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PDM Installation & Configuration: Alternator Failure Output Prioritisation

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Alternator Failure Output Prioritisation


00:00 - One of the key benefits that comes from using a PMU is the ability to design a fault tolerant system that can give a failing car a way to get back to the pits or home to the garage for repair instead of being stranded on the side of the road or track.
00:14 Now while not particularly common, alternator failures are certainly something that should be considered when we're working with a car's electrical system.
00:22 The traditional way this has been tackled is to use an indicator light on the vehicle dashboard that warns the driver if the alternator is no longer outputting any current, meaning the vehicle is now running solely off the battery.
00:34 This was sufficient when high capacity lead acid batteries and simple electronic systems were the norm because there was plenty of energy stored in the battery to run the car for a decent period of time if the alternator failed.
00:46 However with electrical systems becoming more prevalent and complicated, they started requiring much higher current levels.
00:54 Battery technology has also evolved and although modern battery chemistries are far more energy dense than the old lead acid technology, there's also been a reduction in their size and weight.
01:05 This results in a battery that can still supply the large output current required for a starter motor but not for as long because it has a lower overall capacity.
01:14 These batteries are obviously very attractive to the performance vehicle enthusiast as their lightweight, small size and flexible mounting options are all the things we're looking for when we're designing a car to go fast.
01:25 However we need to be cautious when using these batteries as their small capacity means they're drained very quickly when solely responsible for running a vehicle and draining them too far can result in permanent damage, meaning that they will need to be replaced.
01:39 To help with this, in this module we'll look at designing a priority system that will spring into action if the PMU detects a problem with the vehicle's charging system, selectively shutting down non critical systems in order to stretch the remaining battery capacity as far as possible before finally shutting systems down completely when a lower limit is reached to avoid permanent damage to the battery.
02:01 The way this function is going to work is to set up groups of outputs which will be given 1 of 3 priority levels, high, medium and low.
02:09 The PMU will keep an eye on the system voltage level and if it sees this begin to sag, it can select groups of outputs to disable to try and limit the overall current draw of the system.
02:19 As the PMU will have either a direct connection to the battery or an isolator output via its high current input, it can monitor the voltage at this terminal and this will be the input to our function.
02:31 The function outputs are a little less clear as they're not specific output channels but instead groups of outputs.
02:38 This will be different in every application but we'll go through the 3 priority levels we've specified and talk about some common items that might be in each.
02:46 In the high priority group we'll include just the items that the vehicle absolutely requires to move under its own power safely.
02:53 Common items would be things like the ECU, the fuel injectors and pump and the ignition system.
02:59 Most other actuators that control engine functions would also be in this group, like an electronic throttle body but things like boost control or variable valve actuation solenoids might not be included as long as they default to a safe condition when left unpowered.
03:15 I'd also include basic vehicle lighting in this category.
03:18 In particular for a race car as not having lighting at night during a race is not only dangerous for your car, it's dangerous for all the others out on the track and in the pits as well.
03:28 In the medium priority category we'd include any engine function actuators that aren't included in the high priority category.
03:35 We'd also put cooling fans in this category instead of the high priority category as the the goal is to keep the vehicle limping along and while it's in motion there should still be enough airflow through the radiators to keep things from overheating as the engine will not be being pushed hard in this circumstance.
03:51 Windscreen wipers fit into medium priority category as well as it's possible to drive a car at lower speeds without them so they're not in the high priority category.
04:00 But they are more than just a simple convenience feature.
04:03 This brings us to the low priority category.
04:05 This category is for anything the vehicle can operate without over a longer term.
04:09 Things like motors controlling adjustable aero or suspension elements, air conditioning or heating would fit into this category.
04:16 It's possible for the car to finish a race or a long drive without the items in the category, it's just not going to be working at its full potential.
04:24 The logic diagram showing how this function would flow will also be different for every implementation as each case will have different outputs.
04:30 But you can think of it as an and function on every output, the inputs to which are the request to turn that output on and an if statement that looks at the battery voltage and determines if it is above the required level.
04:45 We've described a system with 3 priority levels but you could have more or less depending on your requirements.
04:51 I would advise against getting too granular with this control though as it'll become a headache to troubleshoot down the line.
04:57 Thinking about the devices in the vehicle requiring power in a priority sense like this can also be an excellent way to make decisions around grouping devices together to share output channels.
05:08 In this course module we've discussed a priority ordering system for delivering power to system critical devices in the event of a charging system failure in the vehicle.
05:17 The priority of a device is determined by how critical it is to keeping the vehicle moving under its own power safely.
05:23 As the system voltage reduces, lower priority outputs are disabled, helping to stretch that remaining capacity in the battery as far as possible.

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