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Practical Wiring - Club Level: Actuator Design

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Actuator Design


00:00 - The actuator design stage of our harness design process is typically the least complicated and involves specifying how our actuators will interface to our ECU.
00:10 The majority of the time, our ECU will control an actuator via low side switching.
00:14 That is providing the actuator with a ground connection.
00:18 If the amount of current that the actuator requires is more than the ECU can safely switch to ground, we instead have the ECU control a relay which in turn supplies the required current to the actuator.
00:29 This is where an amount of crossover between the power supply design stage and the actuator design stage is normal.
00:36 In particular, fuel pumps and cooling fans are required to be controlled by the ECU but typically require more current than our ECU is capable of handling.
00:44 In this instance the actuator the ECU controls is the relay with that relay then supplying the current to the EFI system component.
00:53 The first element I consider in this section is the ignition system.
00:56 In particular how we will handle the coil primary winding current.
01:01 It is possible that the ECU we're using has built in ignition modules and is capable of handling the coil primary winding current itself.
01:09 Often this will drive the choice of ignition coil that is being used in the build as if the ECU is able to handle the coil primary winding directly, we can use a standard two terminal coil that is often cheaper.
01:22 If this is the case, we will run a wire from the coil negative post directly back to the ECU.
01:28 The size of which will be the same as the power supply wire that feeds that coil's positive.
01:34 More common however, the ECU will not handle the coil current directly, but will instead trigger an external ignition module which will either be a standalone unit or built into the ignition coil itself.
01:45 In the case where the ignition module is external to the ignition coil, we will wire the trigger input of the ignition module to an ECU ignition output channel using 22 AWG wire, as there will be very little current passing through this wire.
02:00 The coil negative post is then wired to the ignition module high current input using the same gauge wire as we have supplied power to that coil with.
02:10 The ignition module high current output is then wired to a star earthing point, the engine block usually being the best choice for this.
02:18 In the case of our FD3S example, we are using IGN-1A coils, which have the third option for handling the primary winding current, an ignition module which is built into the coil itself.
02:29 As detailed in the power supply section, we've chosen 18 AWG wire to supply power to the positive terminal of our ignition coils, and we will therefore also use 18 AWG wire to connect the negative terminal of the coil back to our engine block star earthing point.
02:46 We will wire the trigger signal pin of the coil to our ECU ignition output using 22 AWG wire as once again very little current will pass through this wire.
02:56 These coils have two more pins that we need to deal with also, and they're one of the reasons that these coils are such a good choice in a performance application.
03:04 As they make them very reliable and help to reduce the possibility of electrical noise from the high voltage secondary winding, getting back into the rest of our electrical system.
03:13 The first of these pins is the reference voltage level the coil would compare the signal on its trigger pin to, to determine if it should be dwelling.
03:21 This is the sensor ground level of our ECU so we will run 22 AWG wire from this pin to the sensor ground splice point and make a connection there.
03:31 The second of these pins is a connection to the secondary winding inside the coil.
03:35 We will wire this pin back to the engine block as this will allow the secondary winding current a short path from the spark plug ground electrode, through the engine block and back to the coil secondary winding.
03:47 We will also use 22 AWG wire for this as the amount of current passing through the secondary winding is actually very small.
03:55 The next element I consider are the fuel injectors.
03:58 The most common setup will use high impedance or saturated drive injectors which are able to be controlled by the ECU directly in a low sign switching fashion.
04:07 In this case we will run a wire of the same gauge that supplies power to the injector back to an injector output on our ECU.
04:15 In the case that low impedance injectors are being used, there will be two possible situations.
04:20 Either the ECU will be capable of driving the low impedance injector correctly and we will wire it in the same fashion as for a high impedance injector, or an external injector driver box will need to be used.
04:33 An external injector driver can be thought of in the same way as an ignition module.
04:37 The injector driver handles the larger injector currents with the ECU providing a low level trigger signal when it requires the injector to fire.
04:46 The wiring of these units can differ slightly depending on the unit chosen, but the documentation supplied with the unit will have detailed instructions on how it should be connected.
04:56 For our FD3S example we're running four high impedance injectors so they are simply wired back to the ECU injector outputs using the same size 22 AWG wire which they are supplied power with.
05:09 Next I will consider the rest of the actuators which can be directly driven from the ECU.
05:14 Most commonly being control solenoids such as cam control, boost control, and ignitions related devices.
05:20 These devices are typically simple to connect, just requiring a wire of the same gauge that supplies power to them run back to an ECU output capable of driving the device.
05:31 For our FD3S example, we have the boost control solenoid, secondary throttle butterfly control solenoid, and idle air control solenoid all run back to the ECU using 22 AWG wire.
05:44 One slightly unusual actuator fitted to the 13B engine in our RX-7 is the oil metering pump.
05:50 This actuator supplies a metered amount of engine oil into the rotor housing to properly lubricate the apex and side seals.
05:57 These pumps are driven by engine rotation with a stepper motor driven valve that meters the supply of oil which then heads out to the rotor housing.
06:06 Our power supply scheme supplies power to the stepper motor with the ECU then providing a ground to the individual coils inside the stepper motor in a specific sequence which can drive the motor in either direction adjusting the position of the metering valve.
06:22 As mentioned in the sensors section of the design process, there is a position sensor on this metering valve that allows the ECU to track its position and control it in a closed loop fashion.
06:31 The wiring for the stepper motor is relatively straightforward, requiring us to run a 22 AWG wire from each of the four stepper motor coils, back to an output on the ECU capable of driving them.
06:43 This type of stepper motor driven metering valve is sometimes used for idle air control also and if your application uses one of these, the wiring is the same as we've outlined here.
06:53 The last actuators I consider are those controlled by the ECU via a relay, which are those that require a higher current level than the ECU is capable of handling.
07:03 Most often these are fuel pumps and cooling fans.
07:05 As outlined in the power supply and power ground design stages, these devices are supplied power from a relay using a larger gauge wire, and grounded directly to a convenient star earthing point.
07:17 All we need to do in the actuator design stage is connect the switching coil of the relay to the ECU.
07:24 Most commonly we will low side switch the relay as this allows us to more easily design a power supply scheme which will be immune to back feeding.
07:33 We connect the relay switching coil negative to an ECU output channel, using 22 AWG wire, as general purpose automotive relay switching coils draw only a small current.
07:43 For our FD3S example, we will be installing only one extra ECU controlled relay which is responsible for supplying power to our fuel pump when commanded to do so by the ECU.

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