Micro Processor Based Mk2 Speed Controller
By Ken Hewitt

Originally Published in 
Radio Control Models and Electronics Sept 95

Introduction

It is one year since the Mk1 speed controller design was
published in the Sept 94 issue of RCM&E, and the response to
that article was very good, but some people did ask if certain
features could be added to the basic design. Well I had already
started the design for the Mk2 which is to be described in this
article, and it included all of the additions that people had
wanted for the Mk1 and quite a lot of new features at the time,
which have now appeared on commercial designs. The Mk2 contains
all of the elements that I hope the average modeller requires
without the more gimmicky features that you can see on some
units, so it does not use any "fuzzy logic" to track the input
or "intelligent learning routines" or "anti glitch profiling".
It was decided at the outset that it would work over a fixed
input signal range, and this range was decided after testing as
wide a range of RC equipment as I could get access to, and to
this end I would like to thank all of the members of the
Bedfordshire Silent Flight Association for their help in this
research.

Specification

	6 - 12 Cells
	40+ Amps  
	Input signal range 1.2 - 1.8mS
	35x59x15mm   
	Weight 40grams (without leads)
	Microprocessor Controlled               High Frequency Switching (1khz)
	BEC                                     Switch selectable BEC cut off voltage
	In flight re-settable BEC cut off       BEC disable jumper
	Fail Safe Motor cut off                 Two throttle response curves
	Max power LED                           Motor off / Brake on LED  
	Timed Motor EMF braking                 EMF brake disable
	Transmitter motor arming                On board arming button
	Soft start/stop switch option


Description of Features

Soft start/stop switch option

The Mk2 can be configured to operate as a speed controller or a
soft start switch with soft stop braking. In the soft start mode
when the motor is switched off it will slow down to minimum
before the brake is applied.

BEC

The Mk2 can be used with BEC over the range 6 - 10 cells, this
upper limit of 10 cells is very common and is caused by the fact
that the linear voltage regulator used in the circuit has to
dissipate more heat the more cells you are using. As the
regulator has an in built over temperature cut out we need to
make sure the device never reaches this temperature or we would
lose our receiver and servo supply voltage.

BEC disable jumper

If it is wished to used the speed controller in some models
without using BEC then it may be disabled by opening this on
board link.

Switch selectable BEC low voltage cut off 

The voltage that the motor will be cut off at is pre-set on a
bank of switches, the setting for this can be seen in the
configuration table. The value is set at 1volt per cell, but if
you wish to discharge your cells to a lower value then this can
be accomplished by setting the switches for a lower number of
cells, ie if you are using a 7 cell pack and set the switches
for 6 cells then the low voltage cut off will be 0.86v per cell. 

In flight re-settable BEC cut off

Once the motor has been cut off by the low voltage cut off
system it can be restarted in flight by closing the throttle and
then opening it up again. This is a feature which will enable
you to stretch out the landing if you are about to under shoot
the landing strip. The motor will cut out again as soon as the
batteries reach the low voltage cut off point.

Two throttle response curves

A linear or non-linear throttle response may be selected to give
the best throttle response to suit your needs. This selection
works for the speed control or soft start switch options.

Max power LED

To confirm that the controller is giving full power and no
longer switching the output on and off, an LED is included to
indicate full power. This LED is also used in the power up stage
of the controller to indicate what is happening.

Timed Motor EMF braking

There is a timed delay after switching the motor off before
applying the brake, this is to reduce the amount of stress
applied to the motor brushes and the braking system that would
happen if the brake was applied straight away.

EMF brake disable

If required the braking function can be disabled by the on board
link. This option is available for the 


speed control or soft start switch options. 

Brake on LED

To confirm that the brake is being applied an LED is fitted to
indicate its operation.

On board arming button

An on board arming button can be fitted to the speed controller
if required, this will stop the speed controller performing any
operations until the button has been pressed. This option may be
omitted if required and a link fitted to the board.

Transmitter motor arming

The speed controller will not start until the throttle has been
set to minimum, even if the arm button if fitted has been
pressed.


Configuration for Different Options

This design can be built in different ways depending on what you
require from the finished speed controller, you can leave out
some options if you wish to save on weight or cost, or if you
just want a unit of a lower spec and lower overall cost.

To BEC or Not to BEC

You may construct the unit we BEC and then have the option to
disable it at a later date by removing link 4 from the board. Or
if you do not wish to use BEC at all then you may omit IC3 and
C1 but you must install link 4. 

Brake or No Brake

The braking function can be enabled and disabled by the use of
link 2. But if you do not require the braking function you can
omit Q6, Q7, R3, R5, R6, D2 and D3.


