25 June 2008

Fabricate - electromechanical computer


I said in a previous post I was thinking about some kind of safety computer to manage the electrical system. I've finally come up with something after a few iterations. Don't be deceived by the complicated looking name! All it means is I used a few relays to make some simple logic.

The operation is such that there are two safety interlocks. The first is the charger interlock. In my case the charge door has to be closed. In a more generic case where there may not be a charge door a switch or 120VAC relay must close to enable the rest of the system.
The next interlock is an internal latch that will disable the system if both the go pedal and the brake pedal are pressed. This could be a problem when trying to start out on a hill. But the easy solution is to use the parking brake instead of the foot brake, which I do in an ICE anyway. I have two design versions: one for dual contactors and one for a single contactor.
With the dual contactor design, the low side contactor is enabled when the key is turned to run (I assume a precharge resistor is mounted across the high side contactor). On the single contactor design, the precharge relay is activated.
When the key is moved to start, the 'active' relay latches on (it turns off when the key is moved way from run or an error condition occurs). Once the controller sufficiently precharges a relay that detects the precharge voltage closes, enabling the high side contactor controls. When the user presses the go pedal it latches the go relay on, activating the high side contactor and controller. This stays latched until the user presses the brake (or an error condition occurs). Then the go relay drops out, dropping the high side contactor and disabling the controller.

I've put all the high voltage parts into a separate enclosure for some safety. All the connectors link up all the electric parts, so future upgrades already have connections to everything. Also built into this system is a reverse speed limiter. When the car is put into reverse, it limits the maximum speed of the controller to about half its maximum.

I have all this designed into one weatherproof enclosure with two connectors and a screw terminal block. It can feed two dashboard indicators for fault and ready. I'm also working on a design to signal a fault when the motor over revs.

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