Servo drives have gotten a lot cheaper in the recent years due to the advances in electronics. Steppers work on a real simple system of transistors, and are mainly suited to low torque kinds of jobs. Historically, servo motors have been for high torque and high speed applications that required lots of acceleration. They were big and bulky, and cost a lot of money. Again, due to progress, servos have decreased in size and cost to the point where you can put together a similar system for the same money.
At the same time, servo systems must be tuned to the load, or they will be unstable and never work. In the past, tuning closed loop systems was an art form practiced only by strange little guys that were unable to communicate with humans. They talked about "the gain structure of the PID loop" and "inherent response due to inertia". Since then, the manufacturers have come up with software that can tune the drives for you once you hook everything up. The strange little pot-tweaking PID tuning guys would say it's not a perfect tune, but it's usually close enough to get the job done. This has also helped to put servos into more common usage.
The word "servo" means that the system is controlled in a closed loop . In a closed loop system, feedback is used to compare the desired condition to the actual condition, and corrective action is taken by the system to make the difference, or error, go away. In other words, if you want a motor to run at 1500 rpm, and the tachometer or encoder attached to it is indicating that the motor's running at 1400 rpm, the system will supply more torque to speed the motor up.
By contrast, an open loop system sets the conditions it thinks will produce the desired result, and expects the system to do that. Older variable frequency drives did this; they would produce some frequency, and expect the motor to keep up. If the motor didn't due to load or slip, the drive would never know, and continue happily to produce the frequency and current it needed. To counteract these variables, the user could put in parameters that would adjust for some of these conditions, but the parameters were always subject to problems. Most stepper systems are the same way. If the step motor starts to lose counts due to excessive acceleration or load, the stepper drive never knows. Some stepping drives do work in a closed loop mode, but there's a hitch. When a stepping motor "loses sync" with the drive pulses, the torque basically goes to zero, and there's not much the drive can do about it but stop.
A servo motor actually is more than just a motor with a shaft sticking out of it. They should really be called "servo drive systems", because the actual iron that turns doesn't have any idea what speed it's turning, or what its position is. So a servo motor is comprised of three parts: the motor which turns, the feedback device, and the drive. We'll talk about these separately before bringing them together.
The actual servo motor is usually a boxy looking thing that has the shaft. It's different from an AC motor in that they are usually designed for much higher RPMs and heat. They are much smaller than normal motors for the amount of torque they can produce, and therefore get real hot during operation. They also can operate well at very low speeds, which makes them suitable for positioning applications. Some of these motors use Hall effect switches in the frame to tell the drive the position of the rotor, so the drive can optimize the current flow to the windings to allow better control.
Feedback comes from either an encoder or a resolver mounted on the back of the motor itself. They basically report the motor position and speed back to the drive. In the case of a resolver, the amplifier will translate this feedback into a more standard encoder emulation feedback for the speed and position controller.
You can't hook a servo rated motor directly up to the wall socket. In order to turn properly, the current must be manipulated in the windings to produce the proper torque. The drive is the little black box in the control cabinet that makes current. There are three parts to the servo drive, and may come in one package, or separately.
1. The Input Stage or Power Supply:
The incoming voltage is rectified into a DC voltage on the DC Bus. The DC voltage is used by the output stage or amplifier to actually send current to the motor. Input voltages can be 120 vac or 240 vac single phase, or 230/460 vac three phase, depending on drive capacity. Some servo drives have this built into the amplifier stage, and some have a single power supply for several amplifiers. Basically if you have more than one drive in the system, it usually makes sense to get only one big power supply for multiple amplifiers. Kollmorgen's BDS4 and BDS5 series amplifiers all take separate power supplies that can be used for more than one amplifier at a time. When you specify a power supply, you need to take into account the current required by each axis, or amplifier attached to it.
2. The Output Stage or Amplifier:
The amplifier is the thing that actually makes current for the motor. It uses some kind of power transistors to switch the DC voltage to the motor windings. At this point, the amplifier only produces current, which pretty much translates directly into torque.
Most amplifiers can take either an analog speed or a torque input.
You give the input a voltage, and depending on the mode of the amplifier,
it either runs that speed, or produces that torque. In order to do
this, the amplifier must be tuned to the motor it is running. Analog
amplifiers have a "personality module" that matches the current responses
to the specific motor you're running. Digital amplifiers can sense
the type of motor hooked to it, and adjust itself accordingly. This
is why you have to buy the amplifier and the motor from the same
manufacturer.
Otherwise, the tuning curves can be all buggered up, and the system will
be unstable.
