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Practical Wiring - Professional Motorsport: Sensor Design

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Sensor Design

07.18

00:00 - The first step of designing the sensor section of our wiring harness is to gather all of the information that we can about the sensors fitted to the engine and chassis, adding it to our documentation library.
00:11 Ideally at the professional motorsport level, OEM sensors will be replaced in favour of more robust and accurate sensors with supplementary sensors also being fitted to monitor additional parameters.
00:22 In these cases we can obtain the data sheets for these sensors and then know that we have accurate information to reference.
00:29 If OEM sensors are to be used, often we will need to trawl the internet to try and find scraps of information on the physical pin out and output format of the sensors.
00:39 For a common engine or vehicle that is a few years old, this information can typically be found fairly easily.
00:45 But for a newer vehicle this information might not yet be available.
00:49 If this is the case, talk to your ECU manufacturer and see if they can assist you with characterising the sensor or create a post on our forums with as much detail as possible and we'll try to assist you there.
01:00 WIth the information about our sensors gathered we can determine how each needs to be wired.
01:06 This tends to group them into three main categories.
01:09 Those that require a power supply, those that don't require a power supply, and those that don't require a power supply but produce a very low level or critical signal and need to be wired using shielded cable.
01:20 For any sensors that require a power supply, this will be provided by the ECU.
01:26 As it needs to be a fixed regulated power supply.
01:29 It's common for motorsports ECUs to have more than one pin dedicated to this task, and the voltage these pins supply can be programmable.
01:38 Most often they supply five volts but some sensors can require a regulated supply at a higher voltage.
01:45 If this is the case, you will need to ensure your ECU is capable of providing this.
01:50 I find a common design will include an ECU that has two sensor supplies.
01:54 Both of which supply five volts.
01:57 I use one of these supplies to power the engine and chassis sensors that monitor critical parameters essential to the operation of the vehicle.
02:05 The other supply I use to supply supplementary engine or chassis sensors which provide signals that the engine can still operate without.
02:14 Sensors typically draw very little current and as such we can use small wiring to supply power to them.
02:21 The size wire we use will most often be determined by the connector that the sensor uses and the size wiring that the connector pins can accept.
02:28 22 and 24 AWG wire are very common options for this.
02:33 You must still read the data sheet for the sensor however to determine its current draw and verify that this size wiring will be safe.
02:41 You also need to determine the overall current draw of the sensors connected to a single ECU supply pin and ensure that this overall current draw is within the limits that the ECU can supply.
02:53 When designing how the sensor supply wiring will be run through the harness, the aim is to only have one splice point per sensor supply pin.
03:02 This means that the sensor supply connector pin will be crimped to a single wire which is then spliced out only once to every sensor that requires a connection to that supply pin.
03:14 Most often this splice point is immediately behind the ECU connector contained within the connector sealing boot.
03:20 This ensures that there is a minimum length of wire between the connector pin and the splice which carries the current of all the connected sensors, minimising any voltage offsets that this may cause.
03:30 Whether a particular sensor requires a power supply or not, it will almost always require connection to a sensor ground pin of the ECU.
03:38 ECUs will typically have a matching sensor ground pin for each sensor supply pin they have, forming pairs.
03:46 It's important that a sensor supplied power from a supply pin is grounded to the matched sensor ground pin as the ECU will be keeping track of the voltage levels at both of these pins to ensure that the potential difference between them remains at the correct regulated level.
04:01 The physical design of how our sensor ground wires run throughout the harness, is similar to our sensor supply design.
04:07 The aim is to have a single splice per sensor ground pin, typically located close to the ECU connector enclosed within the sealing boot.
04:16 The reasons for this are the same as we want a minimum length of wire carrying the combined current of all the connected sensors, to minimise any voltage offsets.
04:25 This strategy also benefits us by minimising the total number of splices in our wiring harness.
04:31 As although failures in a professional motorsport wiring harness are uncommon, if they do occur, splice points are a likely culprit.
04:39 By having our splice points within the sealing boot of the ECU connector, our concentric twist operation is also made much easier as we will not have to contend with any wires which splice out to multiples along their length.
04:51 For the majority of sensors, we run their signal wires back to the ECU input channels with standard wire, the size of which is once again determined by the connector.
05:01 For sensors that have a low level signal or are deemed completely critical to engine operation, we run their signals back to the ECU using shielded cable.
05:10 Examples of this are engine speed and position sensors, and knock sensors.
05:14 The shield around the signal wires within these cables is grounded, allowing any electromagnetic interference to be absorbed by the shield and drained away to ground.
05:25 This shield must only be grounded at one end however as if it's grounded at both ends, a small voltage offset between either end will cause a current to flow along that shield, actually inducing a voltage within the enclosed signal wires.
05:39 Which is exactly what we are trying to avoid.
05:42 Often the ECU will have a specific pin to connect the shields to, or they can be connected to one of the ECU's power ground pins.
05:50 In professional motorsports, the use of exhaust gas temperature or EGT sensors is common place.
05:57 These sensors are a thermocouple and the signal they produce is very small, requiring specialist electronics to interpret.
06:04 Thermocouple sensors are made up of two wires which instead of using copper, for their internal conductor strands, use two dissimilar metal alloys.
06:13 Chromel and alumel in the case of a K type thermocouple commonly use in the automotive applications.
06:19 We won't go into the specifics of how thermocouples work, but they do have some particular wiring requirements that we need to clarify.
06:25 It's imperative that we wire the thermocouple sensors back to the measurement unit be it the ECU itself or an external interface box, using thermocouple extension wire.
06:36 The conductors in this wire are made of the same materials as the conductors in the thermocouple itself.
06:42 And we need to have an unbroken chain of these materials from the thermocouple sensor to the measurement unit.
06:48 This means if the wiring needs to be run through a connector, the connector pins must also be made of these materials.
06:55 Such pins are available for many connectors, Deutsch, DTM and Autosport being the most common I strike.
07:02 As the voltage signals generated are very small, it is best practise to use shielded thermocouple extension wire.