In this last of the Encoder Woes series, (I can hear a collective sigh of relief), I would like to touch on the non-touchy subject of digital commutation. First, a basic explanation of analog commutation in a familiar "brushed" commutator type motor.
Take your hairdryer or cordless electric drill, well, don't take them anywhere, just think about them for a moment. The cordless electric drill is a fine example for our, ahem, study. I am sure you have seen the small sparks that are emitted from the rear end of the motor when the trigger is pulled and released. Those sparks are from the hard carbon brushes that contact the segmented copper commutator on the drill's motor rotor (spinning part).
These carbon brushes conduct DC voltage and current to the many winding coils on the rotor. When this happens, the rotor becomes magnetized. Since the non-spinning part of the motor (stator) has permanent magnets or wired electro-magnetic coils, it does not need a commutator to energize the "magnets". They are energized by direct wire connections or by their inherent magnetic structure.
When the DC voltage is transmitted to the different rotor windings as you pull the trigger, the magnetic coils quickly change polarity from positive to negative and back to positive again as it spins. The stator magnets stay magnetized in one orientation. This switching of polarity causes the rotor to be attracted to, and repelled from, the different stator magnets. Voila! rotation. The more you pull the trigger, the more voltage and current flow, making the rotor spin faster. When you release the trigger, the motor becomes a generator momentarily, and sends DC voltage and current back toward the battery. However, most modern cordless drills "shunt" this voltage to a power resistor. This acts as a brake, and dissipates the regenerated energy into the resistor and ultimately out, as heat.
O.K. so now, you know where those little sparks come from. (Carbon brushes rubbing quickly against a copper commutator.) Now, let's take your "brushless" AC Servo motor. You do have one? Of course you do.
The reason it is brushless is because it is an inside-out motor. The rotor in this case, needs no voltage and current supplied to it because it is made with permanent, (Rare Earth, Neodymium), magnets around its circumference. "So", says you, "how does it spin?" "Well," says I, that's where the encoder, (remember the encoder?) really shines. Unfortunately, more explanation becomes necessary. Sorry.
Not to confuse matters any more than they most likely are already, a Brushless AC Servo Motor is actually a Brushless DC Servo Motor, in that, the AC like sinusoidal wave forms sent to the motor are actually DC voltage that is made into sinusoidal waves by the Servo Amplifier. It is called PWM, or Pulse Width Modulation. It is made by the very fast switching of a three phase transistor (Darlington). Those three phases are connected to the three stator (outside case) windings of the motor.
So, think of those smooth flowing sine waves made by AC, and imagine a close up of the wave edge that looks like a set of rolling stairs going over a gentle hill and down into a deep valley and up and over another hill. From afar, it looks smooth, in fact, it is many on and off cycles of the Darlington transistor in increasing and decreasing magnitudes of voltage and frequency.
Now, how does the Servo amplifier know which of the three phase/s needs to be powered at any given time? Or, how does it commutate the voltage and current? Answer: The encoder. Quite simply, the exact position of the encoder relative to the stator, gives the Servo Amplifier its ability to "know" where the rotor is, and which stator phase or phases to give power to to hold position, rotate forward or reverse and what polarity to provide at any given microsecond.
This is why it is so critical not to disassemble the encoder from the motor. That is, unless you have some expensive equipment handy to realign it. The realignment process uses the encoder itself, the three phases of the stator, software, the amplifier, and possibly an oscilloscope to accomplish this exacting procedure. In other words, don't try this at home. Leave it to the professionals.
If your eyes are still not glazed over, congratulations! I think that is enough on this topic of encoders and their possible woes. Of course, my explanations are rather simple in nature, in hopes to make the ideas a bit more palatable. In fact, the processors and their algorithms necessary to accomplish the powerful, smooth running performance of a modern servo system are fairly complicated. Thankfully, most servo drive manufacturers have done the hard part for us all. They have, for the most part, made the systems "Plug-and-Play".
Until such time as I think of some other earth shatteringly important subject to wax poetic on...I remain your faithful servant. I think I'm tearing up...
Always, do the right thing...
Gaff
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