Abstract
This article deals with the control of two degrees of freedom manipulators that have a flexible and very lightweight link. These robots have a single low-frequency and high-amplitude vibration mode. Their actuators have high friction, and their vibration sensors are often strain gauges that have offset and high-frequency noise. These problems reduce the robot precision and produce noisy control signals that saturate actuators. An efficient control system is proposed to overcome these drawbacks. Actuator friction effect is nearly removed by closing a high gain position control loop around the actuator. It causes the separation of the robot dynamics into the controlled actuator fast subsystem and the link dynamics slow subsystem. Based on that, an innovative control system is designed to remove vibrations using the singular perturbation theory combined with the input-state linearization technique. This control system includes fractional-order controllers that nearly remove unknown sensor offset and sensor ramp disturbances while reducing the high-frequency component of the control signal caused by sensor noise. Simulated and experimental results show the superior performance of these controllers over other standard integer-order controllers of similar complexity and nominal behavior.
Highlights
S OME robotic applications demand lighter and larger robots that can be driven using smaller amounts of energy and can be more transported
The contributions are the following: 1) an innovative control system based on two nested loops that remove vibrations and minimize actuator friction effects; 2) controllers that significantly improve the precision of the robot tip positioning by reducing the effect of the offset and noise of strain gauge sensors; and 3) an improvement to the results attained in 2) by using an fractional-order controller (FOC) rather than an equivalent integer-order controllers (IOCs)
Several IOC are compared with our FOC
Summary
S OME robotic applications demand lighter and larger robots that can be driven using smaller amounts of energy and can be more transported. This number depends on the ratio between the links and the payload masses: the lower this ratio, i.e., the lighter the links and the heavier the payload, the smaller the number of significant vibration modes. Robots with large thin links made of these materials, often have very small link-payload mass ratios that allow us to regard them as having a single vibration mode This greatly simplifies the dynamic models of these robots and facilitates the design of their controllers. One problem is the significant Coulomb friction of the robot actuators, which is a discontinuous nonlinearity that makes difficult the precise positioning of the robot Another problem is the disturbances of the vibration sensors of flexible robots.
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