The motion controller is the core of the entire motion control system. Its function is to execute the written program, collect field I/O signals, realize various calculation functions, control the program flow and I/O devices, and communicate with the operator station and other field devices.
Based on the core technology solutions of motion controllers:
Based on the core technology of motion controllers, they can be mainly divided into analog circuit-based, microcontroller-based, programmable controller-based, general-purpose computer-based, application-specific integrated circuit ( ASIC )-based, programmable logic device-based, and digital signal processor ( DSP )-based types.
Motion controllers are classified according to the controlled object:
Motion controllers can be categorized based on the controlled object into stepper motor motion controllers, servo motor motion controllers, and motion controllers that can control both stepper motors and DC or AC servo motors.
Based on the system architecture of motion controllers:
Motion controllers can be mainly divided into bus-based motion controllers, stand-alone motion controllers, and hybrid motion controllers.
Bus-based motion controllers utilize existing hardware and operating systems, combined with user-developed applications, to achieve motion control, and possess high-speed data processing capabilities. The main bus interfaces include ISA , PCI , VME , RS232 , and USB .
Standalone motion controllers integrate the controller, I/O , user interface, and communication interface into a single housing. Servo loop updates, I/O , and user interface are all handled by appropriate internal software. This type of controller cannot provide the flexible communication and user interface of bus-based controllers, and integration into large systems is more difficult. However, from an application perspective, both types of motion controllers have their advantages: bus-based motion controllers are easy to integrate into systems and have excellent networking capabilities and openness ; standalone motion controllers offer greater flexibility and ease of system upgrades and optimization.
Hybrid motion controllers are assembled from a motion controller and a servo drive. They have the advantages of independent motion controllers, and can also connect multiple servo drives together for coordinated control through various methods and protocols.
Motion controllers are classified according to their position control principles:
Based on the position control principle, i.e. whether there is a detection feedback sensor and its detection device, motion controllers can be divided into three basic types: open-loop, semi-closed-loop, and closed-loop.
1. Open-loop motion controller
Without position detection feedback, the actuator motor is typically a stepper motor. The biggest advantages of this type of motion controller are ease of control, simple structure, and low cost. The displacement command signal flow issued by the motion controller is unidirectional, therefore there are no stability issues. However, because mechanical transmission errors are not corrected by feedback, the position accuracy is generally not high.
2. Semi-closed-loop control motion controller
Position feedback utilizes an angle detection element, directly mounted on the end of the servo motor or leadscrew. Due to its position feedback comparison control, high positioning accuracy is achieved. Since most mechanical transmission components are not included in the system's closed-loop circuit, relatively stable control characteristics are obtained. Errors in mechanical transmissions such as the leadscrew cannot be corrected through feedback, but their accuracy can be appropriately improved using software setpoint compensation.
3. Fully closed-loop control motion controller
Using detection elements such as gratings to detect the position of the controlled unit can eliminate transmission errors in the entire mechanical transmission chain from the motor to the controlled unit, resulting in high static positioning accuracy. However, because the friction characteristics, rigidity, and clearance of many mechanical transmission links within the entire control loop are nonlinear, and the dynamic response time of the entire mechanical transmission chain (compared to the electrical response time) is very large, it is difficult to correct the stability of the entire closed-loop system, and the design and adjustment of the motion controller are also quite complex.
Motion controllers are classified according to the nature of the controlled variable and the motion control method:
1. Position control
Motion controller position control is classified into point-to-point motion control and continuous trajectory motion control according to the control principle.
Point-to-point control is a type of positioning control that does not control the movement trajectory between points, nor does it perform processing or measurement during the process.
Continuous trajectory control is further divided into linear control and contour control. Linear control refers to the controlled object moving in a straight line along a certain direction at a certain speed (single-axis or multi-axis linkage), during which machining or measurement is performed ; contour control controls the instantaneous position and velocity of two or more coordinate axes, forming a contour curve or surface in a plane or space through linkage.
2. Speed control and acceleration control
The speed control of the motion controller can be used alone or combined with position control to form a dual-loop control. However, the main loop is position control, and the speed control is used as feedback correction to improve the dynamic performance of the system.
3. Synchronous control
Motion controller synchronization control involves the synchronized motion control of two or more axes in terms of speed or position, such as systems requiring electronic gearboxes and electronic cams. Some systems, in addition to requiring simultaneous start-up, also demand position synchronization, requiring high synchronization accuracy.
4. Force and torque control
Rollers for plastic film, steel strip, cloth, and paper use constant tension control. In applications such as tightening nuts in automatic assembly machines and automatic drilling, motion controllers should employ torque and position synchronization control.
Motion control system architecture
A motion controller is used to generate trajectory points (desired output) and close a position feedback loop. Many controllers can also internally close a velocity loop.
A driver or amplifier is used to convert control signals (typically speed or torque signals) from a motion controller into higher-power current or voltage signals. More advanced intelligent drives can close their own position and speed loops for more precise control.
An actuator, such as a hydraulic pump, cylinder, linear actuator, or motor, is used to output motion.
A feedback sensor, such as a photoelectric encoder, a rotary transformer, or a Hall effect device, is used to feed back the position of the actuator to the position controller in order to achieve the closure of the position control loop.
Numerous mechanical components are used to convert the motion of an actuator into the desired motion; these include gearboxes, shafts, ball screws, toothed belts, couplings, and linear and rotary bearings.
Typically, the functions of a motion control system include:
Speed control
Point-to-point control . There are many methods to calculate a motion trajectory, which are usually based on a velocity curve such as a triangular velocity curve, a trapezoidal velocity curve, or an S- shaped velocity curve.
Electronic gears (or electronic cams). This means the position of the driven shaft mechanically follows the position of the driving shaft. A simple example is a system containing two turntables that rotate at a given relative angle. Electronic cams are more complex than electronic gears because they make the follow-up relationship between the driving and driven shafts a function. This curve can be non-linear, but it must be a functional relationship.
Motion controller selection
1. Determine the type of servo motor based on the working characteristics of the equipment to be developed.
2. Determine the number of motor shafts to be controlled and the motor operating mode.
3. Determine the position detection and feedback mode, and select whether to use a photoelectric encoder, optical encoder, or magnetic encoder.
4. Determine the number of input/output switches.
5. Based on the above, select a suitable motion controller.
