Electronic components

Sensors

Anything that signals input data to a computer system can be described as a sensor. Sensors may be simple switches that an operator toggles open or closed to ground a reference voltage, modulate a reference voltage (V-Ref), be powered up either by V-Ref, or require power-up outside of the V-Ref circuit.

Types of Sensors

Sensors may generate their own voltage, receive a constant reference voltage (usually 5 V known as V-Ref), or be powered up at a higher voltage.

1. Thermistors. Thermistors precisely measure temperature. They are used to signal oil, air, fluid, and ambient temperatures. The two types of thermistor are defined by whether resistance increases or decreases when the temperature rises:

• Negative temperature coefficient (NTC): Temperature goes up, resistance goes down.
• Positive temperature coefficient (PTC): Temperature goes up, resistance goes up.
  
  The ECU receives temperature data from thermistors in the form of analog voltage values.

2. Variable capacitance (pressure) sensors. These three-wire sensors are supplied with reference voltage and usually designed to measure pressure or linear position values. The medium whose pressure is to be measured acts on a ceramic disc and moves it either closer or farther away from a steel disc, varying the capacitance of the device and thus the voltage value returned (signaled) to the ECU.
3. Potentiometers. The potentiometer is a three-wire (V-Ref, ground, and signal) variable resistor. Once again, these receive a reference voltage and output a signal proportional to the motion of a mechanical device. Potentiometers are voltage dividers. The moving mechanical device moves a contact wiper over a variable resistor. As the wiper is moved over the variable resistor, the resistance path is altered, with the supply voltage being divided between the signal (sent to the ECU) and ground. Potentiometers are used in some types of equipment such as joysticks and throttle position sensors (TPSs).
4. Piezoresistive pressure sensor. These are some-time referred to as Wheatstone bridge sensors because of the printed circuit used to output its digital signal. They are more accurate than variable capacitance sensors and are primarily used in engines, usually as a boost pressure sensor: for that reason, they are only briefly mentioned here.

Switches

Switches complement the sensor circuit and can usually be classified as command inputs. Switches may be electromechanical or ‘‘smart’’; that is, they use signals rather than analog voltage values to signal a change in status. Switches can be subdivided into three groups:

1. Switches grounding a reference signal (V-Ref). A good example would be that of a coolant level sensor. V-Ref from the ECU grounds through the coolant in the upper radiator tank. If the coolant level drops below the sensor level, the reference signal loses its ground, which after a preprogrammed time period, for example, 8 seconds, signals a fault alert.
   
   
2. Manual switches control electrical circuit activity. Many versions of this type of switch are used by the operator to control machine functions. A good example is the standard ignition key.
   
   
3. Smart switches use digital signals to indicate a change in status: these signals may be initiated by a ladder switch (resistor bank) or by an integral processor. The signals produced by a smart switch may be automatically generated by a change in status condition or be generated by a mechanical action such as an operator toggling a switch.

Actuators

A wide range of actuators are used on machine control systems. Most are fairly simple in terms of their operating principles. Here are some of them in brief.

Solenoids. A simple solenoid consists of a coil and an armature. The coil is usually stationary and the armature moves within it. The armature can be integral with devices such as hydraulic poppet valves, spool valves, and levers that effect mechanical movement. They are widely used in machine hydraulics and engines. A simple solenoid has two status conditions: off or on. In most cases, the armature is spring-loaded to default to the mechanical off position when no current is flowed through the coil. When an ECU driver energizes the coil, the magnetic field that builds mechanically moves the armature. Because solenoids use electro-magnetism, they respond more slowly due to the time required to build and collapse electromagnetic fields: This limits their use in applications that require fast response and frequent cycling.

Proportioning Solenoids. Proportioning solenoids are coil and armature devices that function similarly to a solenoid except that they are capable of precise linear or rotary positioning. To ensure the accuracy of the position, most proportioning solenoids have a means of signaling actual position while the ECU driver at-tempts to control current flow to the proportioning solenoid to maintain desired position. An example of a linear proportioning solenoid would be the oil control spool valves widely used in machine electronics.

Piezo Actuators. Piezo actuators have an advantage over solenoids because of their super-fast response rates. The mechanical response of a piezo actuator occurs the instant it is hit with its electrical trigger. The first generation of piezo actuators was bulky due to the size of the stack of piezo wafers required, but this is changing fast. Not only is the physical size of these actuators being reduced (to date, they tend to be a little bulkier than solenoids) but they also require lower electrical actuation pressures (voltages) and current drawn than solenoids. Piezo actuators are often used in the rumble packs integrated into joysticks for sensory feedback, covered a little later in this chapter.

Stepper Motors. A stepper motor is a brushless electric motor capable of the precision positioning of a shaft (and whatever is connected to it). A normal direct current (DC) electric motor spins when current flows through its electromagnetic fields. Stepper motors use multiple toothed electromagnets arranged around a rotor gear integral with the motor shaft. All but one of the toothed electromagnets are slightly offset from the gear teeth on the rotor: This means that when the rotor gear teeth are perfectly aligned with one of the toothed electromagnets, they are slightly offset from the teeth on the others. When the next electromagnet in the rotation is energized, the rotor gear moves slightly to realign the magnetic field and completes a ‘‘step.’’ Typically, four electromagnets are used spaced 90 degrees apart in the rotation. In this way, the rotor can be precisely aligned to any position in the rotation. The precision increases with the number of teeth in the rotor and, correspondingly, on the electromagnets.

Stepper motors are used for precision positioning of valve and door/gate controls on machines, such as those used in exhaust gas recirculation (EGR) mixers. Because a constant DC current flow creates heat (and therefore resistance variables), they are regulated by pulse width modulation (PWM) to reduce resistance generated heat in the controller.