Sercos-based drive systems, with their fiberoptic- ring architecture, are the subject of some controversy. Some of their advantages over analog drives are readily apparent. The ring eliminates tens of wires between the controller and drive, and thus saves wiring costs and installation time. On large machines you can put the drive anywhere without worrying about performance degradation, since there are no long motor and encoder cables to bother about. You can save money here, too, as fully shielded motor and feedback cable together cost as much as $40 per motor.

Another advantage is the drive and controller are electrically isolated. There is little or no potential for ground loops or improper shield connections, which are notorious problems that slow machine start-up.

These are the obvious advantages. The controversy exists over the not-so-obvious advantages. Some people are under the impression that Sercos systems are not as responsive or accurate. These misconceptions arise by comparing Sercos and analog components directly. Instead you need to look at how each system operates as a whole.

Sercos architecture

Sercos, which stands for Serial Real-time Communication System, was started in the mid 1980s in an effort to overcome the limitations of analog interface. It uses digital communication, which eliminates command noise, has better resolution, and stops drift.

Communication between the drive and controller is in the form of "telegrams," which traverse the ring in a set pattern or cycle. At the start of each cycle, the controller posts a telegram called the Master Synchronization Telegram (MST). This telegram ensures that all drives are receiving messages and operating in synch. Each drive then sends an Amplifier Telegram (AT), which indicates its current position, to the controller. The controller then posts a Master Data Telegram (MDT) that tells the drive what to do. The Sercos system comes with many standard telegrams for various combinations of position, velocity, and current. If none of these are suitable, the user can create custom telegrams.

The telegrams are divided into cyclic and noncyclic data. Cyclic data includes command, feedback, and status information. Noncyclic data is transmitted in another portion of telegrams called the "service channel." This channel functions like the serial ports on analog drives and enables the controller to access hundreds of data points and processes in the drives. Both the AT and MDT have cyclic and service channels.

Comparing performance

There are four ways to measure how well a motion system works: accuracy, smoothness, response, and intelligent drive capability. The problem many people have when comparing Sercos and analog systems is that they don't compare equivalent functions. For instance, Sercos systems typically update position commands at a rate of 500 Hz. Analog system users will retort that their systems "execute" at a rate of 2 kHz. However this analog controller "execution" generally refers to the velocity loop update rate. In a Sercos system, the velocity loop is closed inside the drive at a much higher rate than 500 Hz. The point is that users must look at overall system performance to make an accurate comparison.

Command and feedback accuracy. Sercos systems, when compared to analog velocity drives (see box) are more accurate for several reasons.

• They have high resolution. Analog systems commonly have top speeds as high as 30 m/min. A 14-bit analog-to-digital converter on such a system would give a resolution of about 2 mm/min., which is inadequate for some applications. You can overcome this by using a 16-bit a/d converter, which improves resolution by a factor of four, but the cost is significant. Sercos systems use 32-bit velocity signals, so that resolution is not an issue.

• They eliminate signal noise and the effects of component variation. Typical common-mode noise levels in factories tend to be around 1 to 2 V, and analog servo drives reject around 50 to 60 dB, so common- mode noise levels on command signals are 10 mV or more. For a 30 m/min system this noise level is equivalent to 30 mm/min, which exceeds resolution limitations in a velocity drive

Smoothness. Many motion systems designers contend that a 500 or 1,000-Hz update rate causes jerky position and velocity command profiles. However, Sercos drives use micro-interpolation to smooth commands. They can do this in a number of ways.

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The first method fits a polynomial curve to three or four position points stored in the drive's command sequence. Unfortunately, this method requires additional time. If the drive has to wait for three or four position points, then the motion has to be delayed until they are accumulated.

Another method uses the velocity signals in conjunction with the position signals, since a Sercos system can transmit them simultaneously. This method takes position and velocity from two sequential samples to form four constraints and then fits a third-order polynomial to provide smoothness.

Response. Sercos systems can make motion profile changes in 1 msec, since the controller posts the MDT at every cycle. This time is normally sufficient for most industrial applications. A popular misconception here is the system's real-time response is regulated by the telegram cycle rate. Remember, the system can do high-speed capture (reading position feedback synchronized to an external event) in microseconds, and handle limit switches and other monitoring functions faster than the cycle rate.

Analog systems, because of their centralized processing systems, provide very fast servo update rates for one or two-axis systems. However, when four or more axes are involved, the update rate usually falls significantly. Sercos systems typically provide 2-kHz position loop, 4-kHz velocity loop, and 16- kHz current loop update rates that are independent of the number of axes. Many analog users are sometimes surprised to find that the systems they thought would update velocity at 60 msec actually updates every 440 μsec because of the number of axes.

