SENSORS AND ENCODER n encoder that provides feedback on the position of the load. The advantage is that this compensates for backlash and elasticity within the system to manage precise control over the output. In addition to dual encoders, the hardware needed includes a position controller, such as maxon’s Epos4, that can manage a structured three-layer control approach, including a current loop, a speed loop, and an outer load-position loop. The innermost control loop provides fieldoriented control of motor current based on measurement of the current as a feedback signal. A middle control loop, the auxiliary control, modulates motor speed based on the motor shaft’s encoder, while the outermost control loop, known as the main control, manages the position of the load based on the output encoder. Looking at this process in detail, the main control loop relies on a proportional controller, as well as a gain scheduler, and a second-order filter – all of which are in the controller firmware. The loop receives the desired position, velocity, and acceleration from the controller’s path planner, and the actual load position from the outputmounted encoder. The gain scheduler reduces the effects of gear backlash by adjusting the proportional controller gain automatically to correct any tracking error as the load-position error decreases, improving stability when crossing backlash zones. To counteract elasticity between the motor and load, a second-order notch filter suppresses resonant frequencies to stabilise the system, reducing chattering and oscillation. Meanwhile, the auxiliary control loop uses a PI (proportional integral) controller that corrects both the current error and the accumulated error over time, also adding feedforward. The controller also coordinates with an observer that estimates motor speed using motor encoder position data and motor current measurements. To simplify commissioning of dual-loop control, maxon’s Epos Studio software uses an auto-tuning wizard to set the parameters. The tuning procedure can perform identification experiments including motor inertia/friction and main-loop dynamics. Dual-loop control can resolve backlash and compliance challenges with both rotary or linear load encoders, enabling precision in linear stages, actuators and gantries. While a dual-loop control set-up requires robust wiring and a failsafe strategy for encoder redundancy, this approach reduces reliance on mechanical components. In summary then, strainwave gearboxes are the preferred choice where space is constrained and zero-backlash mechanical transmission is essential. Dual-loop control may be more suitable for systems that need belt drives or planetary gearboxes, but precision remains essential, provided there’s capacity to manage the requirements in set-up. n Leuze is setting new standards in the miniaturization and integration of safety laser scanners with its RSL 200. The exceptionally compact size and innovative connection technology allow unprecedented possibilities for sensor integration. The powerful device reliably safeguards machines, systems, automated guided vehicles and autonomous mobile robots. Minimum size, maximum safety. The smallest safety laser scanner on the market. www.leuze.com Maxon’s Epos4 controller can manage a structured three-layer approach to control
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