High bandwidth angular instrumentation system for the measurement and analysis of stepper motor drives

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Cain, C.J., 2004. High bandwidth angular instrumentation system for the measurement and analysis of stepper motor drives. MPhil, Nottingham Trent University.

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Abstract

Modern automatic control systems often rely on the facility to accurately move a tool or simply a payload to a precise position. The requirements of efficiency may demand that the movement is accomplished in the minimum possible time. To achieve this objective the acceleration and deceleration profile must be both smooth and rapid, but at the end of the move the settlement at the position of rest must free from oscillations. Stepper motors are very often used in such applications because they will move a precise number of fixed angle steps in response to a fixed number of digital pulses sent to the motor. This is the basis of numerical control.

The motor is to achieve precise placement without oscillation at the end of the move. The path of the motor must be smooth in transition throughout the entire velocity profile (including phases of acceleration, steady speed and deceleration). However stepper motors are susceptible to slipping out of step with the pulses supplied to the motor. This is most likely to occur during acceleration or deceleration if the motor cannot produce sufficient torque to match the load being moved by the motor. The motor can also suffer from oscillation if the frequency of the steps matches the natural frequency (or a sub-multiple of the natural frequency) of the motor and its load. If these oscillations are large enough, the effect will be cumulative in successive steps and will cause the motor to fall out of step.

The technique of micro-stepping (an electronic means of generating a set number of points of equilibrium between natural steps of the motor) provides a means of more accurate position control. Micro-stepping also reduces (but does not eliminate) resonance due to running at speeds related to the natural frequency of the motor. In order to reduce oscillations, techniques are applied using measurements of disturbances in the electric current in the motor to trace its path of motion and to regulate acceleration to a rate that ensures the motor does not slip out of step. Because this technique tracks the motion of the motor without the use of external transducers, it is known as sensorless feedback.

An instrumentation system has been developed to better understand the efficiency of control systems to facilitate improved algorithms for sensorless feedback. The instrumentation system is able to reconstruct the magnetic flux waveform of the motor. The current and voltage waveforms (the data used for sensorless feedback) have been compared with the PWM pulses, the magnetic flux waveform and with position monitoring from a high bandwidth digital shaft encoder. A test bed was developed to investigate the control performance of a micro-step motor driver supplied by the industrial partner. Some causes of oscillations at low speeds of the motor have been determined and solutions have been proposed. A research platform that integrates the functions of the micro-step driver and the instrumentation onto a common digital signal processor (DSP) has been developed. This increases the bandwidth of the instrumentation and reduces the possibility of quantisation errors in capturing the signals from the motor controller. Alternative methods of analysing the effects of motor motion on the phase current and voltage waveforms have been proposed.

Item Type:

Thesis

Creators:

Cain, C.J.

Date:

2004

ISBN:

9781369316995

Identifiers: