|
|
|
|
|
|
01/11/2010
 Email to a friend
 Comment on this article
|
NUMBER ONE
Open loop or closed loop
Experts agree that position control is the key differentiator between conventional AC drives and servos. "You need to determine how good an AC drive is at position control," says Professor Pacas from the Siegen University in Germany.
 There are basically two types of drive: one with a position control device, such as an encoder, built-in and one without an encoder. These provide the closed loop and open loop modes respectively.
However, for true servo performance – full torque at all speeds – a feedback loop will always be needed. Production machinery needs to hold a position and therefore needs encoders. With an encoder on a conventional AC drive, a lot of functions can be performed that would normally be limited to using only servos. Therefore, for a conventional AC drive to stand any chance of competing with servos, a feedback loop is always required. Open loop drives simply cannot do this.
But while a feedback device is essential it has to be the right resolution according to Professor Pacas. "The functions of motion control have to be implemented in the control unit. The resolution of the encoder has to be appropriate and the implemented position control has to cope with the demands of the application in terms of dynamic behaviour. Otherwise the performance will simply not be met."
Today many applications run with 1024 line encoders which yield a 12 bit resolution. This is also the case if resolvers are used - rotary position sensors, engineered to withstand the punishment of a military application.
"Nevertheless, for a high stiffness velocity control and for very low velocities, a much higher resolution is needed. Today's state-of-the-art sine encoders offer about 1 million "increments" per revolution," says Professor Pacas. "The encoder is the principal limitation. All other limitations depend on the demands of the application such as size, environment, cabling, temperature, mono-axis or multi-axis and communication."
NUMBER 2
Choose your motor wisely
Another key differentiator is the choice of motor. A standard AC induction motor, used with conventional AC drives, generally struggles to achieve the dynamic performance of an AC brushless servo motor.
Traditionally, AC servo motors are permanent magnet motors, the technology of which is vastly different to that of the off-the-shelf standard AC induction motor. With an induction motor a magnetic field is generated dynamically, whereas with servo motors the flux is permanently present. This gives the high performance and dynamic response expected in most servo applications.
In servo applications, consideration needs to be given to motor size, dynamics, protection class, temperature and encoder type. Issues relating to torque density, the ratio of torque to inertia, which relates to maximum acceleration and whether the encoder is totally built in or enclosed, also need careful consideration.
Particularly at low powers, servo drives can be more competitive than AC drives. This is because an AC motor is normally designed for simple fan and pump applications where feedback devices are not needed. It can be expensive to add an encoder. Also the motors do not have mounting brackets for the encoders and unless the application is for a serial OEM, this can add significant cost to the end use.
Servos, on the other hand, come mass produced. And while the actual permanent magnet motor may be higher in cost, it comes with a powerful, built-in encoder and supporting software.
So on smaller sizes, the servo drives would appear to be a simpler solution.
NUMBER 3
The laws of inertia
There is another phenomenon to consider: motor inertia is a limiting factor.
Servo motors have a low inertia rotor compared with standard AC induction motors. Therefore on low power applications the servo motor can accelerate and decelerate faster.
AC drives, such as the ABB industrial drive, can run an AC induction servo motor and be called a servo drive. In this instance, a high performance, closed loop drive is running a low inertia AC induction motor. This would give the same performance as traditional servo drives.
NUMBER 4
Powerful processors
The biggest leap forward for conventional AC drives within the servo arena is the torque quality as a result of the powerful processors. ABB's Direct Torque Control (DTC), for example, means the torque producing current can be controlled in a very short time. This is the issue of computer power – the higher the computer power, the quicker the execution. The result is far better torque control at low speed response. This opens up AC motors to applications with a wide speed range, for instance dropping to a dead speed and then rapidly accelerating to a high speed.
When it comes to fast response times, DTC is superior to other known motor control schemes. DTC and vector control are fundamentally different. One characteristic of DTC is that it does not require a modulator to convert the torque and flux references into voltage and frequency references. This takes away one source for delays in the control and is one of the reasons behind DTC's fast response times.
Whereas sensorless vector control is typically used in low power drives for less demanding applications, DTC is used for the more demanding applications that require a very fast torque response time for optimum performance. DTC is also used in ABB's high power drives, regardless of the application, simply because these high power drives are significant investments that justify the best possible performance.
With the advent of DTC there is little to improve in the control method of variable speed drives: it is no longer the frequency converter that is limiting the performance of a variable speed drive, but the motor. Instead research has shifted towards the application of DTC in other settings.
One interesting development is the application of DTC to permanent magnet motors. The application of a permanent magnet motor with DTC has been very successful in the pulp and paper industry. A major advantage of the permanent magnet motor drive is that the gearbox is no longer required. Getting rid of the gearbox leads to space savings, reduced maintenance costs and improved performance of the paper machine.
"The power electronics and quality of torque control methods of most conventional AC drives are comparable to servos these days," says Professor Pacas. "The quality of the torque can be assessed by its dynamic behavior, that is settling time and by other characteristics such as linearity, accuracy, resolution and long-term stability."
The next processor development, believes Professor Pacas, is the ability to integrate motion control functions in the drive. "This is more of a software issue than hardware. This is the next level of intelligence needed to bring conventional AC drives into the arena, along with the appropriate interfaces and sensors being installed."
NUMBER 5
Field bus technology
With the development of fieldbus standards it is now possible to make drives with universal connectivity, thereby overcoming the limitations of proprietary networks that were typical 15 years ago.
Profibus and CanBUS, for example, open a whole new digital world whereas earlier AC motors were predominantly analogue control and limited with what they could communicate.
Servos are predominantly using Sercos or other synchronized communications systems as their interface. These are critical to Numerical Control. Conventional AC drives do not recognize Sercos or other synchronized communications systems and this could be the next area where research is needed. However, Profibus-DP V2 fieldbus, for example, is designed to replace Sercos-type fieldbus systems, although this technology is still in its infancy and will most likely be overtaken by various Profinet versions, which are coming to the market soon.
High end servo applications use Sercos for the speed of communication but where this level of speed is not important Ethernet offers an attractive solution.
There are Ethernet based protocols which can be used for motion control but in most cases special hardware is also required. Sercos III is on the market and Profinet iRT (isochronous real time) is coming to the market. Already available are slower versions called Profinet sRT (soft realtime) and Profinet IO.
Currently the main portion of the Ethernet drives related market is not in motion control application but in standard variable speed drive applications. The main protocols used are Modbus/TCP and Ethernet IP, although Profinet could change this.
|
|
| |
Author
ABB Ltd
|
| |
| |
|
| |
This material is protected by Findlay Media copyright 2012.
See Terms and Conditions.
One-off usage is permitted but bulk copying is not.
For multiple copies contact the sales team.
|
| |
|
|
| |
To comment on news stories or blogs you need to complete our 60 second registration
process. Once completed this then allows you to download any and all white papers,
register for e-zines and access our detailed supplier directory for FREE.
If you are all ready a registered user then enter your e-mail address and login.
You will need to have logged in prior to entering your comments in the boxes provided.
|
|
|
|