http://homepages.which.net/~paul.hills/SpeedControl/SpeedContro

Speed Controllers

V3.03 5-Oct-2005

1. Introduction

The purpose of a motor speed controller is to take a signal representing the demanded speed, and to
drive a motor at that speed. The controller may or may not actually measure the speed of the motor.
If it does, it is called a Feedback Speed Controller or Closed Loop Speed Controller, if not it is called
an Open Loop Speed Controller. Feedback speed control is better, but more complicated, and may
not be required for a simple robot design.

Motors come in a variety of forms, and the speed controller's motor drive output will be different
dependent on these forms. The speed controller presented here is designed to drive a simple cheap
starter motor from a car, which can be purchased from any scrap yard. These motors are generally
series wound, which means to reverse them, they must be altered slightly, (see the section on

motors

Below is a simple block diagram of the speed controller. We'll go through the important parts block
by block in detail.

2. Theory of DC motor speed control

The speed of a DC motor is directly proportional to the supply voltage, so if we reduce the supply
voltage from 12 Volts to 6 Volts, the motor will run at half the speed. How can this be achieved
when the battery is fixed at 12 Volts?

The speed controller works by varying the average voltage sent to the motor. It could do this by
simply adjusting the voltage sent to the motor, but this is quite inefficient to do. A better way is to
switch the motor's supply on and off very quickly. If the switching is fast enough, the motor doesn't
notice it, it only notices the average effect.

When you watch a film in the cinema, or the television, what you are actually seeing is a series of
fixed pictures, which change rapidly enough that your eyes just see the average effect - movement.
Your brain fills in the gaps to give an average effect.

Page 1 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

Now imagine a light bulb with a switch. When you close the switch, the bulb goes on and is at full
brightness, say 100 Watts. When you open the switch it goes off (0 Watts). Now if you close the
switch for a fraction of a second, then open it for the same amount of time, the filament won't have
time to cool down and heat up, and you will just get an average glow of 50 Watts. This is how lamp
dimmers work, and the same principle is used by speed controllers to drive a motor. When the switch
is closed, the motor sees 12 Volts, and when it is open it sees 0 Volts. If the switch is open for the
same amount of time as it is closed, the motor will see an average of 6 Volts, and will run more
slowly accordingly.

As the amount of time that the voltage is

increases compared with the amount of time that it is

off

the average speed of the motor increases.

This

on-off

switching is performed by power MOSFETs. A MOSFET (Metal-Oxide-Semiconductor

Field Effect Transistor) is a device that can turn very large currents on and off under the control of a
low signal level voltage. For more detailed information, see the dedicated chapter on

MOSFETs

)

The time that it takes a motor to speed up and slow down under switching conditions is dependant on
the inertia of the rotor (basically how heavy it is), and how much friction and load torque there is.
The graph below shows the speed of a motor that is being turned on and off fairly slowly:

You can see that the average speed is around 150, although it varies quite a bit. If the supply voltage
is switched fast enough, it won’t have time to change speed much, and the speed will be quite steady.
This is the principle of switch mode speed control. Thus the speed is set by PWM – Pulse Width
Modulation.

2.1. Inductors

Before we go on to discuss the circuits, we must first learn something about the action of inductive
loads, and inductors. Inductors do not allow the current flowing through them to change instantly (in
the same way capacitors do not allow the voltage across them to change instantly). The voltage
dropped across an inductor carrying a current

is given by the equation

where

di/dt

is the rate of change of the current. If the current is suddenly changed by opening a

switch, or turning a transistor off, the inductor will generate a very high voltage across it. For
example, turning off 100 Amps in 1 microsecond through a 100 microHenry inductor generates

Page 2 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

10kV!

2.2. PWM frequency

The frequency of the resulting PWM signal is dependant on the frequency of the ramp waveform.
What frequency do we want? This is not a simple question. Some pros and cons are:

Frequencies between 20Hz and 18kHz may produce audible screaming from the speed
controller and motors - this may be an added attraction for your robot!

RF interference emitted by the circuit will be worse the higher the switching frequency is.

Each switching on and off of the speed controller MOSFETs results in a little power loss.
Therefore the greater the time spent switching compared with the static on and off times, the
greater will be the resulting 'switching loss' in the MOSFETs.