Output MOS FET's

This is the area where most cost can be saved if you only want
to use the controller at low output currents, you can use five
lower cost devices or just two of the higher cost ones. But I
would recommend using five devices as the overall power
dissipation will be lower and just vary the quality of the
devices you use. I have included a table showing the three
devices I recommend, but new ones are coming out all the time so
you could use any similar devices.


Design Issues

One of the main areas of the design that took a long time to be
finalised was the output stage. This is made up of the power MOS
FET's and the circuit that drives them. The first decision to be
made was to use logic level MOS-FET's since this would remove
the need to drive them with the required 7+ volts need to turn
them on fully, logic level devices only need 4.5v. The next
decision was how to drive the gates, some designs just drive the
gates of the FET's via a resistor from an output pin of the
micro processor, but this can cause the output devices to be in
a partially turned 'ON' state for a short period of time due to
waveform rounding, which can lead to very large amounts of power
being dissipated as heat. One of my first prototypes suffered
this problem and the output devices unsoldered themselves from
the board at only 28 amps load. For this reason it was decided
to drive the MOS FET's with the correct driver IC which is
designed for the job and can switch the MOS FET's on in
fractions of a micro second. The last decision to be made was
how many devices to use. The reason for using multiple output
devices is to reduce the effective 'ON' resistance of the
controller, but there is also an extra plus to this option. If
we had one MOS FET carrying 10amps and it had an 'ON' resistance
of 0.1 ohms then it would be dissipating 10 watts, now if we
used two MOS FET's in parallel then you may think that the
dissipation in each would be half or 5watts, but this is not the
case, as we have two devices in parallel the effective 'ON'
resistance is now only 0.05 and as we are still carrying 10 amps
the total power dissipation is 5 watts or 2.5 watts per MOS FET.
As you can see the power dissipated by each device is only 1/4
of the dissipation of one device, if we use five output devices
we reduce the effective 'ON' resistance by a factor of five but
the power dissipation in each device to 1/25. The whole design
aims for the output stage have been to get the power dissipation
to a minimum and hence have the coolest running speed controller
possible. If you use the best MOS FET's listed in the table you
should be able to produce a speed controller which only
dissipates 3.84 watts at 40 amps, some commercial controllers
have a figure of 80 watts at the same current.


Description of Operation

Hardware

The circuit for the speed controller as shown is very simple as
most of the difficult parts are all handled by the software
programmed into IC1, which is a PIC16C71 micro-controller from
the Arizona Microchip range. This microcomputer controls the
motor with the 5 power MOS-FET's Q1 - Q5 via IC2 which is a
MOS-FET driver IC which makes sure we do not get any waveform
rounding due to the gate capacitance of the MOS-FET's. The
braking MOS-FET Q6 is driven by Q7, and the zener  diode D2 is
to make sure that we do not apply to many volts to the gate of
Q6. Diode D1 is a 3 amp schottky diode and is needed to protect
the main MOS-FET's from the back EMF generated by the motor when
the speed controller is operating at less than full power, the
back EMF pulse can be over 100 volts and this diode clamps it to
about 1 volt above the battery supply. R1 and R2 form a
potential voltage divider to reduce the battery supply down to a
level which can be measured by IC1, C2 smoothes this voltage to
reduce the chances of the low voltage cut off being tripped by
noise on the supply from the motor. IC3 is the linear voltage
regulator used for the BEC supply.   


Software

The PIC16C71 micro-controller used in this design was chosen for
two reasons, first of all it had the ability to handle
interrupts generated by a change of state of one of its inputs,
and this would be very useful in measuring the input pulse width
from the receiver, and secondly it had an on board analogue to
digital converter which could be used to measure the battery
volts for the low voltage cut off. The PIC16C71 also has 13
input/output pins, 1k of program memory, 35 bytes of data
storage and a real time clock/timer.

The program is made up of two main parts, the interrupt service
routine and the main program loop. As inferred from its
description the program spends most of its time going around the
main program loop, but this looping can be interrupted by a
change in logic level on one of the input pins which is
connected to the receiver, the program then passes control to
the interrupt service routine to measure the input pulse width,
after which control passes back to the main loop.

