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But which gear?

ecu-emsAs a young man (many years ago), although my spelling was never entirely satisfactory, the subject of mathematics never daunted me. Matrix manipulations, Laplace transforms and complex differential equations were as meat and drink to me as I endeavoured to find order in this apparently disordered world. It was only later, as I discovered karting, that this world was slightly shaken, when I lost my ability to count.

First, second, third, fourth - the gears would fly by as I accelerated the class IV gearbox machine into the distance and with the humble Villiers 9E engine and equally humble four-speed gearbox I could easily cope. However, when moving on to the more modern 125 Yamaha, six-speed unit, things became more difficult. Moving the lever up to change up and then down to change down was never the problem; it was simply remembering which gear you were in out of the six - was that fourth or fifth? Without a rev counter (for in those days such things were rare), in the heat of the moment there was no way of knowing; and anyway, in the next split-second or so, you were ready to change up (or down) and so there was no time to worry.

Many years later, and in most forms of motorsport today, the four-speed gearbox is but an ancient memory, and six speeds are the accepted norm. In Formula One of course, transmissions have seven speeds, and in 2014 will move up a gear (if you'll pardon the expression) to a mandatory eight. It is little wonder therefore that it is now considered imperative -not only for the driver but also for the engine controller - to know which gear we are in.

In theory, of course, there is little difficulty in working it all out - a simple tailshaft sensor to compare its speed with that of the engine speed can tell you almost immediately. The clutch has to be engaged and the vehicle moving, and the calculation will take but a few milliseconds. Indeed, many aftermarket systems incorporate this as part of their standard package. Another way is to mimic the driver by counting up the box using an inline microswitch in the gear change system, and down the box in a similar way. Storing this information digitally in its memory, the ECU should know where it is at any time, even if the car is stationary.


By far the best method, however, is to mount a rotary potentiometer at the end of the gearbox change barrel. Similar in many ways to the unit mounted at the end of the throttle shaft to determine the degree of throttle opening, rotary potentiometers are simply rotary voltage dividers calibrated to the exact position of the selector barrel. Indicating the position of each gear, in the more sophisticated systems the system can also plot its position at all stages through the gear-changing process.

In sequential systems, neutral can appear in one of two places. Motorcycle-derived gearboxes invariably have neutral between first and second gears, while vehicles that are devoid of any historical precedent tend to slot this in at the beginning. Thus, for a motorcycle, the arrangement on the barrel may be 1-N-2-3-, while for a car it is more likely to be N-1-2-3-. However, knowing where on the barrel the gear should be is one thing, knowing where it actually is, is another, since mechanical wear and the random movement of parts can affect this to a small degree. Limits to the position of the gear therefore have to be added.

It goes without saying that the rotation of the potentiometer - be it 90º, 180º or 360º - must be at least equal to the rotary movement of the barrel. And while the mechanical wiper technology of traditional potentiometers can be affected by the high levels of shock and vibration, not to mention the extreme temperature experienced by a gearbox, newer non-contacting rotary sensors are now available that use Hall effect technology. With these sensors also accommodating duel redundant options, offering the ability for the sensor to be used in two modes - control and position monitoring, or control and gear indicator - there seems to little excuse for not knowing which gear you are in.

But there again, why make it easy for yourself?

Fig. 1 - Non-contacting dual independent output rotary sensor

Written by John Coxon

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