Servo System Overview

This topic provides a quick lesson in servo system - an overview of what it is and how it works.

What is a Servo System?

A servo system essentially comprises an intelligent servo drive and a servo motor that operates with a PLC or CNC to perform complex, specialized moves in one or more directions, or axes. These complex and specialized moves, which are needed in the automation of industrial tasks, are collectively known as motion control.

Servo systems are applied in many different field for automation - in the motor industry, the petrol industry, the textile industry, in packaging systems, warehousing systems and so on.

Closed Loop Servo Systems

In a servo system, feedback information - motor position and motor velocity is sent from the feedback unit of the motor back to the servo amplifier. The servo amplifier analyzes the feedback, makes adjustments as needed, and generates new currents to bring the motor to the commanded velocity. This cycle constantly repeats itself in a closed loop. A closed loop that controls the position of the shaft or load is called a position loop. A closed loop that keeps the velocity of the motor on the commanded value is called a velocity loop.

Servo System Components

A servo system consists of:

Servo motor	A servo motor moves machinery in a single axis of motion. Electrical motors are driven by magnetic fields. Motors have a stationary field generated by the magnets of the motor and a rotating or movable field called stator winding or armature. They operate on the principles of synchronous motors. All rotary motors have some type of bearing that supports the rotor at each end. Every motor has at least two magnetic motor poles, normally four or six. The servo amplifier generates the current in the stator so that a controllable torque is available at the shaft. The servo motors turn (travel) in two directions - positive and negative. Two forms of angular measurement are commonly used in motion control - degree measurement and radian measurement, where 360 degrees constitute one revolution or 2π radians. The servo amplifier operates with standard synchronous servo motors as well as with direct drive motors (rotary or linear), induction machines and DC motors. For more information about these motors see the motor manuals. Motor Stabilizing Stabilizing (tuning) the motor is a fundamental task in achieving best system performance. To stabilize a motor, you must set up initial values for and adjust several motion parameters using . These parameter settings compensate for the difference between the actual motion and the commanded motion - getting the actual as close to the commanded as possible, with minimal oscillation and noise. This difference is called following error. Observe the application notes for Setup of unknown motors.
Load	The load is the machinery and equipment that each motor drives. It is everything connected to the output shaft of a motor, including the shaft itself. A motor must be appropriately sized to its load to ensure the motor is powerful enough to carry out your automation tasks. A servo system delivers and converts motion to a load via one or more of the following mechanical techniques: Direct drive: motor connected to a rotating table Screw drive: motor connected to a lead screw carrying a slide (moving table) Rack and pinion: motor connected to a cogwheel that moves a rack Belt and pulleys: motor connected to rollers that move conveyor belts or chains and sprockets
Feedback device	Every closed-loop servo system needs at least one device to return feedback information from each motor (or load) to servo amplifiers . Depending on the feedback device, feedback is transmitted back to the servo drive in the form of digital signals or analog signals. Two types of feedback devices are supported: Encoder - returns analog or digital signals (optical) Resolver - returns analog signals (magnetical)
Servo amplifier	The servo amplifiers comprise a three-phase, power supply, and high-performance control unit all housed in a single enclosure. The several control loops are realize totally digital in the micro controller.

Feedback Units

Servo motors are available with these feedback units:

Resolver
Encoder
Incremental encoder with HALL

n a closed-loop feedback system, the innermost loop is the commutation loop, which monitors the motor's rotor and ensures that it keeps spinning.

Outer loops are: Position loop, Velocity loop and Current loop Velocity information and the velocity loop are derived from (are computed based on) position information.

The current loop is also known as a torque loop, since amplitude of the electrical current is directly proportional to torque. Torque is force applied in an axis of rotation. All available feedback systems are listed in the KDN on page Multi-Feedback.

Resolver

A resolver can be thought of as a transformer whose output is unique for any given shaft position (an absolute position feedback). The transformer is driven with a sinewave reference signal. Two AC signals are returned from the resolver into the Sine and Cosine inputs. All three of these sinewave signals are low-level and susceptible to noise.

The servo amplifier can use single (two poles) or multi-speed (multiple poles) resolver feedback to calculate primary position, velocity, and commutation information.

Encoder

Encoders direct pulses of light, from a light source at the motor or load, to photo detectors through an encoded disk. These light pulses are then converted into digital feedback information. There are two general types of encoders - rotary and linear. Rotary (rotating disk) encoders are typically mounted to the motor shaft. Linear encoders are typically mounted to the load.

The Motion Profile

Overview

Motion operations are universally embodied in a graph called the motion profile. Understanding and using motion profiles to define your motion application is an important part of achieving best system performance.

The motion profile plots one or more motion operations and measures it against time.

Commanded motion: the motion that is supposed to happen ideally and precisely, without error, when the motor executes a velocity or position command

Actual motion: the motion that really happens in the motor, when a velocity or position command is executed

Closing the Gap between Setpoint and Actual

Best system performance is achieved when you can stabilize or "dampen" the difference or "close the gap" between the commanded motion and the actual motion. This difference is called following error. Stabilizing the servo system means setting the relevant parameters in the servo amplifier, to get as close to the commanded position as possible.

