1. Reconfigurable Mechatronic Robotic Plug-and-Play Controller
- Author
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Jonathan R. Zyzalo, Olaf Diegel, and Johan Potgieter
- Subjects
Flexibility (engineering) ,Engineering ,business.industry ,Emerging technologies ,media_common.quotation_subject ,ComputerApplications_COMPUTERSINOTHERSYSTEMS ,Manufacturing engineering ,Product (business) ,Computer-integrated manufacturing ,Manufacturing ,Process development execution system ,Advanced manufacturing ,Quality (business) ,business ,media_common - Abstract
When the entire system was put together, adequate control of the robot was possible. Overall the system performed well enough to get controlled motion of the six joints of the PUMA robot arm. The system was also subsequently used to control a CNC lathe. There were, however, a few problems with the final system. The system also requires some further enhancement to achieve better control of joint positioning. The main problem occurred with the logic power supply. Every now and then the logic power, which supplies all the encoder circuits and microprocessors, would fail to power the most essential parts of the system. This would result in unpredictable behaviour from the robot arm and make it unsafe. It was found that the grounding of the logic power supply was floating causing differences in the ground of the system components. By using the ground of a bench-top power supply, this problem was solved. The rest of the problems with the final system were a result of the PID control method. The PID control was not able to account for the effects of inertia and gravity because the feedback gains of the PID algorithm were fixed. This meant that the system had a very low level of repeatability and accuracy. The Encoder PID mode of the BrainStem Moto 1.0, although working well for the wrist joints, did not perform very well for the larger joints. When a new setpoint was entered into the PID control loop, the control algorithm would output maximum voltage to the motor until it neared the new setpoint based on the feedback from the encoders and the PID gains. It did this without accounting for the effects of inertia and gravity. In practice this meant, for the shoulder and elbow joints especially, that the joint would move more rapidly in the down direction that it would in the up direction. To correct this problem, a velocity control method needs to be explored. Another problem that exists with the BrainStem Moto 1.0, is that it only has a 16-bit setpoint number and this is reduced to 15-bits as the most significant bit is a directional bit. Due to the resolution of the 250 line incremental encoders, only limited movement of a joint can be completed with each command sent to the BrainStems. This leads on to another problem with the BrainStem program. It is not able to detect when a movement is complete so that the next move of the robot can take place, i.e. there are no flags set to indicate a new setpoint has been achieved. This problem was overcome, in Visual Basic, by introducing a timer delay between selected movements. A problem also occurs when the motors are powered at high speeds. At high motor speeds the PIC fails to read the encoder pulses. This results in uncontrolled motion of the joints. In some cases the encoder circuit that was developed for the incremental encoders fails to supply the pulse train signal to the BrainStems. This may be because there was a frequency limitations on the ICs used in the encoder circuit. This problem could also arise from aliasing. This would depend on the rate at which the PIC samples the input signal from the encoder circuit. The effect of this problem is that robot must operate at reduced speeds in order to maintain control. The only other item of note is that the robot joints require calibration every time the robot is turned on. This is the purpose of the potentiometers in each motor. The potentiometers are input directly to the analogue inputs of the BrainStem Moto 1.0 that provides a 10-bit analogue-to-digital conversion of the potentiometer value. This value gives an indication of the position of the robot when it is powered up. As these problems are overcome, further planned development of the system is to integrate it with RobotWorks, a computer aided manufacturing (CAM) package. RobotWorks is an add-on package for SolidWorks, which is a commonly used CAD program. The idea being that path planning for the manufacture of various parts and 783
- Published
- 2021