Issue link: https://resources.mouser.com/i/1442757
20 Programmable Versus Fixed-Function Controllers: Alternatives for Complex Robotic Motion By Bill Schweber for Mouser Electronics Control of today's sophisticated robot arms, regardless of their size or power, often requires simultaneous management along multiple axes for their motion control. Modern electronics–the motors, power-switching devices (Metal-Oxide Semiconductor Field-Effect Transistors [MOSFETs] or Insulated-Gate Bipolar Transistors [IGBTs]), device drivers, control systems (now digital, formerly all analog), and feedback sensors–now make achieving precise motion control easier than it was just a few years ago (Figure 1). At the same time, however, the demands on system performance have increased dramatically, so the overall project is as difcult as ever. Nonetheless, there's one unavoidable fact: Robotics is largely a mechanical function, so the realities of such systems must be part of the control loop. These include gear backlash, mechanical tolerances, vibration, motor performance, rotating mass inertia, momentum, exing of mechanical structures, variable loads, and more. For these reasons, it is important to decide what type of motor is the best t—usually the choice is between brushless DC motors and stepper motors in low/ moderate power situations. Another necessary decision is related to sensor-based feedback. Most robotic applications use some type of feedback sensor to accurately gauge the end-effector's position, and thus velocity and acceleration (recall that velocity is the time integral of position, and acceleration is the time integral of velocity). This feedback transducer can be a Hall- effect sensor, a synchro/resolver, or an optical encoder. While it is easiest to put the encoder on the motor, placing it there may not provide required data about the end-effector's actual situation, with sufcient accuracy for the application, due to mechanical issues noted above. Therefore, the sensor may need to be mounted closer to the load endpoint. Some motion-control applications operate without a sensor, which reduces cost and mechanical complexity. Rather than using a sensor for feedback, Sensorless Field-Oriented Control (FOC, also called vector control) uses precise, synchronized readings of the current and voltage at each phase of the motor windings; FOCs then perform complicated frame- of-reference transformations and matrix calculations in real time to determine the motor's position. Eliminating the Figure 1: A basic motion-control system for robotics includes algorithm-execution functions, motor drivers, power devices, and a feedback path; mechanical linkages, motor, and sensor (in most cases); and voltage and current measurement and control at key points. (Source: National Instruments)