Course Audit: Introduction to closed-loop control
All motion control systems aim to maneuver loads down paths with a predefined motion. If this intended motion is achieved by comparing actual load motion to desired motion and then making corrections, the system is closedloop. In fact, closed-loop systems today are often servosystems. The word servo — actually an abbreviation of the word servomechanism — is defined in Webster’s dictionary as an automatic device for controlling large amounts of power by means of very small power, and automatically correcting the mechanism’s performance.
There are many servosystem types — electric, hydraulic, pneumatic, and even pure mechanical systems. They all share the same basic feedback and continuous correction characteristics. Electric servosystems are especially common. They consist of five basic elements: Servomotor, comparator, amplifier, feedback device, and trajectory or command generator.
The comparator looks at what the motor is doing — feedback — and compares this against what the motor should be doing — command. If there is a difference — error — the amplifier converts the low-level error signal to high current signals going to the motor in such a way to minimize the difference. This configuration is the basis for most closed-loop servosystems.
Most suppliers of motion control equipment have relatively simple single-axis servo motion control products that are very similar to the basic system described above. Certain controllers even provide this basic servo capability combined with PLC logic functions.
Parameters
There are a number of design considerations for motion systems.
• Speed. How fast does the controlled device have to move? This parameter is typically specified in rpm or inches per minute. It can also be expressed as the time it takes to get from point A to point B.
• Torque. How hard does the motion control device have to work to move the load? This parameter is expressed in rotational units as a force through a lever arm, lb-ft, or lb-force for linear systems.
• Accuracy. How close to the ideal motion path does the motion control system have to perform? How close does it have to be to the ideal command position when it is moving and/or when comes to rest? This parameter is often expressed as an error between actual and desired position. The error units are typically degrees in rotary systems and inches in linear systems.
• Inertia. How much torque is required to change the speed of the moving parts? Inertia is a physical parameter that defines the resistance of all physical parts to changes in speed or direction. The smaller and lighter the parts, the easier it is to change the speed.
Traditional notions about motion control are based on a few assumptions. First, a system that requires high power or torque will be a slow-speed system. Second, a very precise or accurate system will typically run more slowly and be low torque. Finally, a system that is required to perform complex motion also requires more engineering expertise to design.
As motion control system technology advances, the limitations of speed, torque, accuracy, and complexity have become less compromising. Still, there are clear divisions between motion control systems, so that applications determine which controller is best suited for the job.
What “servomotor” means
The term servomotor implies that the motor will be used in a high-performance control system with feedback — in other words, a closed-loop system. The basic principles used in servomotors are similar to other ac and dc motors. The main difference is that servomotors are optimized in the following ways:
1. Size and weight of the rotor is reduced to minimize inertia.
2. Heat buildup within the motor is also minimized. Fins and special materials are used to dissipate heat to the surrounding air or mechanical structure, while motor parts are built with special high-temperature materials.
3. All servomotors are built with provisions to mount feedback devices right into the motor. Feedback devices like encoders and resolvers (to measure shaft speed and position) are commonly mounted inside the motor housing.
The most commonly used servomotors used in industrial applications are dc permanent magnet brush and brushless types, and ac induction type motors. Advances in power electronic devices and microprocessor control have played a major role in the growth of permanent magnet brushless and ac induction servomotors. The elimination of the sliding contacts in brush-type motor commutators has increased motor performance and reliability of servomotors.
Electric servosystem concepts
A typical rotary system includes several components. Its servomotor converts winding current to mechanical torque, producing the motion in a motion control system. A feedback device transmits motor shaft position to the comparator. From there, the position feedback is compared to the command position. The output from the comparator is the difference between the two and is called the position error. Continuing on, the servo-amplifier converts the comparator output (position error) to high current, which is then applied to the servomotor winding.
The output of the servo amplifier is connected in such a way to cause the motor to rotate in the direction to minimize position error. Finally, a command generator provides the desired or command position signal that tells the motion control system how to move the servomotor and load.
These basic system components provide the means for a design engineer to turn a concept into a real working system.
At rest
The block diagram in Fig. 2 is the basis for a broad variety of systems. The basic mechanism used is negative feedback. The output of the system — the actual motor position — is measured and compared to the commanded position. The goal is to make both positions the same for zero error.
Assume that the system has been turned on with the motor shaft and the command generator both at 0°. The system would be at rest, and there would be no motion. The result of position feedback compared to the desired position is called position error; in this case it is zero, so the amplifier would not produce any current to move the motor.
Now let’s assume that some outside force or torque moves the motor shaft 1° clockwise. The position feedback signal is now 1° clockwise, but the command signal is still at 0°. That’s when the comparator detects this difference and responds by commanding the amplifier to produce counterclockwise torque. This torque turns the motor shaft counterclockwise in response to the outside torque (which moved the shaft clockwise.) Since the system activity is continuously monitored, the comparator senses the shaft’s counterclockwise direction and responds by decreasing its signal position error to the amplifier. As the motor rotates back to its 0° position, the position error decreases until the motor shaft is at 0°. Finally, the comparator’s output returns to zero and the system is at rest.
