Motion control generally refers to the transformation of predetermined control schemes and planned instructions into desired mechanical motion under complex conditions, achieving precise position control, speed control, acceleration control, torque or force control of mechanical motion.
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 its application in specialized machines because the latter have simpler motion patterns and are often referred to as general motion control (GMC).
1. Composition of the motion control system
1) A motion controller is used to generate a feedback loop for the trajectory point (desired output) and closed position. Many controllers can also internally close a velocity loop.
2) A driver or amplifier is used to convert the control signal (usually a speed or torque signal) from the motion controller into a higher-power current or voltage signal. More advanced intelligent drives can close their own position and speed loops to achieve more precise control.
3) An actuator such as a hydraulic pump, cylinder, linear actuator or motor is used to output motion.
4) A feedback sensor, such as a photoelectric encoder, rotary transformer, or 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.
5) Numerous mechanical components are used to convert the motion of the actuator into the desired motion, including gearboxes, shafts, ball screws, toothed belts, couplings, and linear and rotary bearings.
2. Motion Controller Principle
Motion controllers perform four basic tasks: transmitting position feedback codes; issuing positioning commands or motion waveforms; closing position loops; and compensating for stability. Of these four tasks, transmitting motor position signals and closing position loops are the most fundamental. The motor position is determined by feedback signals, sometimes with incremental encoders, and by comparison signals with the commanded position. The difference between the actual position and the commanded position is called the position error.
The controller's task is to minimize position errors under conditions of no oscillating load. In most cases, this task can be accomplished by a proportional-derivative (PDT) or PID control algorithm. The PID output signal is fed into an amplifier and the motor via a digit-to-analog converter. The mathematical expression for the PID controller algorithm is:
Among them, the proportional coefficient KP is related to the response speed; KD provides stability and damping; and the integral coefficient KI determines the accuracy of the system. Adjusting these three coefficients can tune the servo system to its optimal response state.
Motion controllers also have the function of generating waveforms, producing time-independent position functions corresponding to the required velocity waveforms. The basic requirements of motion can be expressed as total distance, transition speed, and acceleration. Because the motor position always corresponds to the commanded position, the motion waveform controls the path and speed of the motion. In addition to these basic tasks, an advanced motion controller can perform higher-level functions, such as processing host commands, task sequencing, I/O processing, and error handling. These special functions enable the controller to operate safely and independently.
Working principle of motion controller
A typical motion control system mainly consists of moving parts, transmission mechanisms, actuators, drivers, and motion controllers. The motion commands for the entire system are given by the motion controller, which is the soul of the entire motion control system.
Current motion controllers have at least 256 standard program buffers, allowing up to 256 motion programs to be stored in memory. While one coordinate system is already executing a program, another program can run in any coordinate system. The number of motion programs that can be executed simultaneously is limited only by the number of defined coordinates. While motion programs run synchronously and orderly in the foreground, the motion controller can run up to 32 asynchronous PLC programs in the background. These programs perform some of the functions of a programmable logic controller (PLC).
The role of motion controller
Motion controllers are used to achieve precise position control, speed control, acceleration control, torque or force control of mechanical motion.
Motion controllers can be classified by structure into PLC programmable logic controllers, microcontroller controllers, stand-alone motion controllers, PC-based motion control cards, and network controllers.
A motion controller is an electronic device used to control the motion and position of a motion system. It can receive feedback signals from sensors and output commands to control the motion and position of motion actuators.
There are many types of control methods for motion controllers, including:
Position control: Controlling the position of the actuator, typically using the PID control algorithm.
Tracking control: enables the actuator to follow a given trajectory or curve, typically using advanced control algorithms such as model predictive control (MPC).
Force control: Controlling the movement of the actuator based on external forces or torques.
Torque control: Control is performed with torque as the target, and is usually used in applications that require precise control, such as robotic arms.
Speed control: Controls the rotational speed or movement speed of the actuator.
Pressure control: Controlling the pressure in the hydraulic system and regulating the movement of the actuator.
Current control: controls the current of the actuator, typically used for DC motor control.
Motion controllers are widely used in many fields, especially in AC servo and multi-axis control systems. They can make full use of computer resources, allowing users to easily plan motion paths, execute predetermined actions, and achieve high-precision servo control.
Based on the different power sources used, motion control can be mainly divided into electrical motion control using electric motors as power sources, gas-liquid control using gases and fluids as power sources, and heat engine motion control using fuels (coal, oil, etc.) as power sources. According to statistics, over 90% of all power sources are electric motors. Electric motors play a vital role in modern production and daily life; therefore, among these types of motion control, electrical motion control is the most widely used.
Electric motion control evolved from electric motor drives. Electric drive or electric transmission is a general term for control systems that use electric motors as the control objects. Motion control systems are diverse, but from a basic structural perspective, the hardware of a typical modern motion control system mainly consists of a host computer, motion controller, power drive device, electric motor, actuator, and motion controller feedback detection device. The motion controller is a control device that uses a central logic control unit as its core, sensors as signal-sensitive elements, and electric motors or power devices and actuators as the controlled objects.
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.
In summary, motion controllers have been applied across numerous fields, particularly in AC servo and multi-axis control systems. They fully utilize computer resources, conveniently assisting users in planning motion trajectories, completing predetermined movements, and achieving high-precision servo control. Motion control technology will continue to integrate with AC servo drive technology, linear motor drive technology, and other technologies, driving the continuous advancement of mechatronics technology in my country.
