Motion control involves using motors to position and precisely move actuators. While motion control is not always closed-loop control, it differs from motor control , whose primary goal is to achieve and verify known positions or movements.
01
Stepper motor principle and maintenance
A stepper motor is a type of brushless DC motor whose stator consists of multiple electromagnets arranged around a gear-shaped rotor. These circularly arranged magnets are divided into different groups, called phases. When each phase is energized simultaneously, the motor "steps" to the next position.
Microcontroller -based stepper drivers activate drive transistors in the appropriate sequence. A typical stepper motor has a resolution of 200 steps per revolution, but with a "microstepper" driver , 1600 steps per revolution can be achieved. Stepper drivers are sometimes also called "choppers."
Stepper motors typically operate without feedback devices (such as encoders or decoders), making them a cheaper positioning method than servo motors , but they also lack much holding torque. In addition to the motor and driver, an indexer is required. It can be built into the driver and communicate with the main controller , or it can be indexed by the driver via pulses sent by a controller such as a programmable logic controller ( PLC ). Troubleshooting a stepper system may involve checking the voltage and communication in the control circuitry , or even using an oscilloscope to examine the pulse signals .
02
Servo system components and maintenance
A servo, or servo mechanism, is a device that uses feedback to control position and torque. They can be electric, hydraulic, or pneumatic, but most servo systems used in industrial automation are motor-driven.
Servo motors can be brushed permanent magnet DC motors, brushless permanent magnet AC motors, or AC induction motors. They typically have a built-in encoder or resolver . They are also often integrated with a gear head. The motor assembly has two cable connectors, allowing signals from the encoder/ sensor (feedback cable) and motor power to be transmitted separately.
Servo drives accept pulse input from encoders and monitor torque via current . Temperature sensors and brake control signals are sometimes included in the control cable. Generally, servo drives are more complex than frequency converters and typically have built-in logic functions. Modern controllers are almost always equipped with high-speed communication ports for connecting to other controllers to coordinate motion. This is usually an Ethernet -based communication protocol, but fiber optics are sometimes used.
Servo control algorithms are based on PI (Power Injection) position or torque control. The motor needs to be adjusted according to its own characteristics and the load to ensure optimal performance. For this reason, motors and drives from the same manufacturer are often sold and used together. Some motors have built-in drives and controllers. These "integrated servo" motors can be networked to perform complex tasks or used as standalone positioners.
A key difference between servo motors and typical AC induction motors controlled by frequency converters is that servo motors have holding torque at zero speed. If the motor shaft deviates from its position under control power, it will attempt to self-correct, and if it fails to reach the preset position, it will cause controller failure.
When coordinating motion, a "master" controller or positioner is typically used to adjust the speed of other controllers. The movement of one axis depends on the positional changes of another axis or virtual axis. It is crucial to use a high-speed communication network dedicated to motion systems. Dedicated motion controllers can be used to coordinate servo axes. Integrated machine vision guides the robot to the correct position. Motion controllers can be integrated into a PLC rack or used as a standalone system. Many have individual I/O modules and can be programmed using the IEC 61131 PLC language.
Troubleshooting servo systems typically requires understanding the platform software in addition to typical electrical diagnostic methods. Drives and controllers usually have built-in diagnostic functions to detect potential problems with the motor and its connected loads. Mechanical components such as couplings can also malfunction. In short, please read your documentation first.
03
Path perception in robot motion control
Industrial robots are used for manufacturing and material handling tasks, and their physical configuration depends on the required functions. Payload and speed requirements help determine the type of machine used in a specific application.
Robots 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 needed; to reach any point in the XYZ space, three axes of motion are needed. To fully control the position of the tool at the end of the "arm," in addition to the X, Y, and Z axes, three more axes are needed: the pitch axis, the roll axis, and the yaw axis.
Figure 3 illustrates the six axes required to reach any point and direction in three-dimensional space, but robots use different coordinate systems, and joint configurations may also differ. The origin and orientation also have different definitions depending on the brand , and can usually be changed in the software.
▲Robot coordinate axes and right-hand rule.
X, Y, and Z positions are called Cartesian coordinates, but they can be defined from different reference points. If defined from the robot's bottom or an environmental reference point, it's called world coordinates. In this case, the origin reference frame is stationary. When addressed from the actuator's perspective, it's called tool coordinates, where the origin moves with the end effector. This can also include offsets from the tool connection point to the tool contact part. Local coordinates can also be defined, typically with the origin set within the working area. This allows for replicating reference points for pallets or other in-situ systems.
Individual joints can also be controlled independently, typically defined in degrees. Distances are usually defined in metric units (mm), but can also be scaled in the software to user-defined units. In addition to X, Y, and Z, roll, pitch, and yaw can also be defined using other letters, such as U, V, and W.
The area that a robot can reach is called its working range. 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's working cell.
