Robots perform pre-planned tasks, such as assembly line work, surgical assistance, warehouse picking/retrieval, and even dangerous tasks like landmine clearance. Today's robots can not only handle highly repetitive tasks but also perform complex functions requiring flexibility in direction and movement. With technological advancements, increased speed and flexibility, and reduced costs, robots will be widely adopted. Their lower cost compared to human labor also gives us a glimpse of the future for the robotics industry. Furthermore, advancements in machine vision, computing power, and networks will further drive the widespread adoption of robots.
The realization of these high-performance robots is due to improvements in the following aspects:
1. Complex sensors; 2. Computational capabilities and algorithms for real-time decision-making and action; 3. Motors that rapidly and accurately advance mechanical power to accomplish complex tasks;
When selecting the specific type and model of motor, designers need to consider three primary factors:
1. Minimum and maximum speeds (and acceleration) of the motor; 2. Maximum torque that the motor can provide, and the relationship between torque and speed curves; 3. Accuracy and repeatability of motor operation (without sensors and closed-loop control);
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-sized robot actuators, the drive motor options typically include brushed DC motors, brushless DC motors (BLDC), and stepper motors. (However, in some cases, hydraulic and pneumatic motors are the best choice.)
Brushed DC motors are the oldest DC motor technology, the simplest, and the lowest cost. Due to the contact between the brushes and the rotor, the rotation of the rotor switches (commutates) the magnetic field of the windings wound around the rotor. The motor speed is a function of the applied voltage, so the drive requirements are not high, but managing torque is difficult. Reliability issues also exist due to brush wear, the need for cleaning and maintenance, and the potential to become a source of electronic noise (electromagnetic interference). Because of these problems, brushed DC motors are, in most cases, the least attractive choice in robot design.
Brushless DC motors emerged in the 1860s, benefiting from two key developments: the advent of robust, compact, and low-cost permanent magnets; and the development of small, efficient electronic switches (typically MOSFETs) to switch the current flowing to the windings. Electronic commutation replaced the mechanical commutation of brushed motors in controlling the switching of the magnetic field. The interaction between the fixed switching coils and the magnets on the rotating core replaced the mechanical commutation of brushed motors, utilizing the interaction between the magnetic and electric fields. By changing the switching frequency of the MOSFET, the motor speed can be controlled. Furthermore, compared to brushed motors, its motor controller offers better control over motor performance.
Even better, advanced algorithms such as PID (Proportional-Integral-Derivative) correction algorithms or FOC (Field-Oriented Control, sometimes also called Vector Control) control algorithms can be embedded into the motor controller. This allows the ideal motor operation to be matched with the actual load and load variations, resulting in more powerful and precise motor performance. For example, the motor control algorithm/program can take into account factors such as rotor inertia and enable the motor driver to adapt and gradually reduce errors caused by mechanical factors. Such algorithms make precise control of acceleration and torque possible.
Compared to brushed motors, brushless motors (BLDC) require more complex control circuitry but can exhibit superior performance. Typically, BLDC motors need to be equipped with a position feedback sensor, such as a Hall effect sensor, optical encoder, or back EMF detection device.
Another commonly used BLDC motor in robots is the stepper motor, which utilizes a switching electromagnet located next to the central core of the permanent magnet ring. Stepper motors do not "rotate" in the conventional way; instead, they gradually increase their speed by using a continuously rotating shaft, thus achieving rotation at a specific angle or continuous rotation. Stepper motors have repeatable motion control; they can return to a previous position when needed.
The step angle ranges from 1.8° (200 steps/revolution) to 30° (12 steps/revolution). The step angle or number of steps depends on the number of permanent magnets in the motor, but values outside this range are also possible.