The higher the switching frequency, the more stable is the current waveform in the motors.
This waveform will be a spiky switching waveform at low frequencies, but at high frequencies
the inductance of the motor will smooth this out to an average DC current level proportional to
the PWM demand. This spikyness will cause greater power loss in the resistances of the wires,
MOSFETs, and motor windings than a steady DC current waveform.

This third point can be seen from the following two graphs. One shows the worst case on-off current
waveform, the other the best case steady DC current waveform:

Both waveforms have the same average current. However, when we work out the power dissipation
in the stray resistances in our motor and speed controller, for the DC case:

and for the switching case, the average power is

So in the switching waveform, twice as much power is lost in the stray resistances. In practice the
current waveform will not be square wave like this, but it always remains true that there will be more
power loss in a non-DC waveform.

Page 3 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

Choosing a frequency based on motor characteristics

One way to choose a suitable frequency is to say, for example, that we want the current waveform to
be stable to within ‘p’ percent. Then we can work out mathematically the minimum frequency to
attain this goal. This section is a bit mathematical so you may wish to miss it out and just use the
final equation.

The following shows the equivalent circuit of the motor, and the current waveform as the PWM
signal switches on and off. This shows the worst case, at 50:50 PWM ratio, and the current rise is
shown for a stationary or stalled motor, which is also worst case.

T is the switching period, which is the reciprocal of the switching frequency. Just taking the falling
edge of the current waveform, this is given by the equation

is the time constant of the circuit, which is L / R.

So the current at time t = T/2 (i

) must be no less than P% lower than at t = 0 (i

). This means there

is a limiting condition:

So:

Page 4 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

Let’s try some values in this to see what frequencies we get. A Ford Fiesta starter motor has the
following approximate parameters:

R = 0.04

Ω

L = 70µH

We must also include in the resistance the on-resistance of the MOSFETs being used, say 2 x 10m

Ω

giving a total resistance of R = 0.06

Ω

A graph can be drawn for this particular motor:

Percentage

frequency

42 kHz

8.2 kHz

4 kHz

1900 Hz

610 Hz

Page 5 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

It may help to introduce some terminology here. The

Motor Driver Terminology page

defines some

terms.

4. Regeneration

In this circuit, energy can flow only one way, from the battery to the motor. When the speed demand
of the motor drops suddenly, the momentum of the robot will drive the motor forwards, and the
motor will act as a generator. In the circuit above, this power cannot go anywhere. Although this
isn’t a problem, it is desirable that this power be put back into the battery. This is called regenerative
braking and needs some extra components. The following circuit allows regenerative braking:

In this circuit, Q1 and D1 perform the same function as in the previous circuit. Q2 is turned on in
antiphase to Q1. This means that when Q1 is on, Q2 is off, and when Q1 is off, Q2 is on.

In this circuit, when the robot is slowing down, Q1 is off and the motor is acting as a generator. The
current can flow backwards (because the motor is generating) through Q2 which is turned on. When
Q2 turns off, this current is maintained by the inductance, and current will flow up through D2 and
back into the battery. A graph of motor current as the motor is slowing down is shown below:

If you are driving starter motors, or any type of series-field motor, regeneration is harder to make
work. For a motor to work as a generator, it must have a magnetic field, generated by the field coil.
In a series motor this is in series with the armature coil, so to generate a voltage, a suitable current
must be flowing. The current that will flow depends on the load, which during regeneration is the

Page 7 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

battery, so it depends on how much the battery is charged up - how much current the battery will
draw into it. Alternatively, a dummy resistive load can be switched in at the approriate time, but this
is all a little too complicated for a robot!

5. Reversing

To reverse a DC motor, the supply voltage to the armature must be reversed, or the magnetic field
must be reversed. In a series motor, the magnetic field is supplied from the supply voltage, so when
that is reversed, so is the field, therefore the motor would continue in the same direction. We must
switch either the field winding’s supply, or the armature winding’s supply, but not both.

One method is to switch the field coil using relays:

When the relays are in the position shown, current will flow vertically upwards through the field
coil. To reverse the motor the relays are switched over. Then the current will be flowing vertically
downwards through the field coil, and the motor will go in reverse.

However, when the relays open to reverse the direction, the inductance of the motor generates a very
high voltage which will spark across the relay contact, damaging the relay. Relays which can take
very high currents are also quite expensive. Therefore this is not a very good solution. A better
solution is to use what is termed a

full-bridge

circuit around either the field winding, or the armature

winding. We will put it around the armature winding and leave the field winding in series.

