Development and testing of a high speed hydraulic manipulator with single time scale visual servoing

Abstract

Automation of production processes has enabled to meet the dramatic demand for manufactured products that has grown out of the increase in world population. In existing industries greater production requirements and improvement in product quality call for faster industrial robots. This study details the design and development of a high-speed visual servoing system for industrial applications. The proposed visual servoing system consists of a high speed robotic manipulator, a high-speed camera system and an embedded controller. The proposed robotic manipulator has the configuration of a Selective Compliant Assembly Robotic Arm (SCARA). It uses two custom-designed double vane rotary hydraulic actuators for driving the links of the robot. The SCARA system was mathematically modeled and simulated. Based on the simulation results, the hydraulic actuators were sized for optimal performance. A prototype actuator was subsequently designed, manufactured and experimentally evaluated. The test results show that the proposed actuator is capable of reaching torques of up to 460 Nm in 30 ms with a payload of 12 kg. This is not possible with electric motors of similar size. Then the proposed SCARA was designed and fabricated using the proposed actuators. The end effector of this manipulator was capable of reaching velocities of up to 2.7 ms⁻¹ with a payload of 5.3 kg. Comparable performance is not feasible with contemporary SCARA type robots. The proposed robot was designed for handling payloads up to 15 kg with speeds of up to 2 ms⁻¹. This often results in flexing of the links and twisting of the support column, adding external disturbances to the system. A high-speed camera system was designed and built to obtain the position of the end effector as feedback for the controller. It uses a two dimensional Position Sensitive Detector as the image sensor. An electronic circuit was designed and built for signal conditioning and data acquisition from the Position Sensitive Detector. It was then calibrated to account for non-linearities on the image sensor. The camera was constructed using this Position Sensitive Detector circuit, a lens and an infra red filter.It was then calibrated to estimate the extrinsic and intrinsic parameters. This camera was capable of carrying out measurements at frequencies of up to 1350 Hz. The measurements made by this camera produced an average absolute accuracy of 0.31 mm and 0.37 mm in x and y directions, respectively. A Field Programmable Gate Array was used in this study as the platform for developing an embedded controller for the robot. Using contemporary Field Programmable Gate Array technology, a powerful virtual processor can be synthesized and integrated with custom hardware to create a dedicated controller that out performs some of the conventional microcontroller and microprocessor based designs. The Field Programmable Gate Array based controller takes advantage of both hardware features and virtual processor technology. The input, output interfaces for this controller were implemented using hardware. Complex functions that are difficult to be implemented in hardware were implemented using a virtual soft processor. Four different types of controllers were implemented and tested. These include hardware proportional-derivative, software proportional-derivative, single time scale visual servoing and set point modification type controllers. The proposed implementation carried out single time scale visual servoing at frequencies of up to 330 Hz.