Robot motor selection considerations
The robot performs pre-planned specific tasks such as assembly line work, surgical assistance, warehouse pick-up/retrieval, and even dangerous tasks such as mine removal. Today's robots not only handle highly repetitive tasks, but also perform complex functions that require flexibility in direction and motion. With the advancement of technology, speed and flexibility, and cost reduction, robots will be widely adopted. The cost advantage below labor also shows us the dawn of the robot industry. In addition, machine vision, computing power and advances in the network will also drive the adoption of robotic applications.
The implementation of these high-performance robots benefits from the following improvements:
1. Complex sensors;
2. Realize the computing power and algorithm of real-time decision-making and action;
3. A motor that quickly and accurately advances mechanical power to achieve complex tasks;
When choosing the type and model of the motor, the designer has to consider three primary factors to consider:
1. The minimum and maximum speed of the motor (and acceleration);
2. The maximum torque that the motor can provide, as well as the relationship between the torque and the speed curve;
3. Accuracy and repeatability of motor operation (when sensor and closed loop control are not used);
Of course, there are many other important factors to consider when choosing a motor, such as size, weight, and cost. For almost all small to medium size robotic drives, the choice of drive motor is usually brushed DC motor, brushless DC motor (BLDC) and stepper motor. (However, hydraulic and pneumatic machines are the best choice in some cases.)
Brushed DC motors are the oldest DC motor technology, the simplest and lowest cost. Due to the contact between the brush and the rotor, the rotation of the motor rotor switches (reverses) the winding magnetic field around the rotor. The speed of the motor is a function of the applied voltage, so the drive requirements are not high, but managing the torque is difficult. Reliability issues arise during operation due to wear and tear on the brush, the need for cleaning and maintenance, and the possibility of becoming an electronic noise source (electromagnetic interference). Due to these problems, in most cases, brushed DC motors have become the least attractive option in robot design.
Brushless DC motors appeared in the 1860s, and they benefited from two developments: first, the emergence of a strong, small, low-cost permanent magnet; second, the emergence of small and efficient electronic switches (usually MOSFETs) ) to switch the current flowing to the windings. "Electrical commutation" replaces the mechanical commutation of the brushed motor to control the switching of the magnetic field. The interaction between the surrounding fixed switching coil and the magnet on the rotating core replaces the mechanical commutation of the brushed motor, ie the magnetic field and the electric field are utilized. interaction between. By changing the switching frequency of MOFSET, the motor speed can thus be controlled. In addition, the motor controller provides better control of motor performance than a brushed motor.
Even better, advanced algorithms such as PID (proportional-integral-derivative) correction algorithms or FOC (field-oriented control, sometimes referred to as vector control) control algorithms can be solidified into the motor controller. This matches the ideal motor operation to the actual load and load changes, making the motor performance more powerful and accurate. For example, the motor control algorithm/program can take into account related factors such as rotor inertia and adapt the motor drive and gradually reduce errors due to mechanical factors. Such an algorithm makes it possible to precisely control acceleration and torque.
Compared with brushed motors, brushless motors (BLDC) require more sophisticated control circuits but can exhibit better performance. Usually a BLDC motor needs to be equipped with a position feedback sensor such as a Hall effect sensor, an optical encoder, or a back EMF detection device.
Another type of BLDC motor commonly used in robots is a stepper motor. In this case, a switch-type electromagnet is used, which is located beside the central core of the permanent magnet ring. The stepper motor does not "rotate" in the usual way; instead, the speed is gradually increased by means of the continuously rotating shaft, so that a certain angle of rotation or continuous rotation can be achieved. The stepper motor has repeatable motion control; it can be returned to its previous position when needed.
The step angle range is from 1.8° (200 steps/rev) to 30° (12 steps/rev). The step angle or the number of steps depends on the number of permanent magnets the motor has, but values outside this range are also achievable. of.
For stepper motors, if energized without step pointing, they will remain in place; stepper motors can provide high torque at low rpm. The most straightforward way to turn the stepper motor is to turn the solenoid on and off, but this can cause jitter or vibration. There are some overlaps in the application areas of brushless motors and stepper motors. Stepper motors are better suited for applications that require precise advance and retreat actions (such as picking and placement), rather than areas that require long periods of continuous rotation, as well as small applications where high torque or speed is not required. In addition, stepper motors have lower energy efficiency requirements than brushless DC motors. In addition to the motors listed here, there are many other types to choose from. The motor series is many and complex, with many branches. For example, a permanent magnet synchronous motor (PMSM) is a combination of a brushless DC motor (relative to a rotor) and an AC induction motor (relative to a stator structure). It has the characteristics of high energy efficiency, high relative density per unit volume, torque to weight ratio, fast response time, and relatively easy control, but the price is relatively high.
