e011046.PDF

GENERAL INTEREST

Modular speed controller for R/C models

Speed Controller Duet

part 1: overview and SpeedControl

Design by B. Stuurman

The interest in speed controllers for R/C models is still just as great as

ever, and we can conﬁdently state that the Speed Controller Duet sets a

new standard in this area. It is a universal system that

can be used in all branches of R/C model building,

including electrically powered aeroplanes, boats and

cars. An equally important feature is that almost every-

thing is programmable, thanks to an RS232 interface

that allows the controller to be linked to a PC.

Elektor Electronics

1/2001

GENERAL INTEREST

The Speed Controller Duet is a mod-

ular system. Its intelligent heart is

formed by the SpeedControl unit.

This microcontroller-based circuit

does not contain any power output

stages. These are instead built as

separate units that are controlled by

the SpeedControl. This division of

functions is actually quite logical.

After all, processing the received

pulses has little in common with dri-

ving electric motors with currents of

several tens of ampères.

There are two types of power stages

for the Speed Controller Duet. Speed-

Power 1 is equipped with two relays

for reversing the polarity of the out-

put voltage, for driving or sailing in

reverse. This unit is thus a perfect ﬁt

for applications in model ships and

cars. SpeedPower 2 is fitted with a

‘brake FET’. This can be used to col-

lapse the folding propeller of an elec-

tric glider. The principle is simple:

the motor is short-circuited by the

brake FET, which causes it

to quickly

slow

down.

Once the

rpm’s have

dropped to a

low enough

value, the

force of the air

stream against

the propeller

blades causes

them to fold

back. This con-

siderably

reduces the wind

resistance of the

aircraft.

In principle, both

types of power

stage are usable

with supply volt-

ages between 7.2

and 12 V, with a

maximum current of 50 A. However,

the relays in the SpeedPower 1 are

rated at 25 A, so it is a good idea not

to go too far beyond this limit. The

SpeedPower 1 and SpeedPower 2 are

both ﬁtted with a Battery Eliminator

Circuit (BEC) rated at 5 V and 1.5 A,

which allows the supply battery for

the receiver to be eliminated.

A particularly interesting aspect of

this modular approach is that several

power units can be connected to the

Figure 1. A bird’s eye view of the Speed Control Duet. The SpeedControl is in the fore-

ground, with the SpeedPower 1 at the left rear and the SpeedPower 2 at the right rear.

SpeedControl. For instance, each

motor of a multi-motor aeroplane

could be fitted with its own power

unit in the motor nacelle. The avail-

able power can thus be doubled in

this manner.

The power units are fitted with fast digital

optocouplers, which allows the inputs and

outputs of all power units of the same type to

be connected in parallel, and the distance to

the SpeedControl can in theory amount to

several metres.

SpeedControl specifications

Maximum voltage:

6.5 V

Minimum voltage:

3.75 V

Current consumption:

10 mA (1 LED on, U=4.5 V)

Input:

polarity:

positive

minimum pulse duration:

0.5 ms

maximum pulse duration:

2.7 ms

minimum repetition interval:

16.6 ms (60 Hz)

maximum repetition interval:

28.6 ms (35 Hz)

Outputs:

1: PWM:

positive duty cycle 0-100%

2: for/back:

1 = reverse

Transmitter adaptation:

intelligent three-point timing

User programming:

using a PC, via RS232

Protocol:

19200, 7, n, 2

Options:

practically everything is programmable

BEC:

yes, via power unit; 5 V / 1.5 A

Cut-off:

yes, adjustable

Digital watchdog:

yes

Gliding motor control:

yes, approx. 0.7 s for 100%

Stand-alone option:

yes, using trimpot

1/2001

Elektor Electronics

GENERAL INTEREST

a little while, since many measure-

ments are taken and then averaged,

so be sure to hold the control stick

steady. After the beep, the control

stick can be moved to the ‘neutral’

position. Pressing the pushbutton

again causes the timing of the ‘neu-

tral’ position to be measured (the

‘med[ium]’ LED is illuminated).

Finally, the full throttle timing can be

measured.

