In the age of digital servo drives that use PC based configuration tools, analog servo drives still have a place in motion control systems. While digital drives use configuration software and an on-screen interface, analog servo drives use switches and potentiometers. You can read more about the differences between analog and digital servo drives here.
Today we're going to show you how to configure an analog brushless dc servo drive. By the end of this article you should have a good framework for the steps involved and know what you need to get started.
Hardware and wiring
When it comes to grounding, power supply selection and wiring, the process is the same for both digital and analog. You can refer to your drive model's manual for specific instructions.
Servo Drive Selection
Selecting the best brushless dc servo drive for your needs is also the same as with selecting a digital drive, however there are a few things to keep an eye out for.
First, analog drives are usually limited to what modes they can operate in.
- Current mode is almost always available.
- Velocity mode availability is dependent on the drive model.
- Position mode is usually not an option for analog servo drives.
The mode you need to operate in is usually dictated by the capabilities of your motion controller. Most modern controllers can work with the drive in current mode and this should be your preference if your controller allows it. In short, pick the operating mode based on the capabilities and recommendations of your controller or the demands of your application.
In order to close the control loops, your drive and motion controller will need feedback information from the motor. The two most common feedback devices for analog drive setups are hall sensors and encoders.
All analog brushless three-phase servo drives from ADVANCED Motion Controls use Hall sensor feedback for commutation; make sure your brushless motor has either Hall sensors or commutation tracks on the encoder. Digital drives aren't constrained by this limitation.
When selecting an analog drive, encoder feedback is only needed if the drive will be operating in velocity mode. If the drive will be operating in current mode then the encoder feedback can bypass the drive and connect directly to the controller. For convenience, some drives have a built-in bypass that allows the motor feedback cable to connect to the drive and then pass the encoder signals to the controller cable.
The current limit is used to protect the motor so it doesn't get overloaded. Depending on the model of servo drive, the current limit can be set using switches and or potentiometers. Refer to the drive's datasheet for the settings on your specific model. For AxCent™ servo drives the current limit potentiometers have 12 active turns, with an additional inactive turn at the two extremes. The potentiometer sets the current limit linearly between 0 Amps and the peak current rating of the drive. The continuous current is usually ½ the peak current.
For example the AB30A200 [link to data sheet] is rated to 30A peak. To set the output to 20A peak you would start by turning Pot 1 fully counter clockwise until you start to hear it make a clicking sound (no more than 14 turns total). This will get you to the starting position. 10 turns of the potentiometer will get you 30A peak on the drive, this means there are 3 Amps per turn. So for 20A you would need about 7 turns clockwise, starting from the starting position, plus one additional turn to account for the inactive first turn (so a total of 8 turns clockwise from the starting position).
Current limit switches can also be available to set the current limit.
One of the biggest differences between setting up analog and digital servo drives is getting the motor phasing correct. With a digital drive you just click a button and the drive will go through an auto-commutation procedure to figure out the correct phasing. The motor will slowly spin a couple of times in each direction, then click 'Save' and you're done. With analog drives this is a manual process where you need to try every combination of motor wires to see which one works best. With three wires this means there are 6 combinations. Luckily it's not as hard as it sounds and only takes a few minutes. Here's a video that can walk you through the steps.
As with the motor phases, feedback polarity is another process that digital drives automatically compensate for while analog drives need to be set manually. How do you know if your feedback polarity is wrong? Basically if you turn on the drive and the motor just takes off then the feedback is probably backwards. Hopefully you haven't coupled the motor shaft to the load yet, otherwise you could break something or someone could be injured when the motor takes off (hint hint).
For drives in velocity mode
Some drives have a switch that can set the feedback polarity. Try switching it to the other direction to see if this fixes the problem. If your drive doesn't have a switch you can reverse the polarity by swapping the A&B wires on the encoder.
For drives in current mode
Drives in current mode don't use the encoder feedback, so if the motor runs away, it's your motion controller that is causing the motor to take off. If your controller has a feedback polarity setting then try switching it, otherwise swap the encoder A&B wires.
The servo mode is usually set using a series of switches. Use the mode selection table in the datasheet to determine the correct settings.
Current Loop Tuning (optional)
Most analog servo drives from ADVANCED Motion Controls have current loops that are already tuned for the vast majority of applications. If it turns out that you need to increase the performance of your system, then current loop tuning may be necessary. App Note 15 explains the current loop tuning procedure in detail, as well as this current loop tuning video.
Velocity Loop Tuning
If your drive will be operating in Velocity Mode, then you'll need to tune the velocity loop. If your drive is in Current Mode then you should skip this step.
Velocity mode tuning is pretty simple. Just turn the "Loop Gain" potentiometer clockwise until the motor starts to vibrate and make noise. Then back it off 1 or 2 turns. The motor should stop making noise, be sure to leave some margin in the potentiometer between where the motor stops buzzing and the final setting.
Set the offset potentiometer so the motor doesn't drift in either direction. Adjust clockwise or counterclockwise until you find the balance point. In velocity mode you may not be able to get rid of all of the motion but your controller should be able to compensate.
At this point your drive should be set up and ready to connect to the motion controller. There are still possibilities for the system to go into a run-away condition, for example if the command polarity was reversed on the controller. Therefore, it's strongly recommended that you make sure things are stable before coupling the motor to the load. For the full details on configuration always refer to the hardware manual for the servo drive.
When you're all set up be sure to record all of the switch and potentiometer settings as well as the wiring. In the end some consider setting up analog servo drives to be just as easy as setting up digital drives. Especially if you're setting up multiple machines, the switches and potentiometers can be set quickly and offline, without having to connect to a computer.
by Rene Ymzon, Marketing Manager