Glossary Positioning systems
Part III - Control and feedback electronics
Controller
Integrated into positioning systems, the controller produces electronic control signals to output control commands to the system. If a feedback system has been installed, then the actual position values are sent back to the controller, which compares the actual values with the rated ones and generates a new signal that corrects deviations.
The controller transmits the control signals to the motor driver module. A controller features often several functions, for instance, interfaces, inputs/outputs, memories for movement programs, and evaluation of encoder feedback signals when positioning is governed in a closed-loop setup.
The controller has to work considerably faster since it calculates the control signals, and sends new signals according to the incoming actual values. The time required for this operation is called cycle time.
DC-motor driver
DC-motor driver modules convert a weak voltage signal (mostly ±10 V) output by the controller in current by means of which the motor is driven.
Digital Signal Processor
Digital signal processors (DSPs) are chips, which have been especially designed for high computing capacities to meet the demands raised by complex control algorithms. Conventional processors are too slow for such sophisticated control tasks. In many cases, DSPs are designed in such a way that commands are executed and data processed in parallel and not in sequential mode. Many chips are equipped with a high-speed hardware multiplier and own a memory to exclude many delays in data transfer.
Cycled high-voltage drivers
Simple 4-phase drivers are quite suitable for basic low-capacity applications only. With high speeds, commutating of the current turns out to be a difficult job under inductive loads. Applying a voltage to a coil, the current (and consequently the torque) reaches its rated value on an exponential basis. With high pulse rate, the powering up phase is too short. In such short time, the current cannot ramp up to the rated value and thus, the generated torque reaches only a portion of its nominal value as well.
Three criteria determine the time span until the rated current will be reached: Inductivity of the coils, electrical resistance, and applied voltage.
The inductivity cannot be reduced, but the voltage can be increased for a short while in order to reach faster the rated current. A widely used method means to use clocked high-voltage drivers.
For example, supplying a stepping motor with 20 Volt for a short time which normally consumes 2 Volt only, then the rated current can be reached 10-times faster. As soon as the desired rated current has been reached, the driver is excited to keep the current at its rated value.
Motor driver module
The motor driver receives signals form the controller and converts them into power to drive the motor. As for stepping motors, full-step, half-step and micro-step resolution as well as the desired power can be selected.
Stepping motor drivers - Half step
When operated in half-step mode, the rotor of the stepping motor passes half the way between two stop positions.
In this way, the resolution is increased and the movement gets smoother over the entire speed range. After cutting off the power supply the motor continues rotating until reaching at the next stop position.
Stepper motor driver - Micro step / Mini step
A micro step driver moves the rotor of the motor by a defined angle between two stop positions. Micro-step and mini-step modes are used to improve resolution, to avoid resonance effects, and because of their better smooth running ability, are applied over the whole speed range. After cutting off the power supply, the motor continues rotating as well. Due to this appearance, tables equipped with a brake are installed in Z-design systems (vertical-axis setup).
Stepper motor driver - Full step
When firing a stepping motor in full-step mode then the rotor is moved from a stop position to the next full-step one. After cutting off the power supply the motor stays in that position retained by its strong holding moment.
Central and hierarchic architecture
Controllers can be designed in that way that a single microprocessor controls all kinds of movement, or controllers are integrated in a hierarchic structure in which a central microprocessor co-ordinates specific processors which govern one axis each.