The three timing measurements

must always be made in the

sequence revere, neutral and full

throttle.

If the control stick is at the bottom

when the first measurement is

made, that is the ‘reverse’ position.

On the other hand, if it is at the top

for the first measurement, then

‘reverse’ is at the top.

If the control stick is not moved

between making the reverse and

neutral timing measurements, there

is no reverse (a small amount of hys-

teresis is programmed in for the tran-

sition to reverse).

On completion of the three-point tim-

ing measurements, the controller

resets itself and enters the ‘normal

operation’ mode. This mode begins –

following an initialisation – with

checking the relationship among the

three stick positions. If this is invalid,

the SpeedControl cannot go any fur-

ther, and it lets this be known by

loud beeping.

to the top and measure the ‘full

throttle’ timing. All done!

100%

Example 3

Stick in the middle is stop, stick up

is full throttle and stick down is

reverse (or to switch on the brake

FET).

First set the trim down, since it will

not be used.

Select ‘timing mode’. Move the

stick to the bottom and measure

the ‘reverse’ timing. Then move the

stick to the middle and measure

the ‘neutral’ timing. Finally, move

the stick to the top and measure

the ‘full throttle’ timing. All done!

25%

reversing

20%

neutral

100%

full throttle

Stick position

000070 - 12

Figure 2. Three-point timing illustrated. The

maximum forward and reverse power levels

depend on the choice of the neutral position.

The control characteristics are mirrored about a

vertical line through the neutral point.

Note:

Maximum power is always obtained

in the greater part of the range of

stick motion (see Figure 2 ).

– If the relationship between reverse,

neutral and full throttle is invalid

(which will be the case when the

SpeedControl is switched on), all

three timing LEDs (‘min’, ‘med’ and

‘max’) will be illuminated and the

unit will beep furiously.

– If there are no pulses when the

SpeedControl is switched on, the

‘max’ LED will be illuminated and

there will be a fast beeping signal.

– If the control stick is not in the

‘neutral’ position when the Speed-

Control is switched on, the ‘med’

LED will be illuminated and there

will be a slow beeping signal.

Three-point timing

‘Three-point timing’ forms an essential part

of the SpeedControl concept. This has to do

with how the SpeedControl is linked to the

transmitter, and in particular, to three speciﬁc

positions of the control stick.

Three-point timing is based on the following

three positions of the control stick: reverse,

neutral and full throttle. Neutral must lie

between reverse and full throttle, but it may

also be the same as reverse.

In neutral, the motor is always stopped. At

full throttle, the motor is supplied with maxi-

mum power. If neutral is located beyond

reverse, then when the stick is moved

towards reverse, the ‘for/back’ output is ﬁrst

activated, and after that the power is again

increased from 0%.

In the SpeedPower 1 unit, the ‘for/back’ sig-

nal is used to actuate the relay, while in the

SpeedPower 2 unit it is used to control the

brake FET and block power to the motor.

We can illustrate the versatility of this con-

cept using a few examples. The SpeedControl

unit is assumed to be connected to the

receiver, with the transmitter switched on.

The timing of reverse, neutral and full throt-

tle takes place as follows. If the pushbutton

of the SpeedControl is held down while

power is switched on, the microcontroller will

enter the ‘timing’ mode (under the condition

that the trimmer potentiometer is set to

‘time’), and the three LEDs ‘max’. ‘med’ and

‘min’ will all be illuminated. The control stick

can now be set to the ‘reverse’ position. If the

pushbutton is pressed, the timing of the

‘reverse’ position is measured (as shown by

the ‘min’ LED being illuminated). This takes

Example 1

Stick down is stop, stick up is full

throttle.

First set the trim down, since it will

not be used.

Select ‘timing mode’. Move the

stick to the bottom and measure

the ‘reverse’ timing. Leave the stick

at the bottom and measure the

‘neutral’ timing. Now move the

stick to the top and measure the

‘full throttle’ timing. All done!

During the three-point timing

process, the pause intervals are

measured in addition to the posi-

tions of the control stick. The pulse

repetition intervals are computed

from the measured times, and the

minimum and maximum repetition

intervals (which will normally be the

same) are extracted from the mea-

sured times using a bubble sort.

