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MINOS 04 Software for Stepper Motors Pete Harrison

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Harrison2 Why Steppers Easy to get going Simple Hardware Simple Software Open Loop Easy mechanics

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Harrison3 Why Not Steppers Poor Power to Weight ratio High Current Drain Open Loop Tricky to drive at speed

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Harrison4 Stepper Characteristics Open loop digital control One pulse gives one step Fixed step size Resonances

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Harrison5 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

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Harrison6 Acceleration Accelerate quickly through resonances Dont 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

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Harrison7 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

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Harrison8 Software Requirements Each motor needs independent pulse train. Frequency sets speed Pulse length not critical Frequency changes on the fly to accelerate and decelerate

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Harrison9 Timer Options Software Loops Dual timers – separate interrupts Single timer – single interrupt Single timer – Output compare/PCA Slave Processor

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Harrison10 Software timing Simple to design and execute Step on demand Tricky to coordinate actions Low speeds Poor performance

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Harrison11 Single Timer Frequency division/synthesis 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

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Harrison12 Dual Timers 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

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Harrison13 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

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Harrison14 Trapezoidal Profile

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Harrison15 Calculating Acceleration Steppers need distance instead: Normally work with time as independent variable:

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Harrison16 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

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Harrison17 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 bit values

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Harrison18 Typical Table

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Harrison19 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

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Harrison20 Typical Acceleration

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Harrison21 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]; }

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MINOS 04 Software for Stepper Motors Pete Harrison

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