What are the differences between a PLC and a motion controller? Why is a motion controller better than a PLC?
What is PLC?
A programmable logic controller (PLC) is a digital electronic system specifically designed for industrial applications. It employs a programmable memory that stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, controlling various types of machinery or production processes through digital or analog inputs and outputs.
Features of PLC
(1) High reliability. Since most PLCs use single-chip microcomputers, they have a high degree of integration. In addition, the corresponding protection circuits and self-diagnostic functions improve the reliability of the system.
(2) Easy to program. PLC programming mostly uses relay control ladder diagrams and command statements, which are much fewer in number than microcomputer instructions. Except for mid-range and high-end PLCs, most small PLCs only have about 16 instructions. Because ladder diagrams are visual and simple, they are easy to learn and use, and even programming can be done without computer expertise.
(3) Flexible configuration. Because PLC adopts a modular structure, users can easily change the function and scale of the control system by simply combining them. Therefore, it is applicable to any control system.
(4) Complete input/output function modules. One of the biggest advantages of PLC is that it has corresponding modules for different field signals (such as DC or AC, switch quantity, digital quantity or analog quantity, voltage or current, etc.) that can be directly connected to devices in the industrial field (such as buttons, switches, current transmitters, motor starters or control valves, etc.) and connected to the CPU motherboard through the bus.
(5) Easy installation. Compared with computer systems, PLCs do not require a dedicated computer room or strict shielding measures. During use, simply connect the detection devices to the actuators and the PLC's I/O interface terminals correctly for normal operation.
(6) High operating speed. Since PLC control is executed by program control, its reliability and operating speed are unmatched by relay logic control. In recent years, the use of microprocessors, especially with the widespread adoption of single-chip microcomputers, has greatly enhanced the capabilities of PLCs and narrowed the gap between PLCs and microcomputer control systems, especially for high-end PLCs.
What is a motion controller?
A motion controller is a dedicated controller that controls the operation of an electric motor. For example, a limit switch controls an AC contactor to move an object upwards to a designated position and then downwards. Alternatively, a time relay can control the motor to rotate forward and backward or to rotate intermittently. The application of motion control in robotics and CNC machine tools is more complex than in specialized machines because the latter have simpler motion patterns and are often referred to as general motion control (GMC).
Features of motion controllers
(1) The hardware composition is simple. The system can be formed by simply plugging the motion controller into the PC bus and connecting the signal lines.
(2) Development can be carried out using the abundant software already available on PCs;
(3) The motion control software has good code universality and portability;
(4) There are many engineers who can carry out development work, and they can carry out development without much training.
Control methods of motion controllers
Point-to-point motion control: This only requires control over the endpoint position and is independent of the intermediate process or trajectory of the motion. The corresponding motion controller needs to have a fast positioning speed and employ different acceleration and deceleration control strategies during the acceleration and deceleration phases of the motion.
During acceleration, to enable the system to quickly reach the set speed, the system gain and acceleration are often increased, while an S-curve deceleration control strategy is used at the end of the deceleration phase. To prevent vibration after the system reaches its destination, the system gain is appropriately reduced after the planned position is achieved. Therefore, point-to-point motion controllers often have the capability of online variable control parameters and variable acceleration/deceleration curves.
Continuous trajectory motion control: Also known as contour control, this type of control is mainly used in the motion contour control of traditional CNC systems and cutting systems. The corresponding motion controller needs to solve the problem of ensuring both the contour accuracy of the machining process and the constant tangential velocity of the tool along the contour during high-speed motion. For machining small line segments, multi-segment program preprocessing functions are available.
Synchronous motion control refers to the coordinated motion control between multiple axes. This can involve synchronization across the entire motion process or localized speed synchronization during motion. It is primarily used in systems requiring electronic gearboxes and electronic cams. Industrial applications include dyeing, printing, papermaking, steel rolling, and synchronous shearing. The corresponding motion controller algorithms often employ adaptive feedforward control. This automatically adjusts the amplitude and phase of the control variable to ensure that a control action with the same amplitude but opposite phase to the disturbance is applied at the input, thus suppressing periodic disturbances and ensuring synchronous control of the system.
The difference between PLC and motion controller
Motion control mainly involves the control of stepper motors and servo motors. The control structure is generally: control device + driver + (stepper or servo) motor.
The control device can be a PLC system or a dedicated automation device (such as a motion controller or motion control card). When a PLC system is used as a control device, although it has the flexibility and certain versatility of a PLC system, it is difficult to achieve high precision, such as interpolation control, and requires sensitive response, or programming is very difficult, and the cost may be high.
With technological advancements and accumulated expertise, motion controllers have emerged, embedding some common and specialized motion control functions—such as interpolation instructions. Users only need to configure and call these function blocks or instructions, thus reducing programming difficulty and offering advantages in performance and cost.
Alternatively, it can be understood as follows: a PLC is used as a common motion control device. A motion controller is a special type of PLC, specifically designed for motion control.
Motion controllers are better than PLCs
PLCs are responsible for logic control. They are weak in motion control, although many high-end PLCs now come with motion control modules.
However, the sole requirement for motion control is speed—fast computation, fast response, and fast feedback. Therefore, motion control eliminates many unnecessary components, using a high-speed DSP as its dedicated computing core. In other words, motion control places great emphasis on performance. Therefore, to achieve ideal motion control results, it's best to choose a professional motion control module.
PLCs are weak in motion control, although many high-end PLCs come with built-in motion control modules. However, the sole requirement for motion control is speed—fast computation, fast response, and fast feedback. Therefore, motion control eliminates many unnecessary components, using a high-speed DSP as the dedicated computing core. In other words, motion control places great emphasis on performance. Therefore, to achieve ideal motion control results, it's best to choose a professional motion controller.