6. The full bridge circuit

A full bridge circuit is shown in the diagram below. Each side of the motor can be connected either
to battery positive, or to battery negative. Note that only one MOSFET on each side of the motor
must be turned on at any one time otherwise they will short out the battery and burn out!

Page 8 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

For regeneration, when the motor is going backwards for example, the motor (which is now acting as
a generator) is forcing current right through its armature, through Q2's diode, through the battery
(thereby charging it up) and back through Q3's diode:

6.1. Reducing the heat in the MOSFETs

When the MOSFETs in the diagrams above are on and current is flowing through them in a top-to-
bottom direction, they have a very low resistance and are dissipating hardly any heat at all. However,
when the current is flowing bottom-to-top through the intrinsic diodes, there is a fixed voltage across
them - the voltage drop of a diode, about 0.8 volts. This causes quite a large power dissipation (volts
x amps). A feature of MOSFETs is that they will conduct current from source to drain as well as
drain to source, as long as the Vgs is greater than 10-12 volts. Therefore, if the MOSFETs that are
carrying reversed current through their diodes are turned on, then they will dissipate a lot less heat.
The heat will be dissipated in the wires and the motor itself instead. This extra switching is
performed by the

TD340

full bridge driver.

7. Generating PWM signals

The PWM signals can be generated in a number of ways. It is possible that your radio receiver
already picks up a PWM waveform from the handest transmitter. If there is a microcontroller on the
robot, this may be able to generate the waveform, although if you have more than a couple of
motors, this may be too much of a load on the microcontroller’s resources. Several methods are
described below.

Page 10 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

to Q

just roll around the binary sequence 0000 to 1111 and rollover to 0000 again. These outputs,

which swing from 0v to +5v are fed into a binary weighted summer amplifier, the leftmost LM324
opamp section with the 80k, 40k, 20k and 10k resistors. The output voltage of this amplifier depends
on the counter count value and is shown in the table below as Amp1 output. The opamp following
this just multiplies the voltage by -½ , to make the voltage positive, and bring it back within logic
voltage levels, see the Amp2 output column in the table.

The final, rightmost, opamp compares the voltage with the demand voltage input, which ranges from
0v to 4.6875v, where 0v represents 0% PWM ratio and 4.6875v represents 100% PWM ratio. This
demand voltage may range from –12v to +12v but only the 0 to 4.6875 range will adjust the PWM
ratio.

7.2. PWM generator chips

There are ICs available which convert a DC level into a PWM output. Many of these are designed
for use in switch mode power supplies. Examples that could be used are:

Alternatively, a MOSFET driver which includes a PWM generator can be used. I know of only one
which is not yet released! The SGS Thomson

TD340

7.3. Digital method

The digital method involves incrementing a counter, and comparing the counter value with a pre-

Counter value

Binary value

Amp1 output

(Volts)

Amp2 output

(Volts)

0000

0001

-0.625

0.3125

0010

-1.25

0.625

0011

-1.875

0.9375

0100

-2.5

1.25

0101

-3.125

1.5625

0110

-3.75

1.875

0111

-4.375

2.1875

1000

-5

2.5

1001

-5.625

2.8125

1010

-6.25

3.125

1011

-6.875

3.4375

1100

-7.5

3.75

1101

-8.125

4.0625

1110

-8.75

4.375

1111

-9.375

4.6875

Manufacturer

Normal use

SGS

Thomson

SG1524

,SG1525...

SMPS

Maxim

MAX038

Signal generation

Page 12 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

loaded register value. It is basically a digital version of the analogue method above:

The register must be loaded with the required PWM level by a microcontroller. It may be replaced
by a simple ADC if the level must be controlled by an analogue signal (as it would from a radio
control servo). This method is only really practical if a microcontroller is being used in your robot,
which can preload the register easily.

7.4. Onboard microcontroller

If you have chosen to use an onboard microcontroller, then as part of your selection process, include
whether it has PWM outputs. If it has this can greatly simplify the process of generating signals. The
Hitachi H8S series has up to 16 PWM outputs available, but many other types have two or three.

8. Interfacing to the high power electronics

There are two sides to the electronics: the low-power side, and the high-power side. The low power
electronics includes any onboard microcontroller, the radio control receiver, and PWM generators.
The high-power side includes the MOSFET drivers, the MOSFETs themeselves, and any solenoid or
pump drivers that you may have. Basically anything that is switching large currents.

