Hello, I just joined the forum after reading this thread. I am a certified Master Technician and I am at the other end of the spectrum from all you EE's out here. I would like to offer a few suggestions to your design along with a little tip on your R & D. You may find lots of info out here on the web and get lots of assistance from folks like all the Gents on this Thread but please dont forget the little guy at the other end who has to repair your design after it has been implemented. Or is that important, how many techs in the field repair Control Units (CU's) ie: (ECU, ECM, PCM, TCM, BCM, etc), almost zero. There are a few shops that specialize in certain models and they have a mock up board to diagnose and perform component replacement throughout the entire unit but again, these are mostly specialists to only a couple of CU's. Even though I dont do component replacement, I sure would like to know how to diagnose the CU and understand the Theory of Operation, what pins are inputs, outputs and how do they interact. Which pins are high and low under normal circumstances and what is the logic supposed to be when a certain switch is turned on. You would not believe how many folks dont know why they get 12 volts on their voltmeter on the ground side of a ground side controlled circuit.
Some of the guys talked about over engineering, YES, choosing a chipset that is way too much for the job at hand, I promise, you will want to upgrade your project at some point and it will be sooner than you think, as soon as you complete your project, the boss will want you to add another option to the CU. Be prepared to add a daughter board to the CU as opposed to a complete redesign of your PCB, even better, a completely seperate add on CU (slave). Todays vehicles have upwards of 20+ CU's around the vehicle. One of the reasons was to keep replacement cost down for the consumer in case a specific system had a major malfunction that requires replacement of the smaller CU as opposed to a single PCM that monitors and controls the entire vehicle. Another reason was to speed up and simplify diagnosis for the field tech. I am sure you know all about CAN systems of today. We dont have Logic Analyzers and most dont have oscilloscope above 100MHz.
How will you allow your CU to be diagnosed in the field or in a repair facility, will you allow for the field tech to reflash the programming via a dedicated reprogrammer or via the Web via what type of interface. Will the CU be diagnosed via software on a PC (laptop) that you design or via the OBDII or CAN standards. Emission Standards, this is also where your Map will come into play. Someone mentioned something about sensor inputs, speed of reaction to these inputs and Emissions. Car computers are really slow so dont worry too much about lightning speed reaction times if you are not CAN Complient otherwise I think you will only need something like 9800 baud rate, I may be too slow on that but its slow for sure, but this is not for normal operations of most systems, only the CAN BUSS talking and monitoring systems and components. I highly recommend to build it test points where a graphing multimeter or oscilloscope can tag certain lines without fear of shorting something out in a compact area of the PCB. Not all field techs can afford nice equipment so how user friendly do you want to make your product, how much documentation do you want to put out to the field.
I noticed a few mechanical suggestions. A four channel scope is nice as you will need to tag four different areas of a system, not necessarily all 4 cylinders, creating ignition and injection is a series procedure and that one series line can be tagged to get all four cylinders one after the other. Triggering off of one cylinder (usually cyl #1) will give you your reference point for firing order such as 1, 3, 4, 2 while triggering from #1, just count the pulses series until you see your problem, if its on #1, change your reference trigger point. I know you are not designing the engine but you could ask for some simple additions to their design which consists of a simple stamp in the sheet metal or a paint mark across two mating components.
If you use the flywheel to monitor RPM based on a magnetic pickup or Hall Effect sensor, there are many models that allow the flywheel to be reinstalled in any old manner after completion of maintenance since all the mounting bolt holes are equally spaced, alignment marks or paint marks will help immensely. Components that can only be installed one way such as the front harmonic balancer pulley or crankshaft & camshaft timing gears are not bullet proof. Even the steel alignment pins or keys and keyways wear out. Doing a quick visual inspection of the timing belt or chain will not show the issue at hand as the belt and gear are still in time but with a sheared alignment pin, the camshaft itself is out of time. This is also a major issue with harmonic balancer pulleys that have rubber dividers between inner and outer rings, the timing marks align on the outer ring but the inner ring attached to the crankshaft is out of time. So be aware of where you place your reluctor for a magnetic pick up if you use this type of system. You should publish the waveform and corollation of all your timing signals.
