My last post dealt with the possible failure of the servo system's encoder cable. Since it is most exposed to its environment, physical damage can happen quite easily, and will cause servo system encoder errors. The next most likely cause for Encoder Woes, is the encoder itself.
Many servo systems use optical encoders as the feedback device in their positioning and velocity closed loops. Other systems use Resolver based feedback. A resolver is a more simple device that uses sinusoidal wave forms to establish voltage references at certain positions of the resolver shaft. It is akin to a small motor, in that is has rotor and stator windings and their relative position to each other, as it spins, produces the sinusoidal wave. This "wave" is then use in an algorithm to output the feedback to servo control.
In this blog, we are concerned with an Encoder. Does it have a "pulse?" This device, the encoder, is a digital and mechanical optical feedback mechanism. Its primary component is a precision glass disk that is segmented about its outer edge with an opaque coating. This segmentation produces a number of "windows" that a light beam can shine through. The light beam is produced by an LED. This LED is mounted near the outside edge of the disk, and a receiver is mounted on the opposite side of the disk. As the disk turns, the light beam is alternately allowed to shine through the disk, and is then blocked by the opaque coating. This action produces "pulses" generated by the LED receiver module as the light energy from the LED is alternately "seen" and obscured. The number of "pulses" produced on each revolution of the encoder shaft is its resolution. There are multiple "tracks" of windows on most encoders. One such track is the index mark track. It produces only one pulse per revolution. However, the precise location of this pulse is used by the servo system to determine the encoder's (and Motor's rotor) position, relative to the motor.
Some optical encoders are low resolution, producing say, 360 pulses per revolution. Others are high resolution, these sometimes use special algorithms to produce over 1,000,000 pulses per revolution. A common resolution for servos is between 1000, and 10,000 pulses per revolution. Many servo systems have an additional algorithm that will multiply this resolution by four. This is a Quadrature circuit. With quadrature, a servo motor with 2,000 pulses per revolution (PPR), will deliver to the motion controller, 8,000 PPR. Increasing resolution, means increasing system accuracy.
Now that you have bit of general information on what an encoder does, lets concentrate on what happens when the encoder doesn't do its thing. Those encoder errors we spoke of in the last blog could be generated right at the encoder itself. A servo motor's encoder is attached directly to the end of the motor's rotor (the spinning part), and fixed to the motor's stator (motor frame).
Usually, the motor mounted encoder has some type of "industrial" connector that makes the connection to the encoder feedback cable. This "industrial" connector then connects somehow to the circuit board of the encoder inside the motor's encoder housing.
I hesitate to tell folks to open the back of a brushless servo motor. These precision motors and their encoders are assembled in "clean room" type environments and then sealed. In addition, special digital alignment tools are needed to "align" the encoder to the motor's stator (outer motor frame). If the encoder is loosened or moved in the slightest, the encoder must be realigned. Moving the encoder on a brushless servo motor, will produce system errors, and will result in your purchase of a new motor, or the expense of a servo motor repair.
Hopefully, that last paragraph instilled fear and dread in you. If you decide to open the back of a servo motor, to peek and poke around inside, don't say I didn't warn you. If you do decide to open it, your best tool will be your eyes. If at all possible, look but don't touch.
Remember, most encoder errors are due to faulty connections. Look carefully at the short interface between the "industrial" connector inside the motor and the encoder's connection header. Specifically, look at solder connections. More so, if the motor is attached to a machine with lots of vibration. That vibration can cause those solder connections to fatigue over time. The solder will not fail, it is usually the wire strands immediately adjacent to the solder that break. You might even get lucky and find the encoder's connection plug has worked its way loose. O.K., now you can use your big mitts to get in there and CAREFULLY reconnect the plug.
If all the wires prove firmly engaged and solidly connected, then perhaps the problem is with the encoder itself. Something inside the encoder has failed. Well, unless you have a very expensive alignment machine and motor specific software, you are faced with the fact that a new motor or motor repair is in your immediate future.
If you do have the afore mentioned tools and software, then why are you reading this? You probably know more about servo motors and encoders than I do. However, if you are still reading this, and would like to know why it's so darn important not to move an aligned encoder on a brushless servo motor...stay tuned for part three of Encoder Woes. Commutation explained. That's commutation, not communication.
Till next time...
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