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Stepper Motor Control Techniques
2 - Ramping up
Submitted by paulstreats on March 8, 2009 - 6:31pm.
Ramping Up The Speed
We know from the previous page that if we try to start the stepper motor with fast pulses then it just sits there and hums away not turning.
We need to start the stepper off slowly and gradually increase the speed of the steps (ramping up).
The following C file is the same as the previous. The only difference is the delay between pulses decrements until it the delays between steps are 200us and then it just stays stepping at that level.
So Lets Test It Out
As you can see, the stepper motors speed increased well beyond what was capable in the basic control method. we couldnt get the stepper to move at all at 500us phase changes and now its spinning at 200us changes.
This seems perfect at first until you realise the downside. There is a playoff between speed and torque, the faster your stepper motor is driven to, the lower torque you can get from it. this is because when the phases change, the voltage is being reversed in the coils so the incoming current has to fight against the collapsing field built up by the previous phase. The faster your motor spins, the less time it has to push against the existing current and re-energise the coil. Choosing a faster more efficient h-bridge can help this as can driving your stepper above its rated voltage (a current limiting resistor should be used for each stage) but you will always lose some torque.
An upside to ramping up your steppers is that the mechanical jitter becomes less noticeable. when the motor spins faster the mechanical resonance swaps out for electrical resonance causing a high pitched humming or ringing noise.
This was noticable when i tested it but i dont know how well it came out in the video.
Okay so now weve seen how to use a stepper motor to go forwards continuously, but that isnt what stepper are about. The next page shows how to control the stepper motors positionally.