Robot controllers continuously perform calculations to ensure they know the robot's position relative to a reference point and a path. As the robot moves along the defined path, the axes must work together, making the robot the ultimate form of coordinated motion control. This is why controllers are typically used to support tasks that achieve and maintain position.
One important issue to understand when working with robots is the singularity. In this case, due to physical or mathematical constraints, a robot cannot move its end effector along a specific path. The robot may end up in a position where it cannot rotate the tool around a particular point; this is sometimes referred to as gimbal lock. In other physical configurations, moving a joint in certain directions can damage associated cables or hoses, so caution is needed when moving the robot near a singularity or too far from a rotation axis. Often, there are multiple joint configurations that can achieve the same tool position and orientation; this is often referred to as redundant degrees of freedom.
Robot controllers typically perform logical functions and operate 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 built into the robot's base. Connections can be physical 24 V DC connections, communication links, or "straight-through" ports and connectors routed internally to end effectors or tools. Straight-through ports typically include pneumatic hoses. Robots can be classified according to their physical configuration. Figure 4 shows some common robots used in industrial applications.
Six-axis articulated arms are common in heavy-duty applications, while four-axis SCARAs are typically used for orientation pickup and placement. Delta configurations are very fast and are frequently used in the electronics industry for component placement. Another term to note is collaborative robot (Cobot): these are designed for direct interaction with humans in shared spaces and are not the same configuration shown here.
04
Making robot programming easier
Robots can be programmed using a computer or a teach pendant. Two types of code need to be programmed: program and position data. To perform a task of moving from one location to another, the robot's end effector first needs to define a start and end point, and then a program needs to be written to get there. This may involve additional locations and requires external signals to tell the robot that an object is present or to begin moving.
Positions can be defined by listing them in the software, but using a teach pendant is easier. A teach pendant allows the operator to move the individual axes, "driving" the robot to the desired position. To meet accuracy and safety requirements, it is generally operated at low speed. A 3-position "dead zone switch" also needs to be pressed while manipulating the robot. The spring-loaded switch needs to be held in the middle position; if it is continuously pressed or released, the robot will not move.
A program is a series of movements to different locations. These can be triggered individually or chained together. The languages used in robotics are diverse and often proprietary to the manufacturer. They are typically similar to languages like Basic or Assembly, with JUMP and MOVE statements. Other high-level scripting languages can also be used to build data structures or create mathematical algorithms , such as calculating paths or positions. Some languages allow parallel processing, allowing the robot to perform multiple actions at once, such as calculating movement vectors while a camera tracks a moving object.
The position data table and the programming process reside in separate memory areas, allowing one to be changed without affecting the other. This allows the position to be altered or "modified" by editing the table via a computer or teach pendant. Positions are typically defined in world coordinates, but the positions of the individual axes of a 6-axis robot can vary even if the end effector tool is in the same position. Positions can be taught by driving the robot to a position with a specific axis configuration and selecting "teach" or using a "guided" technique. This technique allows the user to manually push the axis to a series of specific positions while the axis is relaxed, thus describing a path.
05
Precautions for robot troubleshooting and maintenance
Robot troubleshooting and maintenance includes using software or a teach pendant to adjust (micro-calibrate) the position, replacing tools on the end effector, and maintaining electrical or pneumatic connections. Like motion controllers and frequency converters, robot controllers indicate problems in the system by providing fault data. Most faults will cause the robot to stop moving and may require the operator to move the robot to a "safe" position after correcting the fault.
Robotic work cells typically interface with both a PLC and an HMI. The PLC communicates with the robot and displays received fault codes and other data on the HMI. This involves two communication links (robot PLC and PLC-HMI), so ensuring their proper functioning is crucial.
End effectors may have communication interfaces such as M8 or M12 cable connections, junction boxes with terminals , ASI (Actuator Sensor Interface), or Ethernet remote I/O. If the sensor terminal is configured with a cover, it may be helpful to know the type of communication interface beforehand. Check the documentation or inspect the fixture or tooling area to see these connections.
This illustrates a typical layout of a robot's working cell. The different colors of the lines indicate that the connections between different components can be discrete wiring, communication, pneumatics, or, in the case of a robot connected to a controller, a combination of power and feedback wiring. This can complicate troubleshooting because it involves a wide range of knowledge across mechanical, electrical, and control disciplines.
In these systems, there are often actuators that are not controlled by the robot controller, such as in workpiece fixtures. This requires the PLC and robot controller to issue a "handshake" signal. External systems used for material handling and conveying systems can also interface with the PLC, and multiple robots can exist. Preventing collisions between multiple robots and tools can be very complex. Safety devices, such as light curtains, floor scanners, and door switches, can interface with the robot controller and PLC. Machine vision can also be used to locate parts for the robot, thus introducing another level of system complexity.
