Control of Two Link SCARA Robot Using Quantitative Feedback Theory

Abstract

SCARA Robots are mainly suitable for pick-and-place operations such as part handling, assembly, etc. Till now, many controllers are designed (like Trial and error, ZN PID and IMC etc) to control a Scara robot to meet desired position. The main drawbacks of those controllers are if plant parameter changes automatically system performance changes. In this paper, to overcome this drawback Quantitative Feedback Theory (QFT) has proposed for single and two link SCARA Robot. KEYWORDS: SCARA, QFT, Robust Controller, IMC, PID I.INTRODUCTIONSCARA (Selective Compliance assembly Robot Arm) robots are horizontally articulated manipulator with a vertical joint at the wrist end. Generally the robot configuration has one vertical (linear) and two revolute joints, it was designed by Professor Makino of Yamanashi University, Japan, and they are ideally suited for operations in which the vertical motion requirements are small compared to the horizontal motion requirements. Such an application would be assembly work where parts are picked up from a parts holder and moved along a nearly horizontal path to the unit being assembled. Fig.1 shows the Two link scara robot model. Potential problems arise mainly due to the positioning errors in assembly. The Adaptive and model-based control strategies cannot overcome the structure of uncertainties of a robotic system . Dynamic models of robot manipulators consist of highly non-linear coupled second-order differential equations. Non-linearity and parameter variations in real systems prevents, ordinary linear time-invariant control schemes achieving a satisfactory control performance. Linearization techniques for the robust control of robot manipulators with uncertainty have been the subject of many research studies. The SCARA robot is used for clean-room applications, such as wafer and disk handling in the electronics industry. It performs many tasks in a fraction of the time it would take a human. Whether constructing something intricate, such as a computer motherboard, or large and hulking, such as the frame of an 18-wheel semi truck, this tool helps increase production and lower costs because of its efficiency.Many practical systems that have high uncertainty levels in their open-loop transfer functions which makes it very difficult to create suitable stability margins and good performance in command following problems for a closed-loop system. Therefore, a single fixed controller in such systems is found among the 'robust control' family. Quantitative feedback theory (QFT) is a robust feedback control-system design technique which allows the direct design to closed-loop robust performance and stability specifications [4].QFT not only uses transfer function approach but also takes phase information into account in the design process. The unique feature of QFT is that the performance specifications are expressed as bounds on the frequency-domain response. Meeting these bounds implies a corresponding approximate closed-loop realization of the time-domain response bounds for a gi...