MINOS 04
Software for Stepper Motors
Pete Harrison
Why Steppers
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Easy to get going
Simple Hardware
Simple Software
Open Loop
Easy mechanics
http://micromouse.cannock.ac.uk/
Pete Harrison
2
Why Not Steppers
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Poor Power to Weight ratio
High Current Drain
Open Loop
Tricky to drive at speed
http://micromouse.cannock.ac.uk/
Pete Harrison
3
Stepper Characteristics
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Open loop digital control
One pulse gives one step
Fixed step size
Resonances
http://micromouse.cannock.ac.uk/
Pete Harrison
4
Constant speed
• Constant speed implies constant drive
frequency
• Jitter can cause mis-stepping
• A lost step is the last step
• Poor torque at speed
• Some speeds will suffer from
resonances
http://micromouse.cannock.ac.uk/
Pete Harrison
5
Acceleration
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Accelerate quickly through resonances
Don’t start too slowly
Changes only happen at each step
That is – a fixed distance not a fixed
time so cant just add a time interval
• Acceleration has to be adjusted at each
step
http://micromouse.cannock.ac.uk/
Pete Harrison
6
Hardware Requirements
• Digital controls
– Step (one each)
– Direction (one each)
– Enable (shared)
• Accurate timing source for a pulse
generator
• 2 ms-1 probably implies 2500Hz each
http://micromouse.cannock.ac.uk/
Pete Harrison
7
Software Requirements
• Each motor needs independent pulse
train.
• Frequency sets speed
• Pulse length not critical
• Frequency changes on the fly to
accelerate and decelerate
http://micromouse.cannock.ac.uk/
Pete Harrison
8
Timer Options
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Software Loops
Dual timers – separate interrupts
Single timer – single interrupt
Single timer – Output compare/PCA
Slave Processor
http://micromouse.cannock.ac.uk/
Pete Harrison
9
Software timing
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Simple to design and execute
Step on demand
Tricky to coordinate actions
Low speeds
Poor performance
http://micromouse.cannock.ac.uk/
Pete Harrison
10
Single Timer
• Frequency division/synthesis
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Set to a high rate – say 5kHz
On each interrupt add constant to accumulator
On overflow, perform action
ALL motor code must run in the same time slot
e.g. 16 bit accumulator, constant = 3932 =>
f=5000*3932/65536 = 300Hz
• Convenient overflow in assembler
• There will be jitter
http://micromouse.cannock.ac.uk/
Pete Harrison
11
Dual Timers
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The easy way if you have them
Two 16 bit timers needed
One timer interrupt per motor
Independent unless the timers are
simultaneous
• Check interrupt priorities – they need to
be high
http://micromouse.cannock.ac.uk/
Pete Harrison
12
One Timer with Output
Compare
• Fairly common
– 8051 derivatives (PCA)
– AVR (OCRx)
– PIC (Timer 1 CCPx)
• Single 16 bit timer with independent
interrupts at user set rates
• Low overhead
http://micromouse.cannock.ac.uk/
Pete Harrison
13
Trapezoidal Profile
http://micromouse.cannock.ac.uk/
Pete Harrison
14
Calculating Acceleration
• Normally work with time as independent
variable:
1 2
s
2
at
• Steppers need distance instead:
2s
t
a
http://micromouse.cannock.ac.uk/
Pete Harrison
15
Calculating Acceleration
• For each step we need the interval to
the next step
• Either
– Calculate on the fly (square root)
• Or
– Pre-calculate a lookup table
http://micromouse.cannock.ac.uk/
Pete Harrison
16
Lookup Table
• Use Excel or a program and load into
mouse – can live in ROM/FLASH
• Several tables can live in memory
• Calculate whenever we need different
speed/acceleration – needs to be in
RAM
• May need 1024 16 bit values
http://micromouse.cannock.ac.uk/
Pete Harrison
17
Typical Table
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Step
0
1
2
3
4
5
6
7
8
9
10
Time(ms)
Elapsed per Step Frequency Velocity
0
30.834
30.834
32
43.606
12.772
78
53.406
9.800
102
61.668
8.262
121
68.947
7.279
137
75.527
6.581
152
81.579
6.051
165
87.211
5.633
178
92.501
5.290
189
97.505
5.004
200
http://micromouse.cannock.ac.uk/
Pete Harrison
(m/s)
0.015
0.037
0.048
0.056
0.064
0.071
0.077
0.083
0.088
0.093
18
Using the Table
• Acceleration is just working through the
table, picking out values
• Maximum speed is a number that tells
us how far into the table to go
• Each entry is one step so speed index
is also the number of steps to come to a
halt
http://micromouse.cannock.ac.uk/
Pete Harrison
19
Typical Acceleration
0.60
Velocity Profile
Velocity (m/s)
0.50
0.40
0.30
0.20
0.10
0.00
0
http://micromouse.cannock.ac.uk/
100
200
300
400
Tim e (m illiseconds)
Pete Harrison
500
600
20
Sample Code
// motor interrupt
interrupt [TIM1_COMPA] void timer1_compa_isr(void){
UINT temp;
if (!steppersEnabled) return;
// global bit variable
temp = OCR1A;
// remember the counter value
STEP_LEFT=0;
// get the pulse done early
delay_us(5);
// we only need a short pulse
STEP_LEFT=1;
remaining--;
// one more step done
if (remaining <= 0)
arrived = 1;
// global flag
if (currentSpeed < remaining)
// accelerate if we can
currentSpeed++;
else
// be sure we are able to decelerate
currentSpeed--;
if (currentSpeed > maxSpeed)
// not too fast
currentSpeed = maxSpeed;
if (currentSpeed < 0)
// or off the table
currentSpeed = 0;
OCR1A = temp + acc_table[currentSpeed];
}
http://micromouse.cannock.ac.uk/
Pete Harrison
21
MINOS 04
Software for Stepper Motors
Pete Harrison