MOTION CONTROL n current control loops are the key feedback processes for these devices. Because current and torque share a linear relationship in DC brushed and brushless motors, a current controller sets the input torque command, which it converts to a current command, measuring the current applied and correcting it as required. The control hierarchy In the hierarchy of motion control, the motor controller is next in line to the motor. This device includes a power stage that modulates the voltage and current supplied to the motor to execute the motion commands. Motor controllers, such as maxon’s Escon speed controller or Epos4 position controller, are configurable, meaning that their built-in firmware lets you adjust settings such as speed, torque and position limits, as well as communications options. The motion commands are generated by a motion controller, which is the next level up in the architecture. Motion controllers, such as maxon’s MicroMACS6 or MiniMACS6, are programmable, meaning that you can set the motion trajectory plans and interpolation, including parameters such as torque and acceleration. Within this hierarchy, the motion controller commands the motor controller, or coordinates several motor controllers in multi-axis applications, to execute the motion commands. The choice of operating mode with closed-loop operation dictates the architectural responsibility between motion and motor controllers. In a profile-based, point-to-point operating mode, the motion controller sets parameters and commands the motor controller. The motor controller is then responsible for processing motion profiling or trajectory generation, often based on a trapezoidal profile, where it accelerates at a constant rate to a constant speed, maintains that speed, and then decelerates at a constant rate to stop, forming a trapezoid shape when plotting speed against time. This kind of point-topoint mode is not suitable for real-time applications because communications between the motion controller and motor controller exchange pre-defined setpoints at a fixed rate, and cannot react to external events or to real-time control inputs. Real-time control When real-time control is required, a cyclic operation mode can be used, with responsibility for trajectory generation shifting to the motion controller that processes motion profiling and transmits updated set values to the control loops to deal with ongoing inputs. This also requires real-time communications using protocols such as CANopen or EtherCat. Moving up to a higher level, in a strongly coupled application, maximum responsibility is given to the motion controller, which processes motion profiling and control loops across multiple axes. Here, the motor controller is configured purely as a current controller and serves only as a power amplifier, following the commands of the motion controller. The difference is that while the cyclic operation mode might still operate to a preset, point-to-point profile – albeit reacting to external inputs in real time – in a strongly coupled application, the motion controller can calculate advanced trajectories in real time, and across multiple axes. Controller selection Selecting a controller requires a holistic approach, which goes through to the requirements placed on the motors themselves. It is crucial to perform a thorough situational overview of the application, defining the mechanical layout and intended motion control task, along with boundary conditions such as spatial envelopes, compliance factors, environmental conditions, and service life requirements. While system design and controller selection are based on the principles of motion, tailoring an application is an iterative approach, rather than a theoretical study. Describing the drive system clearly in its environment, and defining the requirements clearly from outset, helps designers to arrive at an optimum arrangement more quickly. In fact, the most common challenge we see during system design and selection is insufficient formulation of drive system requirements. Selection tools, such as drive system configurators, can be extremely useful to assist when developing motion systems. But to ensure that all bases are covered, reviewing a comprehensive plan with an engineer dedicated to drive system design will ensure that any design issues are identified and resolved early in the process, without compromising the design at later stages. Most importantly, if the drive system plays a central role in the application, dedicated engineering expertise will make sure that the motion cycle is optimised to achieve the best performance. n Motion controllers, such as maxon’s MiniMACS6 multi-axis devices, are programmable, allowing you to set the motion trajectory plans and interpolation
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