Fabrication

I've replaced the Mini's mechanical speedometer with an electronic unit from a Mitsubishi FTO. This speedometer requires 2548 pulses per km, but my motor controller outputs a 7031 pulses per km. I need to make a box that outputs 1 pulse for every 2.764 input pulses. I made a fairly naive 2.75:1 implementation in an arduino by outputting one pulse for every 3 input pulses except every 4th pulse, where it only waits for 2 input pulses. This jitter doesn't seem to affect the speedo needle even at fairly low speeds.

The current implementation counts to 11 and uses a switch statement to output pulses which isn't really generalizable to other ratios but is probably close enough to what I need.

I printed a 3 part adaptor to mount the Mitisubishi FTO speedometer inside the mini instrument cluster. I printed them in PET-G which hopefully won't soften in the sun -- it's apparently good for 75°C so that does seem unlikely. I must thank Daniel Dillan at www.vivenda.co.nz for suggesting PET-G and advice on getting good results with it (print it hotter and slower than PLA).

I'm not entirely sure why I needed so many prototypes. Good thing I have my own printer.




I now need to create an appropriate speed pulse signal from the inverter. Initial investigation suggests I cannot configure it to make less than 6000 pulses per kilometre, which is a little more than double what the speedometer expects. A simple arduino sketch will fix that.

I reverse engineered the data stored in my Mitsubishi FTO Speedometer in order to re-calibrate it to suit the Mini face. Both faces show 180km/h, but the mini face spreads it over a slightly wider angle than the FTO does. The data is stored in an OKI 16811G 128 byte Serial EEPROM. This is quite an old part which uses Microwire (a predecessor and subset of SPI) to communicate. I had some trouble finding the data sheet for this part so I tried to sniff the communications between the odometer's CPU and the EEPROM with an Arduino running gillham's SUMP compatible firmware. Unfortunately the Arduino only has 2k of ram so recording all the state changes wouldn't fit. The communication is clocked at about 1MHz and I wasn't able to write a parser fast enough to stay in sync.

There is a fair amount of questionable information about odometer EEPROMs on the Internet, mostly related to tools for sale to wind your odometer back. One of these references claimed the part was compatible with a 93C46 so I wrote an EEPROM reader for that part and read out the data. Unfortunately this wiped the EEPROM! Somehow my reader code triggered the erase all command. This isn't entirely surprising because the right data sheet shows this EEPROM uses a different protocol to that used in the 93C46.

With a wiped EEPROM the odometer stop working, so I had to get another one. I also got an Open Bench Logic Sniffer which let me observe the entire communication between the odometer and the EEPROM and confirm that the protocol in the correct data sheet was the protocol actually in use. While re-scaling the speedometer for the new face I also reverse engineered the stored kilometre value and re-set the odometer to zero to celebrate the conversion to electric power. I also made some somewhat comprehensible notes about how this data is stored, an Arduino sketch to read and write the EEPROM which includes some tools to re-format the data, and an Arduino sketch to test an electronic speedometer and find the number of pulses per km.

Surprisingly the new gearbox's speedometer cable connects to the Mini speedometer. Unfortunately it interferes with the body at the gearbox end so I haven't been able to test it's calibration. I've been using the speed display on my EVision which listens to a speed pulse from my motor controller, but there is no odometer or trip meter and I would like to use display other data on the EVision, so I've been looking for an electronic speedometer to mount inside the mini instrument cluster. At the weekend I found a small electronic speedo from a Mitsubishi FTO which was new enough to be electronic but old enough to not be use the CAN bus and be small and separate from the other gauges.

The speedometer input is pulled up to 5v and pulled down by a transistor in speed sensor in the gearbox. I made a simple Arduino sketch and found it requires 2548 pulses per km and very slightly over reads at 100km/h. Interestingly it works at 400km/h! The speedo needle almost touches the other side of the zero stop but the odometer works perfectly. I haven't tested to find the speed at which it stops working.

The mechanism is small enough to fit into the mini instrument pod, but the face is too big. The original mini face will fit if I make the window for the odometer bigger and enlarge the center hole slightly. The shaft is smaller but it shouldn't be too hard to fit the mini needle. I'm not sure if I should mount a remote trip meter button, or drill through everything and try to mount a button in the normal position. I'm now designing an adapter to mount the mechanism. Interestingly both cars have 180km/h speedometers, but the Mini spreads it out over more degrees. I'm not sure how hard it will be to calibrate the needle.

I'm considering whether to wind the speedo forward to 200,000km and call that "electric zero" or to attempt to align it with the car's current speedo -- TachoSoft indicate that the mileage data is stored in a separate serial EEPROM.

