Steve's 'Barn Door' - Electronic Motor Drive

Electronic Drive

Of course, for really accurate control we should be using the Clocks '1 second tick' and the Drive nut should be set up to generate 60 triggers per revolution. This gets our max. error down to fractions of a second and allows the motor to be run at whatever speed it cares to (so long as it's not too slow).

At first sight, it would seem that generating 60 triggers per rev. from the drive Nut is difficult. However it is highly unlikely that we will drive the Nut direct from the motor - rather it will have to be driven via some sort of reduction gearing.

So if the Drive Nut 'gear wheel' has exactly 60 teeth (e.g. Meccano part no. 27d :-) ) it can be driven via a Worm gear (Meccano part no. 32) that will need to be rotated at exactly 60 rpm (i.e. once per second) - and then a simple 'once per turn' operated microswitch fitted to the end of the Worm gear will generate the required 60 per rev. (1 per second) triggers.

What about the Clock 1 second triggers ? Well, unfortunately, simple battery operated quartz clock mechanisms do not generate 1 second electrical 'pulses' - rather they have a micro-motor that 'winds up' the second hand gear against a plastic 'spring' type cog, that after one second, 'snaps' over to cause the hand to 'jump' to the next position (whilst at the same time making an audible 'tick').

So the best approach is to remove the second hand itself and fit a thin plastic or card 'disk' in its place. With 60 holes punched evenly around the edge of the disk, an optical sensor system can then be used to generate the 1 second triggers.

Since high speed Relays (which would be needed to switch at sub-1 second intervals) are both expensive and difficult to obtain, a better approach is to build an Analogue or Digital based electronic circuit.

Designing the speed control circuit

The basic operation is essentially the same as the 'hurry up & wait' circuit, i.e. each clock trigger means 'go' (faster) & each worm gear trigger means 'stop' (slower).

So if we use the worm gear trigger as a 'slow down' control and the clock trigger as a 'speed up' control, then, eventually, the Motor speed will reach a point where the 'slow down' and 'speed up' triggers are in exact balance (i.e. they cancel each other out).

This can be done by Analogue means (a simple 'Op-Amp' integration circuit) or by Digital means (simple 'up/down' counter & R2R "D-A" converter).

If something causes the Motor speed to change e.g.. battery voltage, ambient temperature, friction / resistance, belt slip (or something affects the final PWM Op-Amp circuit) then the triggers will become 'unbalanced' again and the system will self adjust !

In theory it would be possible to design an analogue circuit using op-amps to 'accumulate' the 'up' and 'down' triggers and set the motor voltage after comparing the two. However, with any analogue circuit where some sort of comparison is required, it is very difficult to prevent 'drift' and 'bias' creeping in and upsetting the balance (or 'null') point.

The motor drive circuit must such that the 'correct' speed is obtained at a count of 7 or 8. This allows the maximium 'lag' / 'lead' coount of +/-7 to be supported before the counter 'bottoms out' (or overflows).

Note that the count need only vary the motor speed by a few percent about it's 'correct' speed (i.e. it is not necessary for 0 to correspond to 'stop' or 15 to correspond to 'max speed' .. indeed the count steps need not correspond to equal changes in motor speed = the step from 6 to 7 or 9 to 8 (i.e. near the 'corect' speed) should only cause small changes, whilst the step from 1 to 0 (or 14 to 15) should cause a large change).

Using a belt drive (instead of worm/gear)

If a belt drive is used, in order to avoid problems with belt slippage etc. the 60 per revolution 'trigger' must be taken from the actual drive nut turning (and not from the motor). This can be achieved by fitting a 60 'index mark' grating to the nut drive wheel.

To create the grating, see below (in fact, the number of index marks is not vital, just so long as the clock 'count per rev' == the drive nut 'count per rev'). Note especially re: use of transmission sensors with printed transparent plastic card. Note also that the size of the wheel may have to be limited to less than A4 width (220mm) = printer limit.

Alternative gearing Drive

Meccano gears with exactly 60 teeth are hard to find - and belt drives could slip. They are also difficult to fit  = remember a 14mm hole is required for the drive rod to pass through and the remaining gear then has to be firmly (and accurately) fixed to the Drive Nut.

Plainly a larger gear wheel will be easier to fit. However it is still required to rotate the Nut at 1 rpm i.e. at the same speed as the Clock second hand per minute (1 rpm).

Thus, for example, if a 95 (or 133) teeth Meccano gear is used to turn the Drive Nut, and a worm gear applied to that. is still used to generate 1 'trigger' per tooth, then the Clock 'seconds arm' must be replaced with a disc with 95 or 133 holes in it - i.e. an exact copy of the 95 (or 133) teeth gear wheel.

If a worm gear is used to drive 95 (or 133) then it is still easy to take a 'one per rev' trigger off the worm ..  and since we now have 95/133 'off triggers' this has to be matched by 95/133 'on triggers'. i.e. the clock must be fitted with a 95/133 hole 'grating' used to generate the 95/133 'on' triggers.

Generating an Index Grating

Note that a worm driving 95/133 gear will generate triggers at sub-1 second intervals. At these speeds, using micro-switches or magnetic reed switches is asking for problems and it is recommended that the worm gear sensor be replaced by a transmission optical sensor (with single hole index wheel - i.e. such that the sensor is 'mostly off').

With no concerns that the motor will run too slow, the worry now is that inertia in the system will cause it to run too fast !

So, what about Stepper Motors ? Well that's (almost) my final topic ....