Small margins are applied to these

times, which are subsequently used

as limit values for judging the valid-

ity of the received signal. If the rep-

etition interval (or rate) deviates

from the measured and calculated

value by more than

Example 2

Stick down is stop, stick up is full

throttle. The trim is used to select

forward/reverse, or to switch on the

brake FET.

Select ‘timing mode’. Set the trim

down and move the stick to the

bottom, and then measure the

‘reverse’ timing. Then set the trim

to the top and measure the ‘neu-

tral’ timing. Finally, move the stick

3 %, the pulse is

not allowed to pass, or during oper-

ation it is regarded as erroneous

(this check can be disabled if

desired). It is thus necessary to carry

out the three-point timing process for

each individual R/C installation.

Elektor Electronics

1/2001

GENERAL INTEREST

4...6V

R12

R11

R10

BAT85

D 5

D 4

D 3

D 2

100n

100µ

16V

BC557

BZ1

NMI

PB0

PB1

PB2

PB3

IC1

PA0

R14

560 Ω

PA1

PA2

BC557

PB6

PB7

R15

560 Ω

PA3

ST6260

F/B

10k

BC557

RESET

PC2

PC3

PC4

PWM

10k

R13

10k

TEST

RxD

OSC1

OSC2

TxD

BC547

220p

2V3

BAT85

4MHz

18p

RESET

000070 - 11

Figure 3. Schematic diagram of the SpeedControl unit.

The circuit

The schematic diagram of the

SpeedControl is shown in Figure 3 .

The heart of the circuit is the

ST62T60BB6, which is a powerful

microcontroller with a large number

of on-board peripheral circuits, such

as a standard timer, a PWM timer, a

digital watchdog, an A/D converter

and a serial interface (SPI). There are

128 bytes of RAM present, as well as

the same amount of EEPROM, along

with 3384 bytes of program memory

(user ROM/EEPROM). Although the

maximum clock frequency is 8 MHz,

we have here intentionally chosen

4 MHz, since the controller still

works at this clock rate with a sup-

ply voltage of 3.5 V (instead of 4.5 V

at 8 MHz).

The reset circuit, which consists of

D1, T2 and R5–R8, deserves special

attention. D1 is a 2.3-V voltage refer-

ence. The base of T2 is driven via the

voltage divider R5/R6. The threshold

voltage of T2 is around 0.6 V, so

when the supply voltage reaches

4.2 V, T2 starts to conduct and the

reset signal is disabled. This voltage

lies well above the minimum operat-

ing voltage of the microcontroller, so

we can be sure that it will never end

up in an unstable region. The refer-

ence voltage is also connected to

PA0. This port is configured as an

input, and it can be switched over

by the software to act as an ana-

logue input. This allows the value of

the supply voltage to be measured,

in the interest of the cut-off function.

The RS232 interface is truly

extremely simple: just two diodes

(D6 and D7) and one resistor (R13).

The input signal enters via port PC2,

and the output signal exits via port

PC3. PC2 is configured as the input

to the serial interface (SPI) of the

microcontroller, which handles

incoming data on its own, without

any intervention by the ‘core’ hard-

ware. After reception is complete,

the SPI interrupt sets a ﬂag, and the

core can fetch the data. The SPI is

not used for transmitting RS232

data. A software routine for this pur-

pose is present, and it drives PC3.

As it happens, the durations of the

incoming servo pulses are measured

using their rising and falling edges,

each of which generates an inter-

rupt. The NMI and the PA ports can

detect falling edges, while the PC

ports can detect rising edges. It

would thus be really very nice if one

edge could be captured with the

NMI and the other edge with one of

the PC ports. Unfortunately, this can-

not be done, since the SPI occupies

PC4 (as a clock line), so we have no

choice but to use one of the PA ports.

