Tacho Set-Up

Tacho generator systems can seem quite complicated: they are closed loop feedback systems where the Tacho generator (perhaps a small permanent magnet motor) is connected to the main motor shaft so that the two turn together. Whenever the main motor is rotating, the small motor acts as a generator whose output is fed back to the controller so that the controller can adjust the voltage fed to the main motor so that the shaft speed (which is being measured) is essentially constant and is independent of any load variations.

To get such a system tuned to peak performance can get quite complicated but peak performance is not required and, providing you understand the basic principles involved, even a crudely setup system will result in dramatic improvements in performance.

The tacho feedback board fits into the expansion connector of the Pro or NCC controller. Normally you should order the appropriate controller with expansion connector and Tacho board. A separate page details retro-fitting the expansion connector to Pro and NCC series controllers.

Choosing a Tacho Generator

As already stated, you will probably end up using a standard permanent magnet motor as a tacho generator. The precision, calibrated performance that you pay for in an expensive true tacho generator is not usually necessary.

As supplied, the Tacho board with a Pro-120 or NCC series controller is correct for a Tachogenerator which gives around 7 volts output at whatever is the full speed of the motor being sensed.

So it your main motor is a Bosch 750w motor being used at 36v, the top speed of the motor will be not 3300 rpm, but 4950 rpm (3300 x 36/24).

Let’s say you have 12v motor which has a nominal speed of 5100 rpm. Turning this backwards, if you rotate this 12v motor at 4950 rpm, you should get 11.65v output. That is more than the required 7v, so will do nicely.

You do need to make sure the ‘generator’ motor is symmetrical and does not have offset brushes, see our FAQ sheet.

Setting up

A system incorporating tacho feedback can behave in an odd manner if you do not know what to expect. So it is best to get the system working without the tacho feedback first.

To get the system working without the tacho, unplug the daughter board, then make sure that pins 5 and 6 of the expansion connector are linked together: they were thus linked (by means of a shorting jumper) when the board was supplied. It is least confusing to do this if you have a standard speed pot connected, rather than any radio control system of interface board. Make sure the motor speed control is smooth from zero to full speed, with full speed occurring at full demand speed input.

Only once the system is working correctly without the tacho should you connect the tachogenerator and fit the tacho board.

Since you will probably be using a PM motor as tachogenerator, it is simplest to fit a preset or standard pot as adjustment. A high value pot can be used as a variable resistor in series with the tacho generator or a lower value as a true pot as shown below. In either case, exact values don’t matter as you are going to adjust it!

Tachopot
A 3 pin input connector is used, mainly for mechanical strength: the centre pin is earth and is not used – unless you decide to fit a screened tacho lead (unnecessary).

With the motors unloaded, turn the motor speed controller up so that the motor just starts to turn.

If the tacho circuit gain is low, then you will loose speed controlability: the motor will quickly go from slow to fast. Either you have the adjustment too low or the generator’s output is too low. If the latter, the tacho board’s sensitivity can be increased. To do this, decrease the values of the two input resistors. These are the two 100K resistors connected to the input connector, pins 1 and 3, and are shown on the circuit diagram.

If the tacho gain is too high, the speed will be unstable and the motor will vibrate. At slow speeds the reversing relays on the controller will audibly chatter. Reduce the pot setting until the chattering stops.

With the gain right, the motor speed control will again be smooth from zero to full speed, full speed will occur at the same control setting as without the tacho. Exact correct gain is not important: the setting is fairly tolerant, but it will vary slightly with battery voltage. Higher voltage batteries will cause more gain, so do the initial adjustment with freshly charged batteries!

Once you are happy with the adjustment, try to stop the machine’s wheels turning while they are rotating slowly. Whereas without the tacho feedback, you could easily stop the motors when they were turning only slowly, now you should find then unstoppable – the feedback is increasing the voltage fed to the motors to maintain the rotation speed, so you have full control current available from the controller even at very low speeds. You may find that loading the motor causes the speed to wobble: if so, the gain is still too high.

Try also suddenly releasing the load: the motor speed should not bounce much. If there is a slight bounce it should very quickly settle down.

Conclusion

Fitting the tacho system should have very much improved the system’s performance. The motor voltage now adjusts automatically to compensate for load variations which previously caused speed variations.

The tacho feedback will also compensate for any mechanical variations in the system as, for instance, are caused by asymmetrical motor performance.