The complex, high-speed, high-precision control required for the multiaxis coordinated motion is implemented using a specialized computer called a motion controller. A complete motion control system consists of:
- Human-machine interface (HMI)
- Motion controller
- Transmission mechanisms
The HMI is used to communicate with the motion controller. The HMI may serve two main functions:
- Operating the machine controlled by the motion controller
- Programming the motion controller
Control panels with pilot lights, pushbuttons, indicators, digital readouts, and analog gauges are common hardware-based HMIs to serve the purpose of operating a machine.
Control panels can also be software-based. These panels can have touch screens and embedded computers that run a graphical user interface developed using the software. The advantage of this type of panel is the ease of reconfiguration of the HMI as new features may be added to the machine in the future.
A computer is interfaced with the motion controller for programming purposes. Custom software provided by the manufacturer of the controller is used to write, edit, download, and test the machine control programs. The software also includes features to test motors, monitor input/output (I/O) signals, and tune controller gains.
The motion controller is the “brain” of the system. It generates motion profiles for all axes, monitors I/O, and closes feedback loops. The controller generates the motion profile for an axis based on the desired motion parameters defined by the user or the programmer. While the machine is running, it receives feedback from each axis motor. If there is a difference (following error) between the generated profile and the actual position or velocity of an axis, the controller creates correction commands, which are sent to the drive for that axis. The controller can also generate and manage complex motion profiles including electronic camming, linear interpolation, circular interpolation, contouring, and master-slave coordination.
Motion controllers are available in different form factors. The integrated form factor incorporates the computer, the drive electronics for the axes, and the machine I/O into a single unit. This unit is called a motion controller or drive. In a modular system, the computer, the drives, and the machine I/O are separate units connected to each other via some communication link. In this case, just the computer is called the motion controller.
A complete motion controller consists of the following:
- Interpretation of user programs
- Trajectory generation
- Closing the servo loops
- Command generation for the drives (amplifiers)
- Monitoring axis limits, safety interlocks
- Handling interrupts and errors such as excessive following (position) error
2. I/O for each axis
- Motor power output
- Servo I/O for command output to amplifiers
- Input terminals for feedback signals from a motor or other external sensors
- Axis limits, homing signals, and registration
3. Machine I/O
- Digital input terminals for various sensors such as operator buttons and proximity sensors
- Digital output terminals to drive external devices (usually through relays)
- Analog inputs (often optional) for analog sensors such as pressure, force
- Analog outputs (often optional) to drive analog devices
- Network communications with other peripheral devices, the host computer, and/or supervisory system of the plant using protocols such as DeviceNet®, Profibus®, ControlNet®, EtherNet/IP®, or EtherCAT®
- USB or serial port communications
- HMI communications
The command signals generated by the controller are small signals. The drive amplifies these signals to high-power voltage and current levels necessary to operate a motor.
Therefore, the drive is also called an amplifier. The drive closes the current loop of the servo system as discussed. Therefore, it must be selected to match the type of motor to be driven. In recent trends, the line between a drive and a controller continues to blur as the drives perform many of the complex functions of a controller. They are expected to handle motor feedback and not only close the current but also the velocity and position loops.
An actuator is a device that provides the energy to move a load. Motion control systems can be built using hydraulic, pneumatic, or electromechanical (motor) technologies.
A transmission mechanism is used to connect the load to the motor of an axis. It helps meet the motion profile requirements. When a load is coupled to a motor through a transmission mechanism, the load inertia and torque are reflected through the mechanism to the motor.
Feedback devices are used to measure the position or speed of the load. Also, the drive and the controller use feedback to determine how many current needs to be applied to each phase of the motor. The most common feedback devices are resolvers, tachometers, and encoders. Encoders can be rotary or linear.
In addition, encoders can be incremental or absolute. The selection of the feedback device depends on the machine’s desired accuracy, cost, and environmental conditions. A different type of feedback is provided to the controller from detection sensors such as proximity switches, limit switches, or photoelectric sensors. These devices detect the presence or absence of an object. For example, a photoelectric sensor may detect the arrival of a product on a conveyor and signal the motion controller to start running the conveyor.