In-depth analysis of stepper motors, servo motors and robot motion control-EEWORLD

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Author: Frank Lamb

Troubleshooting and maintenance of motion control and robotics requires a deep understanding of all components and mechanisms in the robot.

Motion control involves the use of motors to position and precisely move actuators. While motion control is not always closed-loop control, it is different from closed-loop control, where the primary goal is to achieve and verify a known position or motion.

Principle and maintenance

A stepper motor is a brushless DC motor with a stator consisting of multiple magnets arranged around a gear-like rotor. The magnets are divided into groups, called phases. Each phase is energized at the same time, causing the motor to "step" to the next position.

The drives are activated in the proper sequence based on the stepper motor's resolution. A typical stepper motor has a resolution of 200 steps per revolution, but using "microstepping", 1600 steps per revolution can be achieved. Stepper drives are sometimes also called "choppers".

Stepper motors are usually run without a feedback device (such as an encoder or encoder), which makes them a cheaper positioning method than a stepper motor, but they also don't have much holding torque. In addition to the motor and drive, an indexer is also required. It can be built into the drive and communicate with the master, or a controller such as a Logic Controller (CLP) sends pulses to index the drive. Troubleshooting a stepper system may include checking voltages and communications in the drive, or even using a DAC to view pulses.

Servo system components and maintenance

A servo or servomechanism is a device that uses feedback to control position and torque. They can be electric, hydraulic, or pneumatic, but most servos used in electronics are motor-driven.

Servo motors can be brushed permanent magnet DC motors, brushless permanent magnet AC motors, or AC induction motors. They usually have an encoder or built in. They are also often integrated with a gearhead. There are two cable connectors on the motor assembly so that the signal from the encoder/(feedback cable) can be transmitted to the motor separately.

A servo drive accepts pulse inputs from an encoder and monitors torque. The servo drive and brake control signals are sometimes included in the control cable. In general, servo drives are more complex than servo drives and often have built-in logic functions. Modern controllers almost always have high-speed communication ports that can connect to other controllers to coordinate motion. Usually this is a communication protocol based on , but sometimes fiber optics are used.

The servo control algorithm is based on D position or torque control. The motor needs to be adjusted according to the motor and load characteristics to ensure optimal performance. It is for this reason that motors and drives from the same manufacturer are often sold and used as a package. Some motors have a drive and controller built in. These "integrated servo" motors can be networked to perform complex tasks or used as a standalone positioner.

An important difference between a servo motor and a typical AC induction motor controlled by a frequency converter is that a servo motor has holding torque at zero speed. If the motor shaft drifts from its position under control power, it will attempt to correct itself, which can cause the controller to malfunction if the preset position is not reached.

When coordinating motion, a "master" controller or position is often used to adjust the speed of other controllers. The movement of one axis depends on the position change of another axis or virtual axis. It is important to use a fast speed dedicated to the motion system. A dedicated motion controller can be used to coordinate the servo axes. Through integration, the hand is guided to the correct position. Motion controllers can be integrated into the PLC rack or used as a stand-alone system. Many have separate I/O modules and can be programmed using IEC 61131 PLC languages.

Troubleshooting a servo system usually requires an understanding of the platform software in addition to typical diagnostic methods. Drives and controllers often have built-in diagnostic capabilities to detect problems with the motor and the load to which it is connected. Mechanical components such as couplings can also fail.

Path perception for robot motion control

Used for manufacturing and material handling tasks, their physical configuration depends on the functionality required. Payload and speed requirements help determine the type of machine used in a specific application.

A robot can have as many as 6-7 axes of motion, or as few as 3. To reach any point in the XY plane, two axes of motion are required; to reach any point in XYZ space, three axes of motion are required. To fully control the position of the tool at the end of the "arm", in addition to the X, Y, and Z axes, an additional three axes are required: pitch, roll, and yaw.

Figure 3 shows the six axes needed to reach any point and direction in three-dimensional space, but robots use different coordinate systems and their joint configurations may be different. The origin and direction are also defined differently from robot to robot and can usually be changed in software.

▲Robot coordinate axes and right-hand rule.

The X, Y, and Z positions are called Cartesian coordinates, but they can be defined from different reference points. If defined from the base of the robot or an environmental reference point, they are called world coordinates. In this case, the origin reference frame is stationary. When addressed from the perspective of the effector, they are called tool coordinates, and the origin moves with the end effector. This can also include an offset from the tool connection point to where the tool contacts the part. Local coordinates can also be defined, usually with the origin set within the work area. This allows reference points to be replicated for pallets or other in-place systems.

Individual joints can also be controlled independently, usually defined in degrees. Distances are usually defined in metric units (mm), but can also be scaled to user-defined units in the software. In addition to X, Y, and Z, roll, pitch, and yaw can also be defined with other letters, such as U, V, and W.

The area that a robot can reach is called the work envelope. Planes and spaces can be defined within the chassis to prevent collisions or ensure safety, and safety devices such as light curtains can be integrated into the robot work cell.

Robot controllers are constantly performing calculations to ensure they know where the robot is relative to reference points and paths. Axes must work together when moving along a defined path, so robots are the ultimate form of coordinated motion control. This is why controllers are often used to support the task of achieving and maintaining position.

An important issue to understand when working with robots is singularities. This is a situation where the robot cannot move its end effector along a specific path due to physical or mathematical constraints. The robot may end up in a position where it cannot rotate the tool around a specific point, which is sometimes referred to as gimbal lock. In other physical configurations, moving a joint through certain orientations may damage associated cables or hoses, so care needs to be taken when moving the robot close to a singularity or rotating an axis too far apart. There are often multiple joint configurations that can achieve the same tool position and orientation, which are often referred to as redundant degrees of freedom.

Robot controllers are often capable of performing logic functions and operating external devices, but they are usually built into the work cell and connected to a "master" controller such as a PLC. The controller can be connected to the robot cell via power and signal cables, or it can be built into the robot base. The connection can be a physical 24 V connection, a communication link, or a "pass-through" and through internal routing to the end effector or tool. The pass-through port usually includes a pneumatic hose. Robots can be classified according to their physical configuration. People.

6-axis articulated arms are more common in heavy payload applications, while 4-axis SCARAs are often used for directional pick and place. Delta configurations are very fast and are often used in industry for component placement. Another term to be aware of is collaborative robots (Cobot): designed to interact directly with humans in a shared space, which is different from the configuration shown here.

Making robot programming easier

Robots can be programmed using a computer or through a teach pendant. Two types of code need to be programmed: programs and position data. In order to perform a task of moving from one location to another, the robot end effector first needs to have a start and end point defined, and then needs to be programmed how to get there. This may involve additional locations, and also require external signals to tell the robot that an object exists or to start moving.

The positions can be defined by listing them in the software, but it is easier to use a teach pendant. The teach pendant allows the operator to move the various axes to "drive" the robot to the desired position. This is generally done at low speeds for accuracy and safety. There is also a 3-position "deadband switch" that needs to be pressed while maneuvering the robot. The spring-loaded switch needs to be held in the middle position, and if it is pressed all the way or released, the robot will not move.

A program is a series of moves to different positions. They can be triggered individually or chained together. There are many different languages used in robotics, and they are often proprietary to the manufacturer. They are usually similar to languages like B or Assembly, with JUMP and MOVE statements. Other high-level scripting languages can also be used to build data structures or create math, such as calculating paths or positions. Some languages allow for parallel processing, allowing the robot to perform multiple actions at once, such as calculating movement vectors while a camera is tracking a moving object.