Stepper motors will remain in their original position if energized but without a stepping direction; they can provide high torque at low RPMs. The most direct way to make a stepper motor rotate is to sequentially switch an electromagnet on and off, but this can introduce jitter or vibration. There is some overlap in the application areas of brushless motors and stepper motors. Stepper motors are better suited for applications requiring precise forward and backward movements (such as pick-up and place-up) rather than those requiring continuous rotation over long periods, and are also suitable for small applications that do not require high torque or speed from the motor. Furthermore, stepper motors have lower energy efficiency requirements than brushless DC motors.
Besides the motors listed here, many other types are available. The motor family is extensive and complex, with numerous subcategories. For example, the permanent magnet synchronous motor (PMSM) is a combination of a brushless DC motor (relative to the rotor) and an AC induction motor (relative to the stator structure). It features high energy efficiency, high relative density in a small volume, high torque-to-weight ratio, fast response time, and relatively easy control, but it is also relatively expensive.
The robot motion system involves more than just motors; it comprises three main functional modules.
1. Real-time controllers take the following three forms: High-speed computational processors for general-purpose applications, running motion-control firmware; FPGAs for DSP applications; and dedicated controller IC circuits with hardwired connections and built-in algorithms.
2. One or more cascaded driver layers to extract lower-level signals from the controller output and then output the high voltage/current required to control the on/off state of the control electronics.
3. MOSFET (or other switching devices, such as IGBT or bipolar transistor), which controls the current flowing to the motor windings.
The specific MOSFET selected depends primarily on the current and voltage requirements of the motor and windings. Once the MOSFET model is determined, the driver is chosen, and the driver selection is determined by the MOSFET's rating; sometimes a series of boost drivers may be needed, depending on the MOSFET size.
Problems that may be encountered when selecting a controller
The selection of a controller model is also highly strategic, requiring a decision before choosing a specific supplier and model. There are many trade-offs to consider when choosing between a general-purpose processor for motor control, a high-performance FPGA, or a dedicated control IC circuit (usually from a specific motor control supplier). Designers need to consider factors such as: what level of control algorithm complexity do you require, and how many I/O ports are needed?
Who provides the control algorithms and code: IC suppliers, third-party partners, or unrelated third-party developers? How do they verify and validate the performance of the motor and its applications?
How much user programming ability do you need? Even dedicated, non-programmable controllers require users to select algorithm type and closed-loop control mode.
(Position, velocity, or acceleration), and some operating parameters need to be set.
Do the motor and application have unique properties that need to be configured? If the answer is yes, then a programmable IC would be a better choice. Conversely, if no algorithm modifications are required, a dedicated IC with hard-wired, fixed algorithms would be preferable to a fully programmable IC.
Does the controller need to support multiple motor types? Even for the same type, does the controller only need to support a certain size of motor within that model, or does it support a range of sizes?
What level of technical support does the supplier provide? What hands-on experience do they have in developing motors? Will they provide specific reference designs that they have previously built and validated, including the interface circuitry between the control IC and the MOSFET driver?
Are there any regulatory issues to be aware of, such as authorized energy efficiency assessments?
(Many motor applications now must meet various "green" environmental requirements.) If so, do suppliers understand these issues, and do their components and algorithms meet these requirements?
The development kit demonstrates the performance of the controller and interfaces.
For many engineers, integrating all the components—including controllers, drivers, MOSFETs, etc., with embedded or standalone algorithms—is a multi-departmental task, one they don't want to start "from scratch." For this reason, many vendors offer evaluation boards or even complete kits that include controllers, example algorithms, drivers, and MOSFETs. For example, the Freescale MTRCKTSPNZVM128 Three-Phase Sensorless PMSM Kit uses sensorless motor control technology to drive three-phase BLDC or PMSM motors. This kit is designed to support rapid prototyping and evaluation using back EMF by leveraging an integrated ADC module on the microcontroller. Furthermore, this kit (featuring an MC9S12ZVML12 microcontroller) can also be configured for sensor-based evaluation using Hall sensors or resolvers.
With technological advancements, including the creation of more precise execution through improved motor control and sensing, the future of robotics looks very promising. Revolutions in key areas such as sensing, control, and motors will continue to influence the evolution of robotics technology.
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