Motion control and robotics troubleshooting and maintenance advice

Motion control and robotics insights

• Motion control and robotics maintenance requires strong understand of all the components and mechanisms in a motor and a robot, which are sometimes intertwined. Even if they are, each requires a specific knowledge toolset.
• Preventing issues from happening in a motion control or robotics application can save companies an enormous amount of money by reducing downtime.

Motion control involves the use of motors for positioning and precise movement of actuators. While it is not always closed loop, it differs from the previous section on motor control where the main goal is to achieve and verify a known position or movement.

Stepper motors and maintenance

Stepper motors are brushless dc motors with multiple electromagnets arranged as a stator around a gear-shaped rotor. The circular arrangement of magnets are divided into groups called phases, each phase is energized together to make the motor “step” to the next position.

Microcontroller-based stepper drives are used to activate the drive transistors in the correct order. Typical stepper motor resolution is 200 steps per revolution, but with “microstepping” drives, up to 1600 steps per revolution can be achieved. Stepper drives are also sometimes called “choppers.”

Stepper motors are typically operated without feedback devices, such as encoders or resolvers, making them a less expensive method of positioning than with servo motors, but they also do not have much holding torque.

In addition to the motor and drive, some type of indexer is required. This may be built into the drive and communicate to a master controller, or a controller such as a programmable logic controller (PLC) can send pulses to index the drive.

Figure 1: Stepper motor system. Courtesy: Automation LLC

Troubleshooting a stepper system can involve checking voltages and communications in the control circuit, or even viewing the pulse train using an oscilloscope.

Servo 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 industrial automation applications are motor driven.

Figure 2: Servo motor assembly. Courtesy: Automation LLC

Servo motors can be brushed permanent magnet dc motors, brushless ac motors with permanent magnets or ac induction motors. They usually have a built-in encoder or resolver. They are also often integrated with a gearhead. There are two cable connections attached to the motor assembly to separate the signals from the encoder and sensors (feedback cable) from the motor power wiring.

Servo drives accept pulse inputs from the encoder as well as monitoring torque with current. Temperature sensor and brake control signals are sometimes included in the control cable. Servo drives are generally more sophisticated than variable frequency drives (VFDs) and often have logic capability built in. Modern controllers also almost always have high-speed communication ports that can be interfaced with other controllers to coordinate motion. Usually this is an Ethernet-based communication protocol, but fiber optics are sometimes used.

Servo control algorithms are PID-based for position or torque control. Motors need to be tuned to the characteristics of the motor and load to ensure peak performance. For this reason, the motor and drive are often specified and sold as a set from the same manufacturer.

Figure 3: Integrated servo example. Courtesy: Automation LLC

Some motors have the drive and controller built in with the motor. These “integrated servo” motors can be networked to perform complex tasks or serve as stand-alone positioners.

An important difference between a servo motor and a typical ac induction motor controlled by a VFD is a servo has holding torque at zero speed. If the motor shaft is moved off its position while under control power it will try to correct itself, faulting the controller if the correct position is not achieved.

Coordinated motion

When coordinating motion, a “master” controller or position is often used to pace the other controllers. Movement of one axis depends on the changing position of another, or on a virtual axis. It is important a fast communication network is used dedicated to the motion system. A dedicated motion controller may be used to coordinate servo axes. As illustrated in Figure 4, machine vision may be integrated to guide manipulators to the right location. Motion controllers may be integrated into PLC racks or be separate systems. Many of them have separate I/O modules and can be programmed in IEC 61131 PLC languages.

Figure 4: Motion control system. Courtesy: Automation LLC

Troubleshooting servo systems usually requires knowledge of the platform’s software in addition to typical methods of electrical diagnostics. Drives and controllers generally have built- in diagnostic functions to detect problems with the motor or attached load. Mechanical elements such as couplings are also subject to failure. As always, read your documentation!

Robotics maintenance

Industrial robots are used in manufacturing and material handling tasks, and their physical configuration depends on the function that is required. Payload and speed requirements help determine the type used in a particular application.

Robots can have up to 6-7 axes of motion or as few as 3. Two axes are required to reach any point in an X-Y plane, and 3 are required to reach any point in X-Y-Z space. To completely control the position of tooling at the end of the “arm,” three more axes are required in addition to X, Y and Z, these are pitch, roll and yaw.

Figure 5: Robot coordinate axes and right-hand rule. Courtesy: Automation LLC

Figure 5 shows the six axes required to reach any point and orientation in three-dimensional space, but robots use different coordinate systems, and joint configurations can differ. The origin and directions are defined differently depending on the brand and can often be changed in the software.

The X, Y and Z locations are referred to as Cartesian coordinates, but they can be defined from different reference points. If defined from the base of the robot or reference point of the environment, they are referred to as world coordinates. In this case the origin’s reference is stationary. When addressed from the view of the manipulator, they are referred to as tool coordinates, where the origin moves with the end effector. This may also include an offset, from the point where the tooling is attached to where it contacts the part.

Local coordinates also can be defined, usually with origins within a working area. This allows references to be duplicated for pallets or other local systems.

Individual joints can also be controlled independently, usually defined in degrees. Distances are generally defined in metric measurements (mm) but also can be scaled to user defined units in the software.

In addition to X, Y and Z, roll, pitch and yaw may be defined by other letters like U, V and W.