When power is first applied the system does a power up reset,
the program initialises things like input and output pins and
enables the interrupt system. Then the switch (SW1) is read to
find out what low voltage cut off point is required, and the
program waits for the arm button (if fitted) to be pressed, it
indicates this waiting state by flashing the green max power LED
about once per second. When this state is exited the program
looks for pulses from the receiver and will say looking until it
has seen 25 pulses between 0.8mS and 2.0mS, this waiting state
is indicated by the green LED being ON continuously. The next
stage will now wait for the throttle to be closed and this is
indicated by the green LED flashing at a fast rate of two per
second. Once all these conditions have been passed then the
status of link 2 is tested to see if the unit is to be a speed
controller or a softstart switch. Then the main program loop is
entered.

In the main program loop the input pulse width value is checked
and the decision made as to whether the motor is OFF, pulsed or
fully ON. Each time around the loop the battery voltage is
checked and if it drops to the low voltage cut off value then
the motor is shut down. Also the input pulse is checked to make
sure that the signal has not been lost and that the pulse is of
a valid width, if not the motor is shut down. Once the motor has
been shut down for either of the two reason given it may be
switched on again by closing the throttle and then opening it
again. 

The softstart switch performs the same checks as the speed
controller except that the motor will switch ON and advance to
full power in about 1.2 seconds and then when you want to shut
it down it will slow down to minimum in about 1.2 seconds and
the brake will be applied (if enabled).


Purchase of the Parts

It has not been able to produce a design that would enable the
constructor to purchase all of the parts from one supplier, so
some shopping around will be necessary. Also some of the
suppliers listed now only supply the small items in packs so
please check this first. The power MOS-FET's are the most
expensive items but savings can usually be made if you buy 10+
so see if someone else you know wants to build one and purchase
the MOS-FET's together to get the saving. Some of the items are
not stocked by the suppliers listed, but they have an equivalent
and so this is the stock number I have given in the parts list.


Identification of Parts

There are quite a few parts that you need to insert the correct
way round, these are C1, C2, C4, D1, D2, D3, D4, RP1, Q7, IC1
and IC2. On C1,2,4 the negative lead is the one with the black
stripe nearest to it. Pin 1 of RP1 is the pin with the dot by
it. The positive ends of D1 and D2 will have a line at that end.
The positive lead of the LED's, D3 and D4 will be the longer of
the two leads. The two IC's have the end which is pin one
indicated by a notch in the plastic case.


Construction

Some of the components (IC1,2, Q1 - Q6) used in this design are
CMOS devices and can be damaged by static electricity. When
handling these items it is advisable to take some basic
precautions, do not wear clothing which builds up a static
charge, do not handle the items until needed and before you
touch them try to touch a water pipe which should earth any
static charge you have built up. DO NOT connect yourself
directly to the mains earth.

The speed controller is made up of two modules, the main PCB and
the heatsink with the five power MOS-FET's fitted to it. The
heatsink can be made of any suitable off cut of material. Mount
the 5 power MOS-FET's on it using M3x6mm counter sunk screws and
M3 nuts, and then bend their leads up at right angels as close
to the body of the MOS-FET as you can. Then Attach a strip of
mylar hinge material as shown so that the mounting nuts do not
short out on the bottom of the PCB.

The PCB can be assembled taking care to insert the parts the
correct way around, and cutting off the component leads as close
to the board as possible. As the PCB has plated through holes
you do do need to have a soldered joint were the solder is built
up around the components lead. D1 should be fitted before Q6 as
it is under it, D1 is a surface mount type device and needs to
be soldered to the two square pads on the top side of the PCB.
Also notice that Q6 is fitted upside down, with the back of the
device pointing upwards. Most of the capacitors actually lay
flat on the PCB and do not sit up right, this is to keep the
profile of the unit as low as possible. If the speed controller
is to be used at high motor currents then it would be advisable
to build up the wide tracks on the top and bottom of the board
that join the MOS-FET's together, with a bead of solder.

Once the PCB has had all of the components fitted the servo lead
can be soldered to the pads on the back of the board, the pads
have +, -, s by them to indicate which is +5v , -v or 0v and
Signal, once soldered the lead should be secured to the board
with a dab of epoxy. The power leads will need to be attached to
the PCB before it is mounted onto the MOS-FET assemble. The
battery negative lead is soldered to the pad on the top of the
PCB nearest IC3. The Motor negative lead is soldered to the pad
on the bottom of the PCB nearest to Q6. The positive battery
lead does not need to be broken as it goes to the motor, but we
need to tap off it to measure the battery voltage and also
obtain our BEC supply, so about 3mm of the insulation is removed
and an off cut of one the resistor leads wrapped around the
inner conductors and soldered, the ends of this lead are then
soldered to the pad on the bottom of the PCB nearest to IC3.

