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4QD-TEC: Electronics Circuits Reference Archive
Tacho generator motor
speed feedback

This page is a bit different from most in that it covers the use of tacho
generators as feedback elements in motor control systems. It also covers a
'Tacho Feedback Board' extra for our Pro-120 and other controllers. It also
includes the circuit diagram. It thus should be of interest not only to
potential customers but also to electronic engineers. If you have come here
straight from 4QD's home page, you might care to read the section of our
'Battery Motors and Controllers' FAQ sheet containing a discussion of Closed Loop Controls.

Most battery motor speed controllers are under manual control and the
operator automatically adjusts speed to match demand. Under these conditions
closed loop motor speed control is an unnecessary expense. However, when it is
required, a tachogenerator can easily be added.

True tachogenerators are usually expensive - which puts off most potential
users. The expense is because they are very accurately manufactured and
calibrated. They are also low volume items. However for most uses a small
permanent magnet motor is perfectly adequate. It won't come with an accurate
'volts per rpm' calibration and it may (just possibly) drift over several years.
For most purposes this simply does not matter!

With any tacho feedback system, the loop gain (controller, motor, tacho,
tacho amplifier) must not be more than unity or instability results.

To put that more simply: the feedback system must alter the output voltage to
the motor so that the motor voltage is raised to exactly match the drop in the
motor armature caused by the current flowing, so the mechanical speed remains
constant.

If the loop over compensates, then the motor will go too fast if loaded and
will 'hunt' - the speed will be unstable and will bounce about. If the
compensation is too little - perfornance will be less than optimum.

However - there are a lot of variables, including battery voltage and motor
temperature. An ideal system would measure everything and compensate for it (and
be very expensive!). In practise, adjustment is quite easy: you will simply
include a pot in series with the motor used as the tacho generator and will
adjust this until the motor starts to hunt (show speed variation), then back it
of a bit until it is slightly below optimum.

Tacho generator rectifier: the circuit

As well as being our TFB, part of this (the 3 LH sections of the quad op-amp)
is actually the circuit that we use in the 4QD series controllers to provide
armature voltage feedback. It is a precision rectifier circuit with error
amplifier and it can either detect the armature voltage or the voltage from a
tacho generator to provide closed loop control.

The output stage is an integrator to remove the d.c. component of the error
signal.

The board is built in two versions (Pro-120 and VTX or NCC) but the only
difference is the connector - 9 way for Pro (black numbers) and 6 way for VTX.

The circuit uses a quad op-amp type LM324. The first two stages are are
straight forward difference amplifiers that compare the two wires from the tacho
generator. One works for positive voltages and the other for negative.

Now the circuit is used from a single supply. Therefore the op-amp outputs
cannot swing negative. Therefore one amplifier gives a positive output for one
tacho generator direction and the other for the reverse direction. The third
section of the op-amp sums the outputs, the result is a rectified version of the
input.

There is a snag in using an op-amp thus. On the inputs which should swing the
output negative, the op-amp is in fact cut-off and the output is not conducting.
The inputs are outside their working range, so they cannot clamp the control
signal. There is then a straight feed-forward path via the 'feedback' resistor
to the inoperative output.

That mechanism explains the 10K load resistors from the outputs of the first
two stages of the op-amp to earth. These, in conjunction with the 3 100K
resistors in the feed-forward path, form an attenuator so that the error from
feed-forward is reduced to around 3%. It may interest browsers to know that the
circuit was first used with an LM3900 (which is a Norton op-amp) where the
inputs can never rise above one transistor Vbe above 0v. With this, the
feed-forward effect is not noticeable and the clamping resistors are not
required. The LM3900
circuit which you will note, does not show an output integrator.

The fourth stage compares the rectified tacho generator voltage with the
demand speed, integrates the difference and gives an output to drive the PWM
modulator.

I don't intend to go into the mathematics but, if you don't get things
correct a tacho feedback circuit can go dreadfully wrong. Motors are mechanical
things and take time to react. If the control signal reacts faster than the
motor then it will over-react and an oscillation loop will result.

Also, the accuracy of the motor speed control depends (as you might expect)
on the gain inside the feedback loop. But if there is too much gain, then also a
feedback oscillator results.

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© 2001-2010 4QD-TEC
Page's Author: Richard Torrens

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