|
DiscoverCircuits.com
-- Hobby Corner
"Hobby Circuits for
your Hobby Projects"
Last Updated on:
08/29/2008 02:09:27 PM
|
Medium Power 12v Brush
Motor Speed Controller
July 13,
2008
designed by David A. Johnson, P.E. |
|
|
|
In many
applications, you would like to hold the speed of a motor constant, even
as variations in the power supply voltage or mechanical load try to
change its speed. In other applications, the average current to the
motor needs to be limited, so the initial in-rush current when starting
the motor is not too high. Also in some applications you would like to
allow the motor to be in locked in a stall condition, without doing harm
to the motor or the drive circuit. These two features can often be
combined in a single control circuit. |
|
There
are many ways you can monitor the speed of a DC motor. Some techniques
connect some type of rotary sensor directly the motor shaft. This
method can often lead to a very accurate speed measurement, since the
sensor may produce multiple pulses per shaft rotation. There are
countless optical and magnetic sensors available for this purpose. When
possible, I like dispense with any external sensor and to tap into the
signals produced by the motor itself, to gage the motor speed. Motors
with a three phase brush commutator (three brush rings) will usually
produce a very clean back emf signal. This is the voltage produced by
the motor, if it was coasting at a specific speed, without any applied
drive. Knowing the relationship between the back emf voltage and the
shaft speed, you can design a circuit to maintain any constant motor
speed by monitoring the back emf voltage. But, few motors use a three
phase commutator. Most DC brush motors you will encounter will be single
phase type. Often these DC motors will produce clean current pulses,
each time the brush commutator changes the polarity of the DC applied to
the motor windings. Using a small resistance in series with the motor
drive circuit, you can extract these pulses to measure the motor speed.
|
|
For many
inexpensive motors, which are often used in toys and some consumer
appliances, neither the back emf method nor the brush current pulse
methods will work. The signals are just too noisy to be practical.
With these motors, only an external shaft sensor will work. You will
have to conduct some experiments on the particular motor you want to
control, to determine the best speed monitoring method. |
|
The
circuit below is a speed control for a medium power 12v DC operated
single phase brush motor. It uses the brush current pulse method to
monitor the motor speed. It includes a method to limit the average
motor current. I have also included a means to turn on and off the
motor from a simple TTL logic signal. |
|
The
circuit uses some rather old op amp ICs I had laying around. Other op
amps and voltage comparators could also be used. The circuit uses a
pulse width modulation (PWM) scheme to control both the motor speed and
the maximum average current. The current control is handy when you want
to limit the motor torque, since motor current and torque are directly
related. |
|
The
circuit uses the current pulses from the motor brushes to extract the
motor speed. For the motor I used, there were exactly 12 pulses per
shaft rotation. The typical average motor current was about 1.2 amps
with a speed of about 1000RPM. At 1000 RPM, the pulse frequency works
out to be 200Hz. |
|
A 10KHz ramp
generating oscillator is used as the timing generator for the PWM
circuit. An op amp compares the ramp voltage with the control voltage
from either the current limiting circuit or the speed control circuit.
When in current limit, the speed control part of the circuit is pushed
out of the way. A multi-pole low pass filter, removes the brush current
pulses from the motor, which appear across a small 0.1 ohm resistor.
The pulses are amplified by a factor of X10 using A4. The signal from
the pulse amplifier is then routed to a comparator circuit, which
generates a square wave type signal. That signal is then connected to a
one-shot circuit, which uses one half of a dual
D-flip/flop IC. The output of the one shot is then filtered, to produce
a voltage, proportional to the pulse frequency. An op amp then compares
the speed signal to an adjustable reference signal. The output of the
speed error amp, then is connected to the PWM circuit, to maintain a
constant motor speed. |
|
In a second circuit, the average DC current is monitored
using the DC voltage across the 0.1 ohm shunt resistor. It too has an
adjustable reference, so the limiting current (torque) can be
controlled. A diode isolates the two control circuits from the PWM
circuit. A second diode routes +5v to the PWM control input, when the
on/off input logic control signal is low. This pushes the PWM output
signal low, keeping the motor turned off. A 5v voltage regulator is
used to provide a stable +5v voltage to the speed and torque reference
circuits. |
|
Click on Schematic below to view PDF version of this Circuit |
|
 |