I made a little load tester with some stove elements. They draw 27A at 260V. The camera is sensitive to near infrared -- they don't look as bright in real life. I think I will have to improve the cooling system before running it at higher voltage. The 3rd element away from the fan runs hottest, probably because the 4th element radiates heat away on it's outside and heats the 3rd element with the other.

Connecting this thing is not entirely straightforward. At low currents, you can use an Anderson disconnect with moderate safety, but things go badly at 27A and 260V. When I connected all 4 elements for the first time, the pins hit end on and the plug didn't go together. I don't know if disconnecting would have gone better with the small run-up that a fully mated connector offers, but I do know that separating from half connected went badly. There was a pop and a ball of white a big bigger than a hand span. When the after-image started to fade I could see the outline of the connector with a jet of white exiting the connector at 45 degrees on each side.

Safe disconnection is easily achieved with the right contactor. A small flash is visible inside the arc chamber during disconnection.

One of the weakest parts of my motor mounting system is the bush which absorbs the torque generated by the motor. This mount was made very quickly from stuff I had lieing around and it turns out I applied force to the bush in different direction to that which it was designed for. I'm now working on a new design involving at least two more mounts which will better control the front-rear position of the motor in addition to it's torque.

The first mount is nearly done and sits between the gearbox and the front rail of the subframe, you can just see the top bolt and the hole for the bottom bolt in the middle left of this picture:

Ed came over and helped me make a temporary battery box for the rear seat. This lets me evaluate performance with 40 more cells, for a total of 76. Knowing how well the car performs at higher voltage with a larger battery lets me decide how hard I need to work to squeeze cells under the bonnet. The intention is still to retain 4 seats in the car, this box is just an experiment.

Building this battery box was harder than it sounds. The seat base is full of curves so the bottom of the box needs a strong base, and holding the cells down with their dangerous high voltage terminals very close to the edges is not trivial. I'll post a picture of the lid which was key to holding the cells down shortly. The box is made of wood which is more flammable than I would like, restraint will be achieved with ratchet tied downs attached to the seat belt anchors.

In other news, the new cable connecting the inverter to the motor has resolved my inverter instability. I'll blog more about this cable soon.

I seem to have successfully moved my inverter to the rear of the car. This will allow me to put a much larger number of cells in the front of the car and improve the weight distribution (I'm currently rear heavy). I found connectors to extend the motor's encoder cable and Ed made one up using some special cable he had left over from a job. I replaced the terminal cover on the inverter with a mounting plate for the new Gigavac GX14 contactors (replacing the large Schaltbau 162 units that wouldn't really fit anywhere), precharge circuit and the EVision shunt. This has a number of drawbacks,

• quite a lot of effort is required to inspect the terminals under the cover
• the positive battery cable connection is partially obscured below the boot opening and is close to the body, working on it is a little scary
• a rear end crash could cause major problems
• I haven't worked out how to form a cover

I may yet move this contactors plate above the inverter or to the right of it.

Pictured is my current test implementation, there are a number of things that need fixing,

• there are some very thin copper bus bars that need to replaced with thicker bars
• the jumpers between the contactors and the inverter are much too thin
• the fuse should be much closer to the battery
• one of the phase cables is completely unsuitable (about 10mm2 total from all three conductors of a household electric oven supply cable (including the oven's on-off switch!)
• the low voltage wires need some tidying up
• putting everything on top of an insulator and standing some insulation between the various components might be a good idea
• I need to make a cover

This worked well for going up and down the driveway, well enough to make me feel confident the inverter will live in the rear of the car without being effected by noise escaping from the phase cables.

I did have one mishap while testing. I know you have to keep the loop area small in the phase cables (this is a clever way of saying "keep the phase cables close together"!) and Ed repeated this several times while I was fabricating, but when it came time to actually put it together, well, I forgot. The inverter was most unhappy to have one phase cable go over, one go under and one go around the side (the latter two going out the boot aperture). It refused to run the motor smoothly with no load and then it blew the 30A test fuse! I re-directed the cables to all go over the inverter and through the hole in the front of the boot and everything was smooth and happy. My testing so far is limited by the thin wires in both the battery and motor circuits but I think (and hope) I put enough current through the phase cables to prove it will work. The encoder cable runs down the side of the car while the phase cables run down the middle. Shortly additional battery cables will also run down the middle, hopefully this won't introduce problems.

There is room for another row of cells in front of the two shown, but this row will have to be slightly lower since the front of the car curves down. There is enough height in the middle to put cells above the motor where it protrudes through the plywood, higher than the rest.

It seems improved Weight distribution can be had by putting the inverter in the boot and most of the battery in the front. This does introduce longer motor cable runs which will reduce efficiency but it is looking worth it -- with the inverter up front there isn't much room for cells and it's 70kg lighter on the front axle than when I started. I'll be mocking up the front to see exactly how many cells fit without the inverter before I commit to this move.