One problem with training or learning is deciding where to start. However, I am going to begin with Digital Signals, which may seem strange because digital signals and digital electronics are generally regarded as being advanced topics; but nearly every system on every vehicle now operates with digital signals and electronics, so even a basic understanding will be of value when working on modern vehicle systems.
Initially, there are two terms that we need to understand – Analogue and Digital. A conventional speedometer or a wristwatch with hands that move around the watch face are Analogue displays, where the movement of the needle on the speedometer or the hands on the watch are an indicator of speed or time. In effect, an Analogue display provides a progressively changing indication that corresponds at all times to a value of something that changes, such as speed or time. The fact that the speedometer or the watch are probably driven by digital electronics is irrelevant for the moment; but the important factor is that analogue displays progressively change and these changes correspond to the changes in speed or time.
If we then think of a watch with a digital display, the display might in fact only change once every minute; so this display is providing an occasional sample of the time at one-minute intervals; and although for general use this occasional sample is more than sufficient, the display only corresponds to the actual time once every minute. Now think what would happen if we had a digital speedometer that only refreshed the display at one-minute intervals; the chances are we would be making a lot of speed cameras flash, but if the speedometer then refreshed the display every second, it would be almost impossible to work out the speed.
If we then look at electrics and electronics, we will frequently encounter Analogue and Digital signals. Vehicle systems sensors, such as Temperature Sensors, Mass Air-Flow Sensors or Crankshaft Speed/Position Sensors pass information to the Engine Management Electronic Control Units (ECU or PCM); and some sensors provide analogue signals, whilst others provide digital signals. One of the most widely used sensors that nearly always provides an analogue signal is a temperature sensor, such as a coolant, air or oil temperature sensors; and there is a good reason for this (apart from simplicity and cost).
When the engine is started from cold and progressively warms up, the engine management control unit will progressively alter the fuel mixture and the ignition timing, as well as adjusting other engine related functions such as emissions control devices. Importantly, however, the fuelling, timing or emissions control functions don’t always just have one setting for cold and one for hot because the fuelling and timing will certainly change progressively as the temperature progressively changes. Therefore, to provide accurate control of the engine functions, the control unit should ideally receive a signal from the temperature sensor that progressively changes and corresponds with the changes in temperature.
Most vehicle temperature sensors are in fact simple resistance based devices that provide analogue sensor signals, where the sensor resistance and therefore the signal voltage change with temperature (Figure1). A typical output voltage provided by a coolant temperature sensor might be close to 4 volts for a cold engine, and reduces down to maybe 0.4 volts for a hot engine; but the change in voltage is progressive and corresponds at all times with the engine coolant temperature.
For an extreme example, imagine what would happen if the sensor only provided a signal to the electronic control unit that changed once very minute or every 5 minutes? The engine temperature would change progressively, but the fuelling and timing would only change occasionally. So, using an analogue signal provides the progressive change in temperature information that enables the control unit to progressively change the fuelling and ignition timing. It is then interesting to note that the change in temperature is obviously progressive, and other engine operating values such as air-flow also change progressively; and this means that many sensors will need to detect the progressive changes (which are not simply ON or OFF). Therefore, many sensors on vehicle systems do in fact produce an analogue signal; but these analogue signals need to be converted into digital signals (either within the control unit or by the sensor) so that the control units can process the information provided by the sensors.
One problem with using analogue sensor signals is that modern electronic control units and computers operate using digital electronic components (we’ll look at this in a subsequent article); but we can for now say that these components operate using signals that are in effect just ON or OFF pulses. If you think of the output voltage from a simple light switch (Figure 2): when the switch is ON, the output voltage is ON, and when the switch is OFF, the output voltage is OFF; and this is effectively a simple digital signal with ON and OFF pulses. In fact, one of the main components used for digital electronics is a transistor that is used as an ON and OFF switch; but there are then millions or billions of tiny transistors functioning as electronic switches that are contained within a micro-chip. So there can be millions of ON and OFF pulses passing through the micro-chips every second.
There are various ways of making use of these ON and OFF pulses and one way is to count them and the different numbers of pulses could then represent different messages or items of information. Therefore, if a light switch is switched ON and OFF 10 times in a minute, the 10 sets of ON and OFF pulses could represent one message, but switching ON and OFF 20 times a minute could represent another message (which is a bit like using a flashing light to transmit messages with the Morse Code system). In effect, the different numbers of ON and OFF pulses are part of a coding system, where different messages or items of information are represented by different numbers or arrangements of ON and OFF pulses. Although this is a simplistic explanation, it is the basic principle of how digital signals can be used to pass information and messages between different computers or different electronic control units. These ON and OFF pulses are of course usually electrical pulses passing through wires, but they can also be flashing light pulses passing through Fibre Optic Cables (Figure 3), which is how information is passed between control units on some vehicle networks such as the MOST in-car entertainment system network.
One relatively simple example of counting pulses is where some types of MAP sensors provide a digital signal to the control units. Although the sensor is measuring Intake Manifold Pressure that changes progressively, the sensor contains electronics that convert the progressively changing air pressure values into a digital signal. For one type of MAP sensor, when the ignition is ON but the engine is not running, the sensor will detect atmospheric pressure in the manifold. With this pressure, the MAP sensor signal will typically contain 150 ON and OFF pulses for every second, which is 150 Hertz; and this equals 15 ON and OFF pulses in one-tenth of a second (as shown in Figure 4). But when the engine is at idle speed with a closed throttle, the intake manifold pressure is LOW (which we often refer to as high vacuum) and the number of pulses then reduce to approximately 80 pulses a second, which is 80 Hertz – this equals 8 ON and OFF pulses in one-tenth of a second. The control unit will therefore count the number of pulses that occur in a pre-determined time period, and this then indicates the manifold pressure.
The CAN-Bus Network…
One other significant application for digital signals is for communication between the different control units on vehicles and also for home and office computers, which includes wireless and Bluetooth communication. One example is the automotive CAN-Bus network system, which links together different vehicle system control units that are generally powertrain related (such as engine management and transmission control units). For example, the engine management control unit could request vehicle speed information from the transmission control unit, which would then pass a message back to the engine management control unit with the requested speed information. There would therefore be two digital signals passing around the network.
With one version of powertrain CAN-Bus signals, a message or individual signal typically contains 108 ON and OFF pulses, where the different possible arrangements of the ON and OFF pulses create a unique coded message. The clever bit is that the messages on a powertrain CAN-Bus network fly around the cables at a rate of around 500,000 pulses a second, which is referred to as 500 kilobits/second or 500 kbits/sec. This is in fact slow because on some vehicle networks, such as the fibre optic based In-Car Entertainment networks, there can be as many as 150 million pulses every second or more. This is all done with millions of tiny little transistors that just switch ON and OFF very, very quickly.
As a final note for this brief topic, CAN-Bus systems actually create two identical signals for each message, except that one signal is a mirror image or upside down version of the other signal; and this is to help reduce the effects of interference on the signal, but that is another story that we can look at in another article.