In English

Development of a Control Strategy for a Percussive Drill driven by a Tubular Linear PMSM

Anton Klintberg ; Jimmy Björkman
Göteborg : Chalmers tekniska högskola, 2014. 69 s.
[Examensarbete på avancerad nivå]

This thesis presents a control strategy for a percussive drill driven by an oscillating Tubular Linear Permanent Magnet Synchronous Machine (TLPMSM). The drill consists of a piston with permanent magnets and a casing with integrated stator windings. The casing encapsulate the whole unit and should be used to hammer into rocks. Between the piston and the casing there are two gas springs used to store the kinetic energy of the masses as potential energy while the oscillating masses change direction. The electric force from the TLPMSM should be used to bring the mechanical system in resonance. When the system is in resonance only a small amount of electric energy is needed to maintain the oscillation. The electric energy injected is needed to compensate for the losses in the system and the energy lost when the casing hits the rock. The developed control strategy consists of a field oriented vector controller for the current and an outer controller for the mechanical oscillations. The outer controller is a proportional controller with positive feedback, which is found to have particularly good properties for the application. The outer controller controls the oscillation at the resonance frequency without any information about the process. This is highly desirable since it is very challenging to make an accurate model of a rock. Furthermore, the controller only demands a current when the the back emf is non zero. This means that when the TLPMSM is unable to produce any output power the current in the windings is zero, which prevents unnecessary copper losses. It is shown that the relative position between piston and casing oscillates twice for each time the casing strikes the rock surface. In the simulated scenario, the impact force from the casing to the rock can reach 645 kN with an oscillation frequency of 38 Hz. To increase the oscillation frequency the gas spring pressure need to be increased.Two methods to make the control strategy sensorless are also presented. The first strategy is a Statically Compensated Voltage Model (SCVM) which is based on the information provided by the induced back emf. The second strategy is based on saliency in the machine and uses injection of a high frequency square wave signal. In this work this strategy is referred to as Square Wave Injection. Both techniques are working in the application but they have different advantages and disadvantages. It is shown that the SCVM is insensitive to measurement noise since 10 % noise could be added to the current measurement without any trouble. It is also shown that it is dependent on good parameter estimation, especially for the rotor ux. An error in the parameter estimations leads to a lowering of the impact force from the casing to the rock from 417 kN per hit to 343 kN per hit. For Square wave injection it is the opposite. It is sensitive to measurement noise since only 0.2 % current measurement noise could be added without that the estimation stopped working. If the oscillation frequency is increased the noise level need to be even lower. However, it is not dependent on parameter estimations since process parameters are almost not used. When each estimation techniques is simulated without measurement noise the square wave injection technique gives a lower maximum position estimaton error than the SCVM, 0.82 mm versus 1.64 mm. But for the square wave injection technique the position estimation error is high at the instant when the current is applied. Since the force generating current is erronously applied an impact force from the casing to rock of 364 kN is reached and this is lower compared to the impact force of 417 kN when the SCVM is used. It is recommended to use SCVM for this application. The SCVM can be used in a larger range of oscillation frequencies for the mechanical system and is more robust against non-ideal conditions.

Nyckelord: Tubular Linear PMSM, percussive drilling, sensorless current control, signal injection, SCVM.

Publikationen registrerades 2014-10-21. Den ändrades senast 2015-01-19

CPL ID: 204610

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