Heat and cold was hit on but dirt and dust and liquids were not addressed. Mainly in high dust areas there can be a lot of dust intrusion into the case of your CU as you need to allow for heat ventilation of some magnitude. Adding on a nice user replaceable air filter would be nice. Air Conditioning systems use them to filter out dust and pollen but sometimes they install them in the damndest places, why. Mounting location of the unit was hit on but putting it under the hood, well, it offers ease of access but its pretty hot and dusty in there. Being in an area of extreme temperature changes promotes high condensation rates and a corrosive environment. Can other fluids be accidentally spilled on it while servicing the brake fluid, oils, or if the radiator decides to let loose, where is all the antifreeze going and if it has not been properly maintained, its corrosive also. Consumers love consumer products like Coke and they love to spill it everywhere if mounted in the cabin, enough said about that. Back to ease of access and length of wire harness and orientation of the CU if mounted up under the dash or down in a kick panel, NOT under the seat, people love to leave their windows down in a rain storm. The field tech would love to be able to pull this thing out and back probe it or attach a breakout box without the need to pull so hard as to pull a few pins out in the process. An EE subject I have seen on many PCB's, if you use electrolytic caps, seal the base with the proper type of glue to prevent the electrolytic from running down the board and eating up part of the PCB traces along with other components. Mitsubishi is famous for this. On another subject I have seen fuel wick up the wire from a fuel injector into the CU and flood it out and destroy it, beware.
Fuel injectors come in all resistance ranges but maybe your design will be model specific, but again, thinking in the future, you will want to make a simple component change to allow for a multitude of applications, this also goes for ignition coils if you go petrol.
Voltage ranges, in the 12 volts systems, you will want to allow from 9 to 18+ volts for weak charging to non regulating dynamo scenarios. I cannot speak for the 24 volt systems.
Maps, this CU is fully reprogrammable and many manufactures today allow for their own mistakes when they design a Map and they also allow for normal wear and tear which will also require a reflash months or years down the road as opposed to a refit of a new expensive preprogrammed CU. You can program multiple Maps designed for multiple applications of your powertrain system, even with only one engine in mind and maybe even only one chassis in mind, along with automatic or manual gear selection, there are always a multitude of driving scenarios such as mostly city, highway, mountainous, granny mode or young punk race mode, work vehicle hauling lots of people or a lot of heavy equipment. Some of these modes could be selected via a user input switch similar to power, normal, towing modes.
Now for my original purpose of joining this thread, get a field tech in with you on the R & D so that when you make your presentation to the Board and they ask for a demonstration of your project, you can also show them how easy it is for a technician in the field to diagnose this CU, its easy of access along with its safety and security of its mounting point and the purpose of its orientation. The need for selectable Maps and the forward looking need of reflashing for a multitude of reasons, a few of which you will name.
I am all over the place on this post so let me recheck and ask a couple more questions. CAN, helps to diagnose issues thoughout the vehicle. Most techs dont understand CAN and I dont even want to get into, scan the system, see what CAN Data says to do, then do it. I always back up scandata via a scope, I just dont trust scanners but they do help to narrow down a system issue to a smaller area of concern. Without a complete understanding of what make the vehicle tick, it does not always come down to experience. I mentioned adding in test points but not only on the PCB. Allow a wire to come out of the main harness to allow an Amp Clamp to be attached for some non intrusive testing, ie: ignition coils, injectors, checking base timing can all be done via an Amp Clamp. Build in some test connections so wires dont need to be probed or backprobing a connector is quite popular also but should not be required, there are some real brutes out here who dont care what kind of a hack job they do, they will just cut up a harness to try to find a short and then rewire two red wires back together just because they are red not caring that there are two sets of red wires in the bundle, then they give up and the next tech has to find that issue as well.
Sorry for the rants, I am out'a'here..............
Duane in Japan
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