One example of a rapid Sercos system successfully controlled a 7-axis horizontalflow wrapping machine that required registration control. The machine wrapped 1,400 items per minute.

Sercos communications rates are improving. Most systems have a communication baud rate of 4 Mbits/sec, and this is expected to increase in the coming months. Very rarely is the communication rate an impediment, though. Most times, it's the controller's processing rate that's too slow. Sercos can support position update rates of 16 kHz.

Another mistake people occasionally make is to compare Sercos to networks that transmit low-level information, such as that used for controlling power devices. Transistor on-off commands must be transmitted at much higher rates than position commands, making direct comparisons difficult. To be meaningful, comparisons must be based on like servo funcions.

Accommodating intelligent drives. These smart drives can take over functions normally performed by the controller, thus allowing it to execute its other tasks faster or control more axes. Intelligent Sercos drives can compensate for friction, gravity, reversal error, and leadscrew error. They also provide velocity and acceleration feedforward for better tracking of motor position to profile, and, if the controller provides the necessary information, drives can compensate for cross-axis coupling on machines with nonorthoganal axes, such as robots.

The drives can also provide dualloop control, where axis position is sensed directly from the load; and axis velocity, from the motor. Dual-loop control is used with rotary motors where a motor sensor by itself cannot measure backlash or other mechanical inaccuracies. Sercos drives can also support other functions to ease the burden on the controller, such as highspeed capture, spindle-orient, homing functions, feed-to-positive stop, simple gearing and camming, and system I/O extension.

A look at analog velocity drives

The analog velocity drive is the basis of the traditional servo system. The drive closes all servo loops except the position loop, which is closed by the controller. Here, the controller sends a speed command to the drive. The drive then closes the velocity loop, commutates the motor, closes the current loops, and operates the power stage. The power stage sends current to the motor, and a resolver or encoder feeds back position information to both the controller and drive.

These systems have several weaknesses, chief among them being command resolution and susceptibility to noise. There are many wires -- about a dozen for feedback, command, ground, shield, enable, and drive-OK. Also, because feedback must go to both the drive and controller, there are two sets of feedback connections for each axis in the system, which adds up to about 6 to 8 wires per axis.

Another problem is that the electrical grounds for the drive and controller must be interconnected. High-powered drives generate a lot of noise, and this can disrupt the computerbased equipment that the drives are connected to.

Still other problems result from the servo system being split. The position loop is in the controller, and the velocity loop is in the drive. Tuning the servo loops can be confusing. The split loops can also make complex compensation moves difficult to do, such as velocity and acceleration feedforward.

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Understanding torque or current drives

This type of motion system uses the controller to close the position and velocity loops. The drive accepts the controller's torque output signal, commutates the motor, closes the current loops, and controls the power stage. Like the analog velocity drive system, the power stage sends current to the motor, and a resolver or encoder feeds back position information to both the drive and controller. One common variation of this configuration combines the position and velocity loops into a single PID position loop.

Sometimes torque drives are referred to as "current" drives, because, in brush motor applications, torque and current are proportional. However, most industrial servo applications use brushless dc motors, where there is a significant different between torque and current. Torque drives should also not be confused with two-phase current drives, where the controller commands the sinusoidal phase currents in the motor.

Torque drives are not as sensitive to noise in the current signal as velocity drives. Also, since the velocity and position loops are united in the controller, resolution is better.

However the number of wires between the controller and drives remains at about a dozen per axis, and, again, the grounds of the controller and drives must be interconnected. The controller has to work harder, too, since it closes both the position and velocity loops.

How power block drives work

Here, the controller closes all servo loops and outputs commands to the power stage. The command signals from the controller, which turn the power transistors on and off, are digital; however, the feedback is analog, with two signals representing motor phase current. The power block drive eliminates some of the weaknesses of the other two drives. First, the position feedback is only connected to the drive, which reduces the number of connections there. Also, since neither the velocity or the torque command signals have to be converted to analog, they remain highly resolved and generally free from noise. This drive is also less expensive.

There are problems, however. This configuration places the greatest burden on the controller. Since commutation and current loops are closed in the controller, the controller wire count grows to about 15 to 20 connections per axis, including the feedback sensor. The grounds of all the components must still be interconnected. Also, the current feedback signals have to be high bandwidth and are more sensitive to disturbances. Even minor wiring problems can undermine the current loop. These problems are usually difficult to solve at the installation site because the controller and drive operate so closely together that it is difficult to isolate the sources of problems.

George Ellis is program manager for new business development at Kollmorgen Motion Technologies Group in Radford, Va. He can be reached at (540) 633-5512 begin_of_the_skype_highlighting (540) 633-5512 end_of_the_skype_highlighting or gellis@kollmorgen.com.

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