The low-power electronic devices may be quite sensitive to noise spikes on the power rails, and may
malfunction or even be destroyed. It is a good idea to isolate the low-power electronics from the
high-power electronics using what is known as

opto-isolators

opto-couplers

, two names for the

same thing! For more information about these, there is a chapter on it

here

9. Interfacing to the radio control receiver

Many roboteers will be using commercial radio control sets. The receivers of these generally connect
to servos, which respond to the radio signal (which may also be PWM):

You may be able to tap into the PWM signal which comes out of the radio receiver before it goes
into the servo, and use this to drive the input to the MOSFET driver. However, this gives you no
choice of switching frequency. Alternatively, the potentiometer can generate a voltage to feed into
the PWM generator.

If you are unsure about your servos, or want to modify them, there is an introduction to them on the
Seattle Robotics Society page

here

Page 13 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

A more advanced method if you have a microcontroller on board the robot is to take the PWM signal
from the radio receiver and connect it to a timer input of the micro. The microcontroller should be
able to decode this waveform, and generate a proportional analogue output value (if it has ADCs, or
if an external ADC is fitted). Another even more advanced method is to send serial communications
data through the radio channel. The radio control handset will need to have a microcontroller in. The
microcontroller should read the pots and switches on the handset, and send suitable commands out of
its UART. This connects to the radio transmitter. At the receiver, the demodulated output is sent to
the robot's microcontroller's UART, and the data is decoded. There is a whole chapter on using
embedded microcontrollers in a robot

here

10. Current limiting

Current limiting is absolutely essential. If the motor is stalled, it can take huge currents which would
destroy the MOSFETs very quickly. The form of current limiting presented here is to measure the
current that the motor is taking, and if it is above a preset threshold, turn the MOSFETs in the bridge
off. If you have a microcontroller on board which generates the PWM ratio, it would be an
advantage if the software could detect the over- current status, and reduce the PWM ratio by, say,
10%.

A circuit to perform this function is shown below.

This circuit shows just the upper MOSFETs of the bridge being driven for simplicity. The lower
MOSFETs are not turned off during a current limit. There is only one sense resistor required for each
motor, and that should be connected immediately from the battery positive terminal.

Circuit description

Page 14 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

The voltage dropped across the sense resistor is amplified by U1A, which is connected in a

differential amplifier

circuit. The gain of this is 480k / 1k which is 480. This is a very large gain

because the voltage dropped across the sense resistor will be very small. The output of the
differential amplifier is then heavily low pass filtered by RxCx. This is because there will be a lot of
noise coming from the motor, and we do not want to limit the current if we don't need to. D13 is
present to make sure that no negative spikes can affect the following circuitry. U2B compares the
filtered signal with a preset value (represented here by V5), and if the current is too high (i.e. the
signal is greater than V5), U2B will turn Q1 and Q2 on which clamps the PWM signals from the
PWM generator. This will force the MOSFET driver to turn the MOSFET off. Q1 must be repeated
four times, one for each of the MOSFET driver channels, but all four transistors can be driven from
U2B. D11, R14 and C4 make sure that the MOSFET doesn't turn back on straight away, but takes a
few milliseconds. This stops the MOSFET being rapidly turned on and off.

10.1. The shunt resistor

The shunt resistor R7 in the cicuit must be a very low value if we want large currents to be able to
flow, up to 100 Amps for example. It must not drop too much voltage, thereby robbing power from
the motor, and it must be capable of dissipating the power without buring out when large currents are
passed through. Some suitable resistors are available from Farnell, code 156-267. These are still too
large a resistance (and too low power), so we can place eight in parallel. The power handling
capabilty is then increased eightfold, and the resistance decreased eightfold.

An alternative is to use a piece of wire of an appropriate thickness and length. This can be calculated
using the data on

this web site

A simulation of the current limiting part of this circuit is shown in the diagram below. The V5
threshold voltage was chosen to set a current limit of 30 Amps. The square wave is the PWM voltage
(MOSFET gate voltage), and the slopey waveform is the drain (motor) current. The spikey bits at the
top of the slopey waveform is when the current limiting is switching in and out.

There is an in-depth document

here

which describes sense resistors in detail.

