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Old 07-05-2012, 11:35 PM
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PStechPaul PStechPaul is offline
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Lightbulb DIY foot throttle

I had an idea for a simple DIY foot throttle for an EV. Apparently the resistive potentiometer types are prone to wearing out and failure, and other types such as hall effect and LVDTs seem to be very expensive.

I don't know what specifications are needed, but for my own purposes using a standard 3 phase VFD controller, it will accept either a potentiometer or a variable voltage which can be set for 0-5 VDC. I have also used a computer joystick which is a 0-200k resistance, and I converted that to a +/- 5VDC signal which also provided reverse. But I think a foot operated throttle will be best.

I have some experience with LVDTs, but they are typically high precision and very expensive position sensors that require special circuitry. Here is a tutorial I found which helped refresh my memory:

But the basic principle is very similar to a steel core moving within a magnetic solenoid. Here are some examples of AC and DC solenoids, and normally they are used to convert electrical power to mechanical power by pulling or pushing a plunger in a linear direction, usually about an inch or so.

However, the other property of a solenoid is that the inductance changes and is increased as the plunger is pulled in, and for an AC solenoid this is desireable because the current draw will decrease as the plunger pulls in. I happen to have a couple of solenoids and an LCR meter, so I ran a little test on one of them:

0.829" 53.6 mH 63.6 uSec
0.700" 62.5 mH 72.6 uSec
0.600" 76.0 mH
0.500" 89.7 mH
0.400" 111.8 mH
0.300" 137.2 mH 147.5 uSec
0.200" 166.9 mH
0.100" 228 mH
0.000" 288 mH 298.6 uSec

It would be very easy to design a circuit which could read and convert these inductance values to a range of voltage suitable for a throttle input. My idea would be to use a Microchip PIC with a 5VDC supply, and measure the inductance by applying a voltage and measuring the time for the current to reach a certain value. For a fixed voltage and a set current, the time is proportional to inductance, so for 5V and a current of 5mA, the times will be as shown in the third column above.

So it can read pedal position at least 3000 times per second, and it can utilize a look-up table to accomplish any linearization or custom response desired. For instance, it may be good to use a logarithmic or square law response, so that the first portion of travel provides very fine control, and the further it is pressed, the faster it responds.

The entire circuit, including the solenoid, could be built for, say, well under $5 in quantity, or maybe $20-$30 for a few prototypes. Maybe this is already available, so I might just build it for my own use, but if it would be a valuable product for the EV community, I can put more time and effort into it and make it available more quickly, like one month compared to 6 months. But I'm not familiar with the requirements and expectations, and maybe this has been done before.
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control, solenoid, speed, throttle

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