Computer aided design and construction of a multi-point positioning actuator system

  • Zan Shi

    Student thesis: Doctoral Thesis


    This thesis investigates the development, control, modelling and analysis of single-phase tubular linear induction actuators (TLIA) for the application of multi-point positioning.

    A novel TLIA has been successfully developed in the research project. This actuator makes use o f the electromagnetic induction principle to produce a force that varies with displacement and can be controlled by the voltage/current applied at its terminals. This enables the position of the actuator to be controlled by a proper external circuitry. Its structure is simpler, compared with traditional TLIA. With the exercise of computer aided design and analysis a monotonously decreasing control characteristic has been obtained. This makes the actuator inherently suitable for stepless positioning under both open loop and closed loop control.

    The positioning was realised by changing the terminal voltage applied to the actuator in the actual system. Two forms of power supply units (PSU) have been studied. One is a triac controlled a.c. line phase controller. The other is a single-phase variable-frequency-variable-voltage a.c. converter using PWM technology. Both PSUs’ output voltage can be controlled by a DSP board configured as a closed loop control system. Although the PWM PSU is a little more complicated than the triac based one, it offers better performance and more flexibility in control methods.

    A second order spring-mass-damper system was used as the fundamental model of the actuator system. The control algorithm consists of a digital controller, a non-linear correction based on the inverse function method and an explicit friction compensation with the Coulomb friction model. With the digital control technique, the restrictions in the characteristics of the actuator have been improved significantly.

    To get an in-depth insight of the electromagnetic phenomena happening inside the actuator and to function as a CAD tool, a general-purpose axi-symmetric FEM package has been developed for the computation of transient electromagnetic field. It can deal with various forms of power supply from standard current excitation to voltage excitation by coupling an external circuit with the electromagnetic field equations. A decomposed magnetic vector potential (DMVP) method, which is based on the indirect coupling of external circuit equations, has been proposed to deal with transient eddy current problems under voltage excitation coupled with external circuit equation. Comparisons between the direct coupling method and the DMVP indirect coupling method have been made. It has shown that the proposed new method can reduce the CPU time and storage considerably. Also a new method to improve the accuracy of Maxwell’s stress tensor based force calculation has been proposed along with a fundamental analysis of various sources which cause the error in force computation. The proposed force calculation scheme can achieve two goals: elimination of the path-dependent sensitivity and improvement of the accuracy in the force calculation.

    Computation of transient temperature field has been studied as well by FEM. The FEM package for electromagnetic field computation has been further expanded to incorporate the function of transient temperature computation. The coupling between the electromagnetic field and the temperature field was treated in an indirect way. Experimental tests have been carried out on a prototype TLIA to verify the FE simulation. A good agreement between the computed and the tested results has been obtained.

    Finally the newly developed TLIA, P SU and DSP-based controller have been put together to form a complete integrated positioning system. Application as a multi-step positioning control system has been successfully achieved.
    Date of AwardApr 1997
    Original languageEnglish
    SponsorsNCR Scotland

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