch is now the most commonly used engine position sensor. There are several good reasons for this. Unlike a magnetic pulse generator, the Hall-effect sensor produces an accurate voltage signal throughout the entire rpm range of the engine. Furthermore, a Hall-effect switch produces a square wave signal that is more compatible with the digital signals required by on-board computers.

Functionally, a Hall switch performs the same tasks as a magnetic pulse generator. But the Hall switch's method of generating voltage is quite unique. It is based on the Hall-effect principle, which states: If a current is allowed to flow through a thin conducting material, and that material is exposed to a magnetic field, voltage is produced.

The heart of the Hall generator is a thin semiconductor layer (Hall layer) derived from a gallium arsenate crystal. Attached to it are two terminals - one positive and the other negative - that are used to provide the source current for the Hall transformation.

Directly across from this semiconductor element is a permanent magnet. It's positioned so its lines of flux bisect the hall layer at right angles to the direction of current flow. Two additional terminals, located on either side of the Hall layer, from the signal output circuit.

When current is passed through the Hall layer, a voltage is produced perpendicular to the direction of current flow and magnetic flux. The signal voltage produced is the direct result of the magnetic field's effect on electron flow. As the magnetic lines of force collide with electrons in the supply current, current flow in the crystals is distorted and, as a result, electrons are deflected toward what becomes the negative Hall voltage terminals. It is this creation of an electron surplus at the Hall voltage terminal that results in the production of a weak voltage potential.

The Hall switch is described as being on any time the Hall layer is exposed to a magnetic field and a Hall voltage is being produced. However, before this signal voltage can be of any use, it has to be modified. After leaving the Hall layer, the signal is routed to an amplifier where it is strengthened and inverted so the signal reads high when it is actually coming in low and vice versa. Once it has been inverted, the signal goes through a pulse-shaping device called the Schmitt trigger where it is turned into a clean square wave signal. After conditioning, the signal is sent to the base of a switching transistor that is designed to turn on and off in response to the signals generated by the Hall switch assembly.

The shutter wheel is the last major component of the Hall switch. The shutter wheel consists of a series of alternating windows and vanes that pass between the Hall layer and magnet. The shutter wheel may be part of the distributor rotor or be separate from the rotor.

The shutter wheel performs the same function as the timing disc on magnetic pulse generators. The only difference is with a Hall switch there is no electromagnetic induction. Instead, the shutter wheel creates a magnetic shunt that changes the field strength through the Hall element. When a vane of the shutter wheel is positioned between the magnet and Hall element, the metallic vane blocks the magnetic field and keeps it from permeating the Hall layer. As a result, only a few residual electrons are deflected in the layer and Hall output voltage is low. Conversely, when a window rotates into the air gap, the magnetic field is able to penetrate the Hall layer, which in turn pushes the Hall voltage to its maximum range.

The points where the shutter vane begins to enter and begins to leave the air gap are directly related to primary circuit control. As the leading edge of a vane enters the air gap, the magnetic field is deflected away from the Hall layer; Hall voltage decreases. When that happens, the modified Hall output signal increases abruptly and turns on the switching transistor. Once the transistor is turned on, the primary circuit closes and the coil's energy storage cycle begins.

Primary current continues to flow as long as the vane is in the air gap. As the vane starts to leave the gap, however, the reforming Hall voltage signal prompts a parallel decline in the modified output signal. When the output signal goes low, the bias of the transistor changes. Primary current flow stops.

In summary, the ignition module supplies current to the coil's primary winding as long as the shutter wheel's vane is in the air gap. As soon as the shutter wheel moves away and the Hall voltage is produced, the control unit stops primary circuit current, high secondary voltage is induced, and ignition occurs.

In addition to ignition control, a Hall switch can also be used to generate precise rpm signals (by determining the frequency at which the voltage rises and falls) and provide the sync pulse for sequential fuel ignition operation.

Photoelectric Sensor

A fifth type of crankshaft position sensor is the photoelectric sensor. The parts of this sensor include a light-emitting diode (LED), a light-sensitive phototransistor (photo cell), and a slotted disc called a light beam interrupter.

The slotted disc is attached to the distributor shaft. The LED and the photo cell are situated over and under the disc opposite each other. As the slotted disc rotates between the LED and photo cell, light from the LED shines through the slots. The intermittent flashes of light are translated into voltage pulses by the photo cell. When the voltage signal occurs, the control unit turns on the primary system. When the disc interrupts the light and the voltage signal ceases, the control unit turns the primary systems off, causing the magnetic field in the coil to collapse and sending a surge of voltage to a spark plug.

The photoelectric sensor sends a very reliable signal to the control unit, especially at low engine speeds. These units have been primarily used on Chrysler and Mitsubishi engines. Some Nissan and General Motors products have used them as well.

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