When the PCB is finished it can be tested before fitting it on
to the MOS-FET assembly as stated in the section 'Testing'. If
the unit seems to function correctly then it can be fitted on to
the MOS-FET's. Slide the PCB down over the leads of the
MOS-FET's until the bottom of the PCB is touching the body of
the MOS-FET's and the PCB is parallel to the heatsink, then
soldered the leads of the MOS-FET's from the top side of the
PCB. You can now test the complete unit driving a motor. 

If all is OK then the completed unit can be wrapped in some
heatshrink tubing as used for making up battery packs, it is
important that the heatsink is not able to short to any wiring
in the installation as it is not at 0 volts. The heatshrink
tubing should be cut away around the switch SW1 to enable the
settings to be changed.


Testing

The completed PCB assembly can be tested before fitting to the
MOS-FET's by connecting a battery to the power leads, if all is
OK the green LED should flash about once per second, if the arm
button is then pressed the green LED should the light
permanently, if the servo lead is then plugged into a receiver
and the transmitter is turned on the green LED should start to
flash at a high rate, close the throttle on the transmitter and
the red LED should come on, open the throttle and the red LED
should go out, advance the stick to full power and the green LED
should come on to indicate full power.

Once the PCB is fitted onto the MOS-FET assembly the above tests
can be repeated this time with a motor connected to the output
leads from the unit. Once the red LED comes on then the motor
should start when the stick is moved towards full power.


And Finally

I hope that this design fulfils all of the shortcomings of the
Mk1 and that it meets the needs of the electric flight
enthusiast, without being too complicated or expensive. The
software is in revision 1.0 at the moment and if anyone has a
feature that they would like to see in a speed controller then I
will look into the feasibility of including it in a future
version of the software.


Table of MOS FET's

Cost    Device          Rds     ON Res  V Drop  Power   V Drop  Power
					@ 20A   @ 20A   @ 40A   @ 40A
Low     BUK555-60B      0.055   0.011   0.22v   4.4 W   0.44v*  17.6 W* 
Medium  BUK556-60H      0.02    0.004   0.08v   1.6 W   0.16v   6.4 W 
High    IRL3705         0.012   0.0024  0.048v  0.96 W  0.096v  3.84 W 
							* NOT RECOMMENED

Configuration Tables                                                            

Settings for SW1                                                         

	5 Cells 6 Cells 7 Cells 8 Cells 9 Cells 10 Cells 11 Cells  12 Cells
	5.5v    6v      7v      8v      9v      10v      11v       12v
SW1-1   Off     On      Off     On      Off     On       Off       On 
SW1-2   Off     Off     On      On      Off     Off      On        On 
SW1-3   Off     Off     Off     Off     On      On       On        On 
SW1-4   Off=Linear Throttle Response   On=Non Linear Throttle Response                       

Function of Option Links                                                        

	Link Open                               Link Closed                     

Link 1  Connect Arm button across Link 1        Disable Arm Button
Link 2  Speed Controller                        Soft start switch                       
Link 3  Enable EMF Brake                        Disable EMF Brake                       
Link 4  Disable BEC                             Enable BEC                      


MkII Speed Controller Parts List
				
R1              8k2  1%                 
R2              3k9  1%                 
R3              4k7                     
R4              1k                      
R5              120R                    
R6              10k                     
R7              120R                    
RP1             SIL 10k x 4             

C1              100uF 16v               
C2              10uF   16v              
C3              470nF                   
C4              100uF 16v               
C5              10nF                    

D1              Schottky diode          
D2              4.7V Zener 1.3w         
D3              Red LED                 
D4              Green LED               

Q1-Q5       or  BUK555-60B              
See Text    or  BUK556-60H              
	    or  IRL3705                 
Q6              2SJ174                  
Q7              2N3904                  

IC2             TC4428                  
IC3             LM2940CT                

X1              4MHz 3 Leg resonator    
SW1             4 Way DIL switch        
Arm Button      SPST Push Button        

Servo Lead      Available from local model shop 

IC1             PIC16C71-04/P   Available from author   7.25 inc P+P
PCB                             Available from author   5.00 inc P+P   
				or 11.50 UKP for both items
                                Post and packing is included in the price for
                                the UK and Europe. Please add 1.00 UKP per
                                order for the rest of the world
                                Make Cheques/ PO payable to K.Hewitt.

Author

Ken Hewitt,
21 The Brambles
Welwyn
Herts, AL6 0PG
England

or
C/O RCM&E,Nexus House, Broundary Way, Hemel
Hempstead, Herts. HP2 7ST.  

Email  ken@welwyn.demon.co.uk