On disengaging a peg from a hole

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Lim, Joey Zhong Yi ORCID: https://orcid.org/0000-0002-7576-0501 (2022). On disengaging a peg from a hole. University of Birmingham. Ph.D.

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Abstract

In the field of manufacturing and remanufacturing, robots are employed in assembly tasks. Robotics researchers often use a cylindrical peg and a cylindrical hole as a model to understand high-precision insertion operations. During those operations, two main obstacles were identified, namely, jamming and wedging. Jamming occurs when the force is applied in the wrong direction and can be rectified easily by changing the direction. Wedging occurs when the peg appears to be stuck in the hole. The wedging of a peg is more complex than jamming, and it involves the deformation of the components. Many studies have been performed in the area of peg-hole assembly. Although researchers have mentioned the necessary conditions for wedging, the peg-hole jamming problem was the main focus.
This thesis aims to better understand the peg-hole wedging problem to find methods to dislodge a wedged peg and to design a remote-centre-compliance (RCC) device to avoid the wedging and jamming of a peg that can be used in both assembly and disassembly. Using the definition and necessary conditions of peg-hole wedging, the systematic process of wedging a peg is analysed and illustrated. There are four steps to wedge a peg in a hole. First, the peg and hole must be in 2-point contact, and the two contact points must be within each other’s friction cone. A force or moment is then applied to deform the peg and hole, and the peg tilting angle increases. The force or moment is then released in the third step, and the peg tilting angle will be reduced by a small amount. Finally, when the peg tilting angle reduces, the reaction forces at the contact points will be collinear, and the peg is wedged. In the simulation and experiment in this research, the hole is divided into two sides, and a force-torque (FT) sensor is installed beneath each hole. The readings obtained from the sensors have shown that the hypothesis of the wedging process is correct, and when the peg is successfully wedged, the resultant force experienced by the FT sensors is balanced.
The dislodging of a peg is also investigated in this thesis. To dislodge a wedged peg, intuitively, the peg is either shaken, twisted or knocked. Depending on the application, some would use a low force to dislodge the wedged peg to avoid damaging the components, while others would prefer a quicker disassembly process. In this investigation, the wedged peg is dislodged using different methods, such as applying a constant force and pulsating forces with different frequencies and magnitudes. The time needed to dislodge the peg is recorded to compare the effects of different combinations of parameters used. The result from the simulation shows that the peg can be dislodged at low impulses within a specific range of pulling force magnitudes. Adopting a pulsating force helps reduce the impulse required to dislodge the peg compared to using continuous force in the low magnitude region. However, in the lowest magnitude region, using a continuous force resulted in a lower impulse as the time for dislodging the peg was shorter compared to when a pulsating force was employed.
Many techniques have been proposed and investigated to aid the peg-hole assembly process, and one of them is by using an RCC. At the University of Canterbury (New Zealand), researchers designed a passive compliant device, which was an inverted Gough-Whitehall-Stewart mechanism, to assist the peg-hole insertion process. This thesis analyses a modified version of that compliant device, where the legs do not meet in pairs at the platform but at points located remotely from it. This allows the device to have the features of an RCC mechanism, which has been proven by other researchers to be effective for precise peg-hole assembly tasks. This device is also suitable for both assembly and disassembly processes. Unlike the currently available RCC design, which can only withstand high compressive forces, the proposed compliant device can resist both compressive and tensile forces. The compliance matrix of the new design and the location at which it is diagonal are derived using small approximations, proving that the centre of compliance is situated away from the platform. The correctness of the small motion assumptions and the RCC properties of the new compliance device have been confirmed by performing the sensitivity analysis.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):