Some circuits you may see sample the current going through the main power MOSFET by placing a
much lower power MOSFET in parallel with it. There is a circuit on the 4QD site which does this

here

. This works OK, but the problem is the actual limiting current is dependant on the value of Rds

Page 15 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

(on) of the MOSFET. If Rds(on) was only half the value we were expecting it to be, then twice as
much current would flow before the limiting circuit took effect. Also the Rds(on) value depends very
much on the current that is passing through the MOSFET, and on the temperature. Any variation in
Rds(on) will change the limiting current.

The Rds(on) figure is quoted as a maximum value on the datasheet, but it is not a design-safe
parameter. This means that it is not within defined limits which are published on the datasheet. For
example, CMOS digital logic guarantees that the output voltage, Vo, will be between Vcc-0.5v and
Vcc, and that figure can be used to design circuits which rely on that figure. However, with Rds(on),
we only know that it will be between 0 and the quoted value. We cannot rely on a minimum value of
it, yet it is the minimum value which controls the current limit. Therefore, using a separate shunt
resistor is a much safer method.

One problem with the circuit presented above is that you may want to provide a larger current during
acceleration, or in emergencies. This can be solved by disabling the current limiting using a separate
line from any onboard microcontroller, or by adding a circuit which allows an over-current condition
for just a short time. The amount of time that this is allowed must be carefully calculated to prevent
damage to the MOSFETs, and must take into account the cooling system that you have provided.

An alternative to using the op-amp differential amplifier circuit used above is to use an integrated
current sense monitor IC. Several companies make these, I have used the Zetex ZXCT1010. Zetex's
range of current monitors can be found

here

10.2. Current limited torque speed characteristics

If a DC motor is being driven by a speed controller with current limiting active, what happens to the
torque speed characteristic graph?

The

DC Motors

page describes the normal motor torque speed graph, and how the torque of a

permanent magnet DC motor is proportional to the current. If the current is limited however, the
torque must also be limited, at the value coincident with the limited current on the torque-current
graph. The effect that this has on the torque speed graph is shown below:

As the load torque increases, the speed drops - we are following the line in the torque speed
characteristic from the left hand side towards the right, drooping down. This is the same as the
uncontrolled motor. The motor torque always equals the load torque when the motor is running at
constant speed (this follows from Newton's first law - "

An object in motion tends to stay in motion

Page 16 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

with the same speed and in the same direction unless acted upon by an unbalanced force.

" The

motor torque and load torque must be balanced out if the speed is not changing).

Let's call the current limit value

and the equivalent torque value on the torque-current graph at this

current is

. When the load torque exceeds

, the motor can no longer create an equal and opposite

torque, and so the load will push the motor backwards in the opposite direction - we are now
following the line as it drops downwards into negative speed.

Let's take an example; an opponent's robot is more powerful than ours (or his current limit is set
higher), and we are in a pushing match. As each pushes harder, our speed controller reaches its
current limit first. Our robot is now pushing at a constant force (since the motor torque is now
constant at its highest value). As the opponent pushes harder, our wheels start to rotate backwards,
and the pair of robots accelerates backwards at a rate given by Newton's second law:

F=ma

a=F/m

where

is the difference between the forces of the two robots pushing, and

is the total mass of the

two robots.

11. Feedback Speed Control

To stop a robot swerving in an arc when you want it to go forwards, you need to have feedback
control of the motor speeds. This means that the actual speed of each wheel is measured, and
compared with all the other wheels. Obviously to go in a straight line, the motor speeds must be
equal. However, this does not necessarily mean that the speed demand for each motor should be the
same. The motors will have different amounts of friction, and so a ‘stiffer’ motor will require a
higher speed demand to go as fast as a more free-running motor.

A block diagram of an analogue feedback speed controller is shown below

The speed demand is a DC voltage, which is fed to the PWM generator for motor A. This drives
motor A at a speed dependant on the demand voltage. The speed of motor A is sampled using an
optical encoder. This has a frequency output, which is proportional to the speed of the motor.

If we assume that motor B is already running at some speed, then the optical encoder on its shaft will
be producing a frequency also. The phase comparator compares the two frequencies, effectively
comparing the speeds of the two motors. Its output is a signal which gets larger as the two input
frequencies get further apart. If the two frequecies are the same, it has a zero output.

Page 17 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

The integrator adds the output of the phase comparator to whatever its output was before. For
example, if the integrator output was previously 3 volts, and its input is 0 volts, then its output will
be 3 volts. If its input changed to –1 volts, then its output would change to 2 volts.