In fact, the same sequence of events occurs in reverse if the motor shaft is displaced in a counterclockwise direction. In closed-loop servosystems all components are bidirectional and provide equal response in both the clockwise and counterclockwise directions.
Houston, do you copy?
Closed-loop motion systems make no distinction between shaft disturbances and command signal changes. Both cause the motor shaft to move to the commanded position. It follows then that if the command signal is changed to any position, the system would respond by moving the motor shaft accordingly. It’s important to keep something in mind, though. Real-world servo motion control systems have certain limitations that do not yield the ideal performance described above.
1. Amplifiers and motors can only produce a certain amount of speed and torque. If the load on the motor shaft or speed is more than what the motor and amplifier can produce, the system will have a large position error and cease to be a practical system.
2. Feedback devices are not perfect and cannot detect changes in shaft position that are less than the resolution of the sensing mechanism. This limitation produces motor shaft position errors that go undetected and uncorrected.
3. All servosystems have limitations on how rapidly they can respond to changes. This can cause the motor shaft to respond imprecisely to rapid command sequences or changes in load. In extreme cases, the system can become unstable; system response may be so slow that by the time it responds, the motor shaft may already be moving in a different way. This type of problem is called instability or oscillation and can actually cause a system to never come to rest or remain hunting for the commanded position indefinitely.
To a large extent, these system limitations can be compensated for in the design of an actual motion control system. Positional accuracies of a few millionths of an inch are within today’s design capabilities.
Microprocessor-a-run-run
No single component has enhanced motion control technology like the application of microprocessors. Fig. 3 shows an expanded block diagram of a typical motion control system. The key element added is a microprocessor. Microprocessors can provide both simple and complex commands and implement the comparator section. As we’ve seen, these commands provide precise movement of the motor shaft. Because microprocessors can store a group of commands in memory, it can execute them in a sequence or as individual commands based on logical decisions concerning external events.
Fig. 4 shows the block diagram of a system in which the microprocessor’s role has been expanded to control and coordinate the functional relationships between the motion requirements, machine input and output, communications with other devices, and the human/machine interface (HMI). In many applications this coordination requirement is just as demanding as the motion control.
1. The input/output or I/O functions of a motion control system usually involve interfacing with the balance of the machine. Push buttons, limit switches, indicator lamps, and solenoids are examples of I/O functions frequently required in industrial applications. Another use for I/O in motion control systems is the interface to other machine control devices such as programmable logic controllers, temperature controllers, remote I/O, and even connect to the Internet.
2. Modern industrial control systems often coordinate the activities of many types of industrial controllers. Motion, temperature, logic, and measurement systems may need to be coordinated to produce a particular part. This type of coordination is typically done by linking all these controllers to the host computer via communication ports. Communications between the industrial controllers is presently one of the most debated topics in the industry. Hardware configurations and communications protocols are still not fully standardized from vendor to vendor and user to user.
In today’s global economy it is imperative that manufacturers of industrial control equipment conform to one of the “world’s standards” for communication because users expect seamless system integration. Several world-standard protocols have emerged by shear volume of the installed base: SERCOS, DeviceNet, ProfiBus and Ethernet fall into this category. An argument can be made that these defacto standards are not the highest speed, the most efficient, or even the most cost effective — but they are accepted by the global marketplace and have to be respected.
3. HMIs allow motion controllers to talk to operators. Typical operator interfaces are:
• Push buttons
• Indicator lamps
• Thumbwheel switches
• Alphanumeric displays
• Keypads
• Touchscreens
These devices are located near the machine operator and are used to make changes in the control process, such as production rate or even which product the machine is to make. They also allow operators to monitor the complete machine and provide diagnostics and production data.
Multiple axes
In more and more applications, it’s necessary to coordinate two or more axes to create complex parts. Two basic types of multi-axis architecture are common.
A central control architecture is shown in Fig. 5. This architecture uses a single high-power microprocessor to accomplish all the tasks needed for servo control, I/O, operator interface and communications. This single processor is typically housed in a single box and uses amplifiers that do not have any inherent intelligence.
The distributed control architecture shown in Fig. 6 uses multiple microprocessors to accomplish the tasks. In this embodiment a high-speed communication bus such as SERCOS is used to connect multiple intelligent boxes, each with a specific role in accomplishing the overall machine control.
The central control architecture is in some ways simpler but not as robust in communicating to other control elements without adding additional communication means. Caution must be used to not bog down the single microprocessor with time intensive tasks.
The distributed architecture generally has more processing power and more easily connects to a wider range of control devices. Generally, distributed architecture is a better choice for demanding applications.
The modern motion control system is much more complex than the overview presented here. In reality a microprocessor is required to do other tasks as well, including:
1. Monitor and report on the internal operation of power supplies and high-speed logic operations.
2. Monitor and report motion control errors such as motor and amplifier overloads, hardware failures, disconnected wires, or servo instability.
3. Communicate with the outside world of host computers and operator interface devices with a myriad of formats and protocols.
4. Monitor and tune the servo response parameters so the motion amplifier is stable and provides smooth operation.
All of these tasks must be done continuously to provide a reliable motion control system that is easy to use.
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