Monday, December 7, 2009

Why The Stepper Motor

We have to look back to the type of control system that we select.

Two main types of control system, Open-Loop & Close-Loop.
Open-Loop control:-
The Brain send an instruction to the Muscle and expect the instruction to be carried out perfectly. and never bother to go back to check if the job had benn done properly as told.
Close-Loop Control:-
Commonly named Servo-System. The Brain will send instruction to the Muscle, after completing the instruction, by using encoders the Brain will cross check if its been done correctly,  if there is error, the brain will instruct the muscle to make the correction/adjustmet, & repeat the cross-checking until the brain feels happy.

Another advantage of a Servo system is they can move much faster then steppers.

The first reaction for most will be to use an close-loop system, error correction, faster speed... simply because that is how a healthy person will think. BUT after a quick search, one will learn that a close loop system will cost an arm & leg.... Not willing to hurt the pocket & further complexity, one normally settles for Open-Loop System...

Anyway, most CNC router employ a stepper system as their muscle and they had been proven competent mostly.

A compromise must be made, Generally,
  1. One must not skim on specification of the motor, such as power & torque.
  2. Eliminate or minimise backlash in the system.
  3. One must tune the machine to ensure the motor doesn't
    •  loose step, i.e. motor turn but the machine doesn't move. The cause are generally asking the motor to perform acceration which the motor torque are not sufficient to achieve, mechanical friction, Load of machine too heavy.. etc.
    • Backlash, this is the term to describe the "free-play" between the to & fro motion, one can introduce mechanical solution to minimise the error or use software to compensate.
    • Electronic inteference, one must ensure the instruction signals are transmitted uncorupted by external noise such as EMI, RFI... etc.

There are no free lunch! Stepper motors system is generally a fraction of the Servo-system, so more brain-juice & body sweat is required to compansate the situation. BTW, servo system do have their unique sets of headache to tackle... but that is another story for another article...

When looking at the Stepper motor specs sheet these are the things to look
  • number of phase, the modern steppers are normally hybrid 2-phase.
  • number of wires,
    • 4 wires means 2-coils,
    • 6 wires means 2-coils with choice of Bipolar in series or uni-polar (half coils)
    • 8 wires means 4 coils with choice of Bipolar inseries, Half coils or parallel half coils.
  • Coil inductance of each coil. Lc
  • Torque.
  • Max. Current rating of each coil.
Stepper motor normally runs over 10 times the rated Voltage, so the voltage rating isn't important. To find the actual motor Voltage rating, here is a simplified and safe estimate.

Stepper motor rated DC voltage = 32 x sqareroot(Lc)
where, LC is in mH.

Actually, if the motor doesn't heat up too much, one can go for higher Voltage.

Don't ask me why as the answer will come in a few pages or tech ladden write-up which I am too lazy to do.

  • Higher the V, we can get higher speed from the motor, if high speed is a necessary, seek out ahigh coil inductance motor.
  • Higher the A, we can get higher torque from the motor, check out the coil rating.
  • The limiting factor is always the heat during operation, IMHO it is always desirable to have as low a Coil Resistance no matter what...
  • Sufficient Torque, too little can't do the job, but too much doesn't always better. just be rational.

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