Let’s assume that motor B is running slower than motor A. Then the output of the phase comparator
will be positive, and the output of the integrator will start to rise. The speed of motor B will then
increase. If it increased to a speed greater than that of motor A, then the output of the phase
comparator would become negative, and the output of the integrator would start to fall, thereby
reducing the speed of motor B. In this manner, the speed of motor B is kept the same as the speed of
motor A, and the robot will go in a straight line (as long as its wheels are the same size!).

This method can be expanded to use any number of wheels. One motor will always be the directly
driven one (in this case motor A), and the others will have their speed locked to this one. Note that if
the directly driven motor is faster, or more free-running, than the others, then when it is driven at its
fastest speed (the PWM signal is always ON), then the other motors will never be able to keep up,
and the robot will still swerve. It is best, therefore, to directly drive the slowest motor.

An analogue feedback speed controller such as this is quite difficult to make, and keep stable. It is
easier to perform this function using software in an onboard microcontroller....

11.1. Software feedback speed control

To perform the same function as described above in software requires that the software has digital
representation of the speed of each wheel, and can finely control the width of the PWM signal sent to
each wheel. To get the speed of each wheel, an optical encoder must be used as in the analogue
method, but the output of it must be sent to the microcontroller. This is achieved using a counter,
clocked by the speed controller, which the microcontroller can read, and can clear. At regular
intervals, the microcontroller must read the counter, then clear it. The interval depends on the
maximum speed of the robot, the diameter of the wheel, the number of slots in the speed encoder’s
disc, and the number of bits of the counter.

A complete design using this technique is being worked on and will be presented here when it is
complete.

11.2. Speed encoders

To start with, we need a device that will measure the speed of the motor shaft. The best way to do
this is to fit an optical encoder. This shines a beam of light from a transmitter across a small space
and detects it with a receiver the other end. If a disc is placed in the space, which has slots cut into it,
then the signal will only be picked up when a slot is between the transmitter and receiver. An
example of a disc is shown below

Page 18 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

circuitry. However, because this device is right next to a DC motor, which generates a lot of
electrical noise (since it is switching high currents at the commutator into the inductive windings –
hence high voltages and sparks!), the output must be low-pass filtered. The phase comparator should
have as little noise as possible at its inputs.

The cutoff frequency for the filter is determined by how many slots are in the disc, and by how fast
the disc (and hence the wheel) is intended to rotate. It is given by the equation

where

is maximum speed of the wheel in rpm, and

is the number of slots in the disc. The filter

can be made from a simple RC circuit as follows:

Then the R and C values chosen such that

For example, if the maximum wheel speed is 10rpm, and there are 12 slots in the disc, then RC =
0.08, so suitable values of R and C might be 8k2 and 10uF.

An alternative way to measure the speed is using a magnetic sensing device. If your motor or gearing
has steel teeth, then sensors are available that can detect and count these as they go by. The

Infineon

TLE4942

is such a device. I will not go into this anymore since the datasheet at that link describes

how to use the device.

Allegro also make many magnetic devices for speed measurement. The Allegro Microsystems
website is

here

12. Links to other relevant pages

Basic guides to what speed controllers do

4QD manufacture speed controllers, and publish this basic technical guide:

http://www.4qd.co.uk/faq/index.html

SGS Thomson produced a good document about current limiting in a full bridge circuit.

http://www.st.com/stonline/books/pdf/docs/1668.pdf

DC motor driving including methods of speed regulation.

http://www.st.com/stonline/books/pdf/docs/1656.pdf

Driving DC motors

http://www.st.com/stonline/books/pdf/docs/1704.pdf

Information about MOSFETs

Page 21 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html

International Rectifier have several application notes on MOSFETs and how to use them:

Good ones are "

Current Ratings of Power Semiconductors

", "

Paralleling HEXFET® power

MOSFETs

", "

Gate Drive Characteristics and Requirements for HEXFET® power MOSFETs

", and

The Do's and Don'ts of Using MOS-Gated Transistors

", all of which are included at the following

site:

http://www.irf.com/technical-info/appnotes.htm

Practical speed controller projects:

The Open Source Motor Controller (OSMC) project.
A project to make a high current speed controller using 4 paralleled MOSFETs in each bridge arm,
and an HIP4081A driver/controller:

http://groups.yahoo.com/group/osmc/

A converter for the above project to convert an analogue in signal into direction and PWM signals:

Back to main index

Page 22 of 22

Speed Controllers

7/27/2008

http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html