3. The Controller:
The controller is the brains of the outfit. It's the part of the system that takes your position input and tells the amplifier what to do to make it do that. There's a whole bunch of controllers out there, some integrated, some not. All of them basically have the same makeup. There are some discreet inputs that can be used for stuff like enabling the drives, hard limits, and initiating moves. There are discreet outputs that can be used to indicate the end of a move, drive faults, and other stuff. There are an analog interface to the amplifier that the controller uses to indicate how fast to go or how much torque to produce. Nine times out of ten, this is also where you tune the drive. The manufacturer will provide some kind of tuning software to let you stabilize the system to your load. This is all the PID stuff you've been abused with in the past.
The other piece of the puzzle is the program in the controller. Most often, the controller has a proprietary programming language that looks something like BASIC. This is where you tell the motor what to do and when, like, "When this input is on, go forward 10 inches and then return. Then tell me when you're done by setting this output". In order to This program would look *something* like this in MX2000 language:
IF IN(101) THEN ; When the INPUT turns on
MOVE(1) = 10 ; MOVE axis 1 10 units
END IF ;
WAITDONE(1) ;WAIT UNTIL axis 1 is finished.
OUT(102) = 1 ;Set output
Your programs will probably be a lot more complicated than this, but that's the basic idea.
An example of a controller that has no amplifier is the Superior's MX2000. It can run up to 8 axes simultaneously, and coordinate two. Coordinated moves are moves that take two axes to produce, like diagonal lines or circles. If you are going to do coordinated moves, you need to get a multi axes controller that can deal with them. Incidentally, most common PLC's are not up to this task. Research this carefully if you are going to use a PLC as a controller.
Again, some amplifiers have their own controllers built into them, making them a "smart amplifier". The Kollmorgen BDS5 and Silverline have brains built into it, as does Superior's TDC line. They have their own digital I/O and analog inputs. Be careful though, because these controllers are single axis, and can't do coordinated motions very well with other axes.
Okay, in a nutshell, here's the deal. Servo drive systems can replace most stepper applications and do a better job because:
1. Servos don't lose sync. They always know where they are.
2. Servos can produce a lot of torque. This is good for high accelerations.
3. Servos are close to the same size as stepper motors, and come in frames that directly replace most stepper motors.
4. Servo controllers are typically a lot more flexible than stepper controllers. The motions they can produce are a lot more complicated.
DO NOT put a servo drive on a mechanical system that is loosely coupled to the load. Sloppy gearboxes, Lovejoy couplings, belts, chains, and long frames that flex are recipes for disaster. It is almost impossible to tune a servo drive to these systems, because the inertia changes in the load. Because of the lightning fast response time of the servo system, it will never settle down, and will sit there and vibrate until something breaks. It may be the coupling, it may be the motor shaft, or the amplifier may melt. Make sure that your mechanical system is tight. Use zero backlash couplers and gears. Use high quality ballscrews for linear motions. If it's not tight, you're asking for a lot of frustration and time spent on a project that will not work.
This is almost impossible to really figure out without a degree in physics, so both Kollmorgen and Superior have software packages that can assist you in selecting your motors and drives. Call us and we'll give you a copy. They ask basic questions about how the mechanical system is configured. Keep in mind what I said above though; the selection software can't make up for bad machine design or specification.
This is true for any motion control system, stepper or servo. If you just have one or two positions to hit, maybe you need a cylinder. Motion systems can do a lot, but the tradeoff is that they add a level of complexity to your system that you may not have to endure.
Most motion controllers can control some discreet I/O, such as valves and the like, but they don't replace PLC's. The ones that come close cost a whole bunch of money. The I/O count of a large machine will exceed the available I/O of the motion controller, so you'll need a logic controller too. Typically, the I/O count of motion controllers is in the 16 - 32 point range.
Most OPI's and PLC's will support serial communications, but almost none of them support the command protocol for any motion controller directly. This means you have to write a serial port driver for that controller. If you're pretty adept at coding, this is the way to go.
The majority of your interfacing can be done over discreet I/O. You write a program in the motion controller that will take triggers from the PLC and then perform moves. This is by far the easiest way to integrate the controller into a system.
The other option is to use Profibus, DeviceNet or some other industrial protocol to do your interfacing, and you can generally extract a bunch of information from the controller, like speed and position. Depending on your application, you may need all this data. If you just need to do some simple moves though, it may not be worth the trouble and expense.
One of the ways to guarantee frustration and failure is to rush into a system
design. Just because the motor can turn the load doesn't mean the system is
going to work. Define your project first without regard to brands, then see if
the equipment you like will do the job.
We're not trying to scare you. Motion controls can be tricky, but it's not
rocket science. We have the experience to help you define your project and get
you running! Give us a call, we'll be happy to help!