Tutorial: Servo systems

In the early 1980s, when machinery was producing engines and transmissions for automobiles and downtime costs were in excess of thousands of dollars a minute, manufacturers needed a solution to protect uptime and, in case of failure, return to production as quickly as possible. Satisfying this need for maximum uptime was the catalyst for the servo systems we know today. Historically, servo-based motion control has been viewed as a costly collection of components for high-performance applications that demand precision and process efficiency. Today, due to decreasing costs brought about through today's high-volume production and technological advances, servo-based solutions are frequently being implemented in low-performance applications, where their advantages can be realized without increasing overall costs.

To better understand the benefits that servos bring to a variety of packaging applications, it's important to have a basic understanding of a motion control system and, more specifically, an understanding of servo motors and servo drives.

Basic motion control system A basic motion control system includes:

Motion controller-The motion controller is the brain of the motion control system. It provides the instructions that the servo motor executes. Servo motor-The servo motor is the muscle of the motion control system, converting the electrical power from the servo drive into the mechanical power that moves the machine.

 
Servo drive (also called an Amplifier)-The servo drive, or amplifier, receives low-level commands from the motion controller and greatly amplifies the commands to provide the necessary power to the servo motor.

Feedback device-The device, such as an encoder or resolver, provides real-time position and velocity information as feedback to the motion controller.

Servo motors
Servo motors are best known for their rapid acceleration and deceleration capability, made possible by delivering high-peak torque in conjunction with a high torque-to-inertia ratio. Serving applications with power demands ranging from hundreds of watts to more than 75 kw, servo motors are famous for their high dynamic response and precision accuracy in traditional motion control applications, such as machine tools and robotics.

Servo motors may be categorized by these four criteria: magnet type (induction or permanent), mechanical technology (rotary or linear), electrical technology (AC brushless or DC brushed), and construction (housed or frameless).

An induction, or asynchronous, servo motor generates its torque through the difference in rotational velocity of the rotor's magnetic field and the stator's magnetic field-the greater the difference, called slip, the greater the torque. AC voltage is applied to the stator of the servo motor, which in turn induces a magnetic field in the rotor-the reason this type of motor is referred to as an "induction" motor. Asynchronous servo motors are most frequently encountered when high power is required.

A permanent magnet, or synchronous, servo motor has a stator similar to an induction motor. Unlike the induction servo motor, however, this type of motor incorporates permanent magnets mounted to the rotor. The magnetic fields of the rotor and stator rotate together, synchronously, at the same rotational velocity. As load is applied to the motor, a slight positional alignment difference between the magnetic poles takes place, causing torque to be produced-the greater the alignment difference, the higher the torque produced.

Servo motors most frequently create rotary motion, which is then translated into linear motion via mechanical components; however, linear servo motors are also available that generate linear motion directly without the need for additional mechanical components. Linear servo motors are available in both induction and permanent magnet-based technology. Their strength lies in their ability to achieve even higher acceleration/deceleration rates, greater potential accuracy, and higher speeds than their rotary counterparts. However, unlike rotary servo motors, which work with gearboxes and similar drive train components to multiply the torque available to the application by trading off speed for torque, these linear servo motors must directly develop the full force required by the application. Consequently, their use is usually limited to smaller machines.

The AC brushless servo motor, the workhorse motor for modern positioning applications, is really the permanent-magnet servo motor previously described. This is in contrast to the DC brushed servo motor (also a permanent-magnet synchronous motor), which dominated the servo motor scene two decades ago. The DC brushed servo motor had the magnets mounted to the stator with DC power applied to the rotor through carbon brush assemblies. As the motor rotated, the polarity of the voltage applied to the rotor through the brushes was constantly reversed by a mechanical assembly called the commutator. Without this mechanical commutator, the motor would lock in a position and stop moving. Because commutation was accomplished through mechanical means and power supplied to the rotor through the sliding contact of the brushes, mechanical wear and maintenance were always a problem. The AC brushless servo motor eliminates the need for brushes, and commutation takes place electronically inside the servo drive, yielding zero wear and zero maintenance.

Finally, housed- versus frameless-construction servo motors represent some of the more recent developments in the packaging of servo motors. The traditional servo motor is of housed construction and is a standalone component bolted onto the machine. Mechanical power is transmitted to the machine through the motor's shaft, which is connected to the machine using various methods, such as a coupling, a belt and pulley arrangement, or a gearbox.

Available today for the most demanding applications are frameless, or "kit," motors, which are integrated directly into the machine structure. The machine builder must now integrate the rotor, stator and feedback components. These motors represent the pinnacle of dynamic performance and occupy significantly less space than their housed counterparts.

Servo drives
Like servo motors, servo drives have distinct features. For example, servo drives are available with a sealed construction, functioning as a standalone unit that mounts directly to the machine, or with an open construction, which is intended for mounting within a cabinet. Servo drives are also available with a distributed power supply or a self-contained power supply, and with the option of regenerated energy handling through storage (capacitors), "burning up" (bleeder resistors), or line regeneration that returns energy to the power line.

Each application is unique; for best results, the motion system designer should match the servo system to the application. For axes that make hundreds of strokes per minute, it makes sense to store regenerated energy in capacitors, making it available again for the next acceleration. For axes running at a high speed and making fewer cycles per minute, the sensible solution is to burn off excess energy in resistors. Axes, such as those doing tensioning, are running in constant regeneration, making the ability to send this energy back up the power line the best choice. When a machine involves many axes being controlled from a central location, a distributed-power-supply-type system makes the most sense. While some axes are accelerating, others are decelerating, thus reducing the power drawn from the power line as well as reducing (or eliminating) the need for additional capacitors or bleeder resistors to handle the regenerated power. Selecting the right servo for the application lowers costs, improves performance, maximizes robustness and reduces downtime.

Servo benefits
The modularity and space-saving size of servos provide OEMs and end users with greater machine flexibility, reductions in makeready time and related waste, and improved quality by eliminating the shortcomings of mechanical components. The long-term, reliable, repeatable performance of servos can add increased throughput and speed to a broad range of motion control applications. Overall, the cost of ownership is significantly offset by the savings resulting from decreased waste during startup and changeover, as well as from reduced wear as a result of fewer components.

In addition to their attractive productivity and accuracy advantages, servo-based systems are typically operator-friendly and provide networking options that span a wide range of protocols such as Ethernet TCP/IP, Profibus-DP, DeviceNet, INTERBUS-S, ControlNet, and the Rexroth standard, SERCOS. SERCOS is the only open standard that exists for coordinating and synchronizing digital-based motion controls-it is the digital interface between the motion control system and its drives. This communications standard (recognized by the IEC as IEC-61491) allows many servo drives to be connected to a motion controller via a common, noise-immune fiberoptic cable. Once in place, the SERCOS network connections form a loop, as opposed to hard wiring each drive directly to the motion controller. What makes the SERCOS standard truly unique is that it solves the coordination of multiple axes of motion on automation equipment through a protocol that guarantees nanosecond synchronization. It also provides standardization of the format used to exchange variables between control and drive.

Networking options like SERCOS facilitate increased communication speeds, noise immunity, open, standardized hardware, and less cabling and wiring, which further simplify component replacement and system updating. Servo-based systems can also provide valuable diagnostic functions designed to aid troubleshooting and greatly reduce the time required for repairs, and most are available with a PC interface and HMI options.

Essentially, servo technology is nothing new. However, in the past few years its affordability has made it an option for smaller applications, and with the ever-present speed and precision demands that are placed on manufacturers, servo-driven designs are quickly becoming the new standard.

This tutorial is provided by Bosch Rexroth U.S., 847/645-4073.