This means that the incoming signal

must be inverted twice, first by T1

for buffering and level conversion

(since the amplitude of the servo

pulse is only 2.5 V with some receivers), and

then again by T5 in order to generate an edge

interrupt via PA2. The rising edge of the

incoming pulse thus generates an NMI inter-

rupt (which takes precedence over all other

interrupts), while the falling edge generates

a PA2 interrupt. The durations of the pulses

and the pauses between pulses are measured

by the built-in timer, which works pretty

much on its own, under control of the inter-

rupt routines.

The other inputs that are present are the trim-

pot P1, whose voltage can be read by the

software, and pushbutton S1. If S1 is held

down while the power is switched on, then

one of three options is chosen, depending on

the previously selected setting of the trimpot.

These are ‘time’, ‘program’ and ‘speed’.

‘Time’ puts us in the three-point timing mode,

which means that after you have released the

pushbutton you will have to press it three

more times in succession (and wait for the

beep) for the reverse, neutral and full throttle

positions. In the ‘program’ mode, the con-

troller can be programmed by the user

entirely according to his or her own prefer-

ences, using a PC. In the ‘speed’ mode, no

servo signal is necessary; the trimpot takes

over the control function, as though it were a

control stick.

All port B lines are configured as outputs.

Each of them can sink or source up to 20 mA.

The buzzer, the LEDs and the outputs to the

power units (‘PWM’ and ‘for/back’) are con-

nected to these lines (the latter two via open-

collector drivers).

In addition to providing status information in

1/2001

Elektor Electronics

GENERAL INTEREST

Pulse durations and cables

There is little in the way of standardisation to be seen in the area of radio control, but fortunately the differences are not that large, and

nowadays a positive servo pulse is used nearly universally. The following table lists the pulse durations used by the best-known manufac-

turers, along with the repetition rates. The order numbers and colour codes for the servo cables are shown to the right.

Make

Pulse duration (ms)

Servo cable

min

neutral

max

order no.

+ batt

– batt

pulse

Futaba

0.9

1.5

2.1

F1439

red

black

white

Graupner/Jr

0.8

1.5

2.2

3941/6

red

brown

orange

Multiplex

1.05

1.6

2.15

890412

red

black

yellow

Robbe

0.65

1.3

1.95

8182

red

black

white

Simprop

1.2

1.7

2.2

0101745

red

blue

black

We used servo cables with Futaba plug/socket sets for the various connections, but you are naturally free to use whatever you wish. If you

want to make up your own cables, bear in mind that genuine servo cables contain very ﬁne stranded wire leads inside a silicone rubber

mantle. The contacts are pressed into the leads, which yields more reliable connections than does soldering. If you solder your own con-

nections, therefore, always ﬁnish them with sleeves of heat-shrink tubing.

the core of the software while the

motor is stopped. If this pulse were

to correspond to ‘full throttle’, the

motor would give a tremendous jolt.

Gliding motor control prevents this!

The PWM timer cannot generate out-

puts with 0% or 100% duty cycle,

since spikes are always present in

the output. The software thus

switches PB7 over to operate as a

normal output in such cases, and

sets it to the appropriate state – with

no spikes. The power MOSFETs in

the power units appreciate the

favour.

(000070-1)

special cases, the LEDs have the following

significance in normal operation: the green

LED is illuminated in the ‘no power’ state, the

red LED D3 at ‘full throttle’ and the yellow

LED for intermediate settings. The red LED

D2 is connected to the ‘for/back’ output and

is illuminated when this output is active.

In addition, the three ’power’ LEDs have an

additional meaning if the user option ‘fail’ is

enabled. In this case, these LEDs are illumi-

nated if errors are detected in the incoming

pulse stream (as long as the user has not dis-

abled the ‘reptest’ check).

The final item to be noted in the schematic

diagram is the PWM signal. The PWM timer

(auto-reload timer) is not fed directly from the

main program loop, but instead indirectly via

a ‘sliding’ delay. This means that the outputs

signal can never abruptly change from

stopped to full throttle – this transition is

always ‘sliding’. This reduces the strain on

the motor(s), but what we ﬁnd even

more important is that this function

enormously increases safety. Sup-

pose that against all odds, a false

pulse should manage to penetrate to

Next month, we will continue with

construction, programming and the

power units.

Elektor Electronics

1/2001

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