Basic Motion Profile Characteristics

Commanded and actual motion profile shapes have the following characteristics that are also universal to all motion operations:

Characteristic	Meaning
Moving	Moving refers to the execution of a motion instruction that makes the motor move. A motion profile's moving portion represents most of the profile - the motion itself. The motor is considered moving for as long as the motion controller is commanding new positions. The point at which motion stops is known as the target position.
In Position	When a motion command stops executing, and the motor slows to within a few counts of its target position, the motor is considered to be stopped, or "In Position." A range of positions, typically plotted in a motion profile, represents in position. That is, In Position is signaled when the motor gets close enough to the target position -- within its In-Position range that you have specified, via its parameter. An In-Position signal is often used to make sure the motor stops before the machinery continues its operation.

Characteristic

Meaning

Moving

Moving refers to the execution of a motion instruction that makes the motor move. A motion profile's moving portion represents most of the profile - the motion itself. The motor is considered moving for as long as the motion controller is commanding new positions. The point at which motion stops is known as the target position.

In Position

When a motion command stops executing, and the motor slows to within a few counts of its target position, the motor is considered to be stopped, or "In Position." A range of positions, typically plotted in a motion profile, represents in position. That is, In Position is signaled when the motor gets close enough to the target position -- within its In-Position range that you have specified, via its parameter. An In-Position signal is often used to make sure the motor stops before the machinery continues its operation.

Limits and Ranges of Operation

Overview

Another important task in achieving best system performance is setting certain motion limits and ranges of operation to protect equipment from damage and to optimize operational efficiency.

Two Types of Settings

There are two types of settings for motion limits and ranges of operation:

Type of Setting	Meaning
Fault limit	Fault limits are settings that signal errors when certain limits on motor movement, such as speed and position, as well as electrical current, are exceeded. Fault limits are designed to protect equipment from damage and can cause the drive and motor to shut down. For example, every motion control system has hardware limit switches, which are used in the position loop to set a limit on how far the actual motor position can deviate from the commanded position before a fault is signaled. You may also program software limits. The difference, or gap, between commanded position and actual position is known as following error. Such a limit protects against motor runaway and stalling.
Tolerance band	Tolerance bands are set and specify the safe, efficient physical ranges for the equipment. Some of these tolerance bands do the following: In the current loop, set a limit on the amount of electrical current to the drive and motor. This protects the motor from damage that would be caused by excessive current. In the position loop, place a limit on how far the motor can travel in a positive or negative direction. Set a range of positions that are considered to be In Position. That is, this range specifies how far the motor can deviate from its commanded position and still be considered in the correct position.

Type of Setting

Meaning

Fault limit

Fault limits are settings that signal errors when certain limits on motor movement, such as speed and position, as well as electrical current, are exceeded. Fault limits are designed to protect equipment from damage and can cause the drive and motor to shut down.

For example, every motion control system has hardware limit switches, which are used in the position loop to set a limit on how far the actual motor position can deviate from the commanded position before a fault is signaled. You may also program software limits. The difference, or gap, between commanded position and actual position is known as following error. Such a limit protects against motor runaway and stalling.

Tolerance band

Tolerance bands are set and specify the safe, efficient physical ranges for the equipment. Some of these tolerance bands do the following:

In the current loop, set a limit on the amount of electrical current to the drive and motor. This protects the motor from damage that would be caused by excessive current.
In the position loop, place a limit on how far the motor can travel in a positive or negative direction.
Set a range of positions that are considered to be In Position. That is, this range specifies how far the motor can deviate from its commanded position and still be considered in the correct position.

Acceleration and Deceleration

Overview

If the servo amplifier is operated with motion tasks under position control, different acceleration/deceleration profiles can be chosen. It depends on the mechanical structure of the machine and the required dynamical quality, which profile should be chosen. If the machine tends to sway (e.g. robot arm), sine² would be the best choice. Here the torque is altered linear and the velocity characteristic becomes square. This reduces the excitation to sway. Disadvantage of this profile is the double up of the acceleration/deceleration time.

If the machine is mechanically stiff and there are high requirements in dynamics, the linear profile should be chosen. This leads to a torque step at the beginning and the end of each acceleration/deceleration ramp.

Two Types of Acceleration and Deceleration

The following table describes the two fundamental acceleration and deceleration types, linear and square. A motion profile may accommodate a combination of these two types.

Type	Description
Trapezoidal	Trapezoidal is a rate of acceleration and deceleration that represents a steady speed-up and slow-down.
Sine²	To limit any jolting, the drive is accelerated/decelerated within the acceleration time along an acceleration ramp without any discontinuities. The resulting speed characteristic corresponds to a sine² curve.

Further acceleration curves are stored in profile tables. You can create your own profile tables, information for that can be found in KDN.