The area which a robot can reach is called the work envelope. Planes and boxes can be defined within the envelope to prevent collisions or for safety purposes, safety devices such as light curtains also can be integrated into a robot workcell.

Path awareness for robotic motion control

Robot controllers constantly perform calculations to ensure they know where the robot is relative to defined points and paths. Axes must work together when maintaining positions along a defined path, so a robot is the ultimate form of coordinated motion control. This is why controllers are generally dedicated to the task of achieving and maintaining position.

An important problem to understand when dealing with robots is that of a singularity. In this case, the robot cannot move its end effector along a certain path due to either physical or mathematical constraints. Robots can end up in positions where it cannot rotate the tooling around certain positions, this is sometimes referred to as gimbal lock. There are other physical configurations where moving joints through certain orientations can damage associated cables or hoses, so care is needed when moving robots close to singularity points or rotating axes too far. There are usually multiple joint configurations that can achieve the same tool position and orientation, this is often referred to as redundant degrees of freedom.

Robot controllers are often able to perform logic functions and operate external equipment, but usually they are built into workcells and connected to a “master” controller, such as a programmable logic controller (PLC). The controller may be a separate unit connected to the robot with power and signal cables or be built into the robot base. Connections may be physical 24 Vdc, communication links, or “pass through” ports and connectors with internal routing to the end effector or tooling. Pass through ports often include pneumatic hoses.

Robots can be categorized by their physical configuration. Figure 6 shows some of the common types of robots used in industrial applications.

Figure 6: Common robot types include 6-axis articulating arm (left), SCARA (center) and Delta (right). Courtesy: Automation LLC

The 6-axis articulated arm is very common in applications with heavy payloads, whereas the 4 axis SCARA is often used for oriented pick-and-place. The Delta configuration is very fast and often used in the electronics industry for component placement. An additional term to be aware of is a collaborative robot (cobot). They are designed for direct human interaction within a shared space and have a different configuration than those shown here.

Robot programming is easier with a teach pendant

Robots can be programmed using a computer or by means of a teach pendant. Two types of code need to be programmed, procedures and positional data. For a task where a robot end effector needs to move from one location to another, the starting and end points need to be defined, then a procedure needs to be written on how to get there. This may involve additional positions, along with external signals telling the robot an object is present or to begin the move.

Positions can be defined by listing them in the software, but using a pendant is easier. A teach pendant allows an operator to move individual axes, “driving” the robot to the desired location. A low speed is usually used for precision and safety. A 3 position “deadman switch” also needs to be pressed while the robot is being maneuvered. The spring-loaded switch needs to remain in the middle position, if it is depressed all the way or released, the robot cannot move.

Procedures are series of movements to different positions. They can be triggered individually or linked together. There are a variety of languages used in robotics, generally proprietary to the manufacturer. They often resemble languages such as Basic or Assembly, with JUMP and MOVE statements.

Additional high-level scripting languages also are 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 more than one action at a time, such as calculating movement vectors while a camera follows a moving object.

Positional data tables and programmed procedures reside in different memory areas, so one can be changed without affecting the other. This allows positions to be changed or “touched up” by editing the table with a computer or teach pendant.

Positions are often defined in world coordinates, but the positions of individual axes of a six- axis robot can differ with the tooling of the end effector being in the same position. Positions can be taught by driving the robot to a location with a specific axis configuration and selecting “teach,” or by using a technique called “lead-by-the-nose.” This technique allows the user to manually push the axes to a specific series of positions while the axes are relaxed, describing a path.

Considerations for robotic troubleshooting,  maintenance

Troubleshooting and maintenance involves using the software or pendant to touch up (slightly correct) positions, replace tooling on the end effector, and maintain electrical or pneumatic connections. As with motion controllers and VFDs, robot controllers will indicate problems with 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 a fault.

Robotic workcells interface with the robot, often with a PLC and HMI. The PLC communicates with the robot, displaying received fault codes and other data on the HMI. This involves two communication links, (robot-PLC and PLC-HMI) so ensuring they are working properly is important.

End effectors may have M8 or M12 cable connections, junction boxes with terminals, communication interfaces such as ASI (Actuator-Sensor Interface) or Ethernet remote I/O. If the sensor terminations are under a cover, knowing what kind they are ahead of time can be helpful. Check documentation or examine the gripper or tooling area to see these connections.

Figure 7: Robotic workcell example. Courtesy: Automation LLC

Figure 7 shows a typical layout for a robotic workcell. The different colors of the lines illustrate that the connections between the different elements may be discrete wiring, communications, pneumatics, or a combination of power and feedback wiring in the case of the robot to controller connection. This can make troubleshooting complicated, as a wide range of knowledge is required in mechanical, electrical and control disciplines.

There are often actuators that are not controlled by the robot controller in such a system, such as the workpiece fixturing. This requires “handshaking” signals from the PLC and the robot controller. External systems for material handling and conveyor systems also may interface with the PLC, and multiple robots also may be present. Preventing collisions between multiple robots and tooling can be very complicated. Safety devices, such as light curtains, floor scanners and gate switches, may interface with the robot controller and the PLC. Machine vision also can be used to help locate parts for the robot, introducing another level of complexity to the system.

– This was featured in the “Maintenance and Troubleshooting in Industrial Automation” book by Frank Lamb, the founder and owner of Automation Consulting LLC and a member of the Control Engineering editorial advisory board. Edited by Chris Vavra, web content manager, Control Engineering, CFE Media and Technology, cvavra@cfemedia.com.

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