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Analog Servo Amps Versus Digital Servo Drives

The traditional analog servo amplifier receives ±10V analog commands from a motion controller, translating the command into  current to the motor.  Most servo amps have potentiometers for setting gain, offset, and such.  Brushless servo amps typically add hall sensors inputs for commutation.

I use the term digital servo drive for a servo amplifier combined with digital control.  A typical servo drive that supports the CiA 402 motion profile receives commands from a fieldbus such as CANOpen, Ethernet PowerLink, or EtherCAT.  It provides power to the motor and receives the motor’s feedback, typically including hall sensors, encoder, and/or resolver inputs.  Typically these drives can run in torque mode, velocity mode, position mode, or PVT (position-velocity-time) mode.

Some models support alternate modes, including mimicking an analog servo amp (±10V command), a stepper drive (step/direction commands), or encoder (follow a master encoder input).

If your needs are simple, you can use CiA 402 drives directly connected to a computer with your control logic creating and sending the motion commands to the drives.  For complex motion profiles, it may make sense to put a dedicated soft (on PC) or hard (embedded CPU) motion controller between control logic and the drives.

I really like using digital servo drives on a fieldbus.  The biggest reason: the wiring is much simpler.  For example, PC to CAN interface to servo drives to motors instead of PC to PCI board to breakout boards to analog servo amp to motors.  Another big improvement: it’s much easier to add motors later, unlike with a typically motion controller which is setup for a fixed number of axes.

Even using digital servo drives in analog mode has some advantages:

  • Setup is much easier:
    •  Typically no DIP switches or pots to worry about.
    • With many digital drives, phasing the motor is so much easier: connect the motor’s power wires in any order, connect the hall sensors in any order, spin the motor in the direction you want to be positive, and let the drive figure out the correct order.  With analog drives: keep swapping wires until you get the right setup, for the desired direction of motion and hall phasing.
    • With many digital drives, you can run motors in sinusoidal mode, with encoder feedback but without hall sensors.  I’ve found that a significant number of my older motors have one or more faulty hall sensors, but they’re still usable on my digital drives.
    • With many digital drives, you can watch the hall sensors inputs so if the hall sensors are flaky, you can quickly identify which ones are the problem.
  • Because all the settings are stored in the drive, not controlled by pots, it’s easier to replicate the setup, and harder to screw up.
  • On the negative side, digital drives are often more expensive than analog servo amps, and may require encoder feedback to the drive, which complicates the wiring when they’re used as analog servo amp replacements.

I’ve been setting up a variety of motors for both a AMC BE15 analog brushless servo amp and a Copley Accelnet digital drive:  motor setup is so much easier on the Accelnet.

Sometimes the digital drive might add complications:  the first digital drive I used was an IDC B8000 analog input digital drive.  I was using a pair of them for some extra axes on a Adept robot system, and I could not get the whole system tuned to perfection.  The tuning was OK, but I think the B8000’s feedback loops were interfering with the Adept’s feedback loops.

Note: post expanded 7/31/2012

 

3 comments

1 Doug { 11.28.12 at 10:50 am }

Hi Tony,

I posted the following questions to Phranc on the automation primer blog. You were helpful to me before in suggesting some books about PLC program design. So I wanted to ask you the same question:
I’m interested in the electrical design steps after all the major components of the machine have been spec’d (PLC, cylinders, valves, motors, etc.). Steps on how to create an electrical schematic after choosing the components and at the same time how to know what other components are needed for the cabinet (terminal blocks, fuses/circuit breakers, etc.).
Do you know of any practical resources on this subject? Text books, websites, tutorials…?
I guess I’m looking for a practical electrical engineering design guide for automated equipment. Everything I’ve found so far is to specific to one area, or does not seem very practical, or does not give a systematic design approach from the beginning. Any suggestions will be appreciated.

Thanks,
Doug

2 Tony { 11.29.12 at 12:43 pm }

Hi Doug,
Thanks for reading! Here are some quick comments:
— After searching around a bit, I don’t think anyone has written a book on this topic. Your best bet is asking questions and getting good at advanced Googling.
— Part of the problem is that the requirements vary so much, depending on the type of machine (clean room, hazardous, max voltage, max current) and where it is placed (CE standards for Europe, possibly NEC and UL for the US).
— What we do at work probably isn’t typical: use SolidWorks to verify everything will fit, let our techs figure out the best layout, then update the SolidWorks model to match. We make extensive use of DIN rail mounted components, including custom break out PCBs. Typically we don’t have separate electrical cabinets; instead, it’s inside the machine. I try to minimize voltages over 50V, and typically try to keep the AC in one area, with finger safe connections; obviously, if you’re doing a 480VAC 3-phase system you will have to do things differently.
— Schematic specifications vary, too. We typically use the ladder style, currently with DraftSight, although I’m looking for an affordable schematic package (since our Via software died). Most electrical schematic software is pretty expensive (e.g. AutoCAD Electrical is >$5K, plus annual maintenance), and PCB schematic software won’t work well. Straight AutoCAD or clone (e.g. DraftSight, DoubleCAD XT, NanoCAD, BrisCAD) is pretty painful if you do a lot of schematics. So right now I don’t know of any perfect schematic solution.

I hope this helps a bit.
–Tony

3 Tony { 11.30.12 at 5:06 pm }

Doug,

Another possible source of info is standards for various industry groups. For example, the Nestle and the OMAC group are working on common requirements, including specifying the electrical labeling, grounding, shielding, wiring, documentation, etc. On the negative side, the OMAC standards aren’t done yet, and might cost quite a bit when they are finished; I know the SEMI standards aren’t free.

–Tony

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