UPTEC F 14051 Examensarbete 30 hp November 2014 Design and implementation of a servo system by Sensor Field Oriented Control of a BLDC motor Per Eriksson Abstract Design and implementation of a servo system by Sensor Field Oriented Control of a BLDC motor Per Eriksson Teknisk- naturvetenskaplig fakultet UTH-enheten A servo system intended to steer antennas on board ships is designed, built and tested. It uses a Brushless Direct Current (BLDC) motor with an encoder to keep Besöksadress: track of its position, and Field Oriented Control (FOC) implemented on Toshibas Ångströmlaboratoriet Lägerhyddsvägen 1 microprocessor TMPM373 in order to control the current flowing to the motor. The Hus 4, Plan 0 servo system will be connected in cascade to another already existing servo system and controlled with two input signals. The first signal determines if the antenna axis Postadress: should rotate clockwise or counter clockwise. The second signal is a stream of pulses, Box 536 751 21 Uppsala where each pulse means that the motor should move one encoder point. Telefon: A printed circuit board is designed and built to complete these tasks. A proportional 018 – 471 30 03 -integral regulator is used to control the position of the motor, using the position Telefax: error as the controller input. The servo system is tested. The performance of the 018 – 471 30 00 resulting servo system is sufficient to satisfy the required position error limit of 0.5 degrees. In order to reduce the periodic disturbances presented in the system in Hemsida: experiments, Iterative Learning Control (ILC) is implemented. It is shown that using http://www.teknat.uu.se/student ILC further decreases the position error. Handledare: Sven-Åke Eriksson Ämnesgranskare: Ping Wu Examinator: Tomas Nyberg ISSN: 1401-5757, UPTEC F14 051 Sammanfattning Ett servosystem tänkt att användas för att rikta antenner ombord på båtar är designat, byggt och testat. Det använder en borstlös likströmsmotor (BLDC) med en encoder som håller reda på dess position, och Field Oriented Control (FOC) implementerad på Toshibas mikroprocessor TMPM373 för att kontrollera strömmen som går in i motorn. Servosystemet kaskadkopplas tillsammans med ett annat redan existe- rande servosystem och styrs med hjälp av två signaler. Den första signalen bestämmer om antennaxeln ska svänga medsol eller motsol. Den andra signalen är en ström av pulser, där varje puls betyder att motor ska ytta sig ett encodersteg. Ett kretskort med all nödvändig hårdvara för detta är designat och byggt. För att styra positionen av motorn används en proportionell-integrerande regulator där styrsignalen är positionsfelet. Servosystemet testas och resultatet är ett servosystem vars positionsfel nästan alltid håller sig under kravet på 0:5 graders fel. För att minska de periodiska störningarna i positionsfelet som visat sig i systemet under experimenten så implementeras algoritmen Iterative Learning Control, som visar sig minska positionsfelet ytterligare. 1 Contents 1 Introduction 4 1.1 Background . 4 1.2 Purposes and goals . 5 1.3 Methodology . 5 1.4 Thesis outline . 5 2 Working principles and theories 7 2.1 Servo systems . 7 2.2 Brushless Direct Current Motor . 8 2.3 Field Oriented Control (FOC) . 10 2.4 Encoder . 12 2.5 Space Vector Pulse Width Modulation . 12 2.5.1 The three-phase inverter . 12 2.5.2 Space Vector Pulse Width Modulation . 13 2.6 Current Measurement . 15 2.7 Torque ripples . 16 2.8 Controllers . 17 2.8.1 PI controller . 17 2.8.2 Iterative learning control (ILC) . 17 3 Implementation 18 3.1 Project Specications . 18 3.2 Motor Specications . 18 3.3 Pulse-width modulation . 18 3.4 Computer Communication . 19 3.5 Current Measurement . 19 3.6 First design . 21 3.7 Second design . 21 3.8 Programming . 23 3.8.1 Main method . 23 3.8.2 Vector Engine . 25 3.8.3 Key press detection loop . 26 3.8.4 Field Oriented Control loop . 26 4 Experiments 29 4.1 Setup . 29 4.2 Testing the rst design . 29 4.3 Testing the second design . 31 4.3.1 Determination of current PI controller settings . 31 4.3.2 Determination of speed PI controller settings . 32 4.3.3 Iterative Learning Control in order to reduce periodic disturbances . 33 2 5 Results and Discussion 36 5.1 Dead-time aecting the performance . 36 6 Conclusions and suggestions for future work 38 6.1 Conclusions . 38 6.2 Suggestions for future work . 38 3 Chapter 1 Introduction 1.1 Background Research Electronics is a company that delivers electronics tailored to customer requirements. These include control and regulation devices that require high speed and precision, or measuring equipment used in extreme environments. The company has developed a two-way satellite communication system in order to provide broadband for ships out in the sea. The motion of the ships' decks makes this quite a dicult task, and most existing antenna system solutions are driven from the center of rotation of the antenna dish. The solution presented by Research Electronics is not following this norm, and the servo motors are instead placed as far as possible from the center of rotation. This improves the pointing accuracy but requires the servo system to be able to achieve a very low positioning error. There are solutions available today on the market, that are able to meet the requirements. Unfortunately they are quite expensive and often comes with extra features not needed for this purpose, such as various lter types used to modify the behaivour of the system and analog inputs to control the speed and torque. As an example, the hardware required for KVHs 4 Mbps maritime broadband solution TracPhone V11 costs 75000 USD. Therefore, Research Electronics has decided to build this system itself. This could reduce the cost of the servo system by 90% or more by using cheap but ecient components and creating the control systems. The servo system that the present thesis project is concerned with is schematically shown in Figure 1.1. The system consists of two servo systems connected in cascade in a closed feedback loop. Both systems have a similar setup, with encoders that keep track of positions. The inner encoder is connected directly to the motor while the outer encoder is connected to the antenna axis. For a satellite dish control system, each cascaded servo system needs to be connected to an antenna axis. A basic component in any servo system is a variable speed drive, which consists of a motor and a controller. Vector control of AC motors was rst developed by K. Hasse and F. Blaschke in the early 1970s. Vector control is a variable speed drive control system where the stator currents of three-phase AC motors are identied as two orthogonal components which can be visualized as a vector. Two dierent branches of vector control emerged during the 1980s, Field-Oriented Control (FOC) and Direct Torque Control (DTC). In DTC the speed is controlled by estimating the magnetic ux and torque of the motor based on the measured voltage and current. The drawbacks of DTC are that it is dicult to use at low speeds, and produces high torque and current ripples [1]. You also need a very good current sensor if DTC is to be used in a high performance application, which increases the cost of the required hardware. FOC can be used in many dierent ways. It is possible to implement FOC using either synchronous (brushless) or induction (brushed) motors. In both of these cases, you can also make the choice of using sensors or not. Sensorless control requires derivation of rotor speed by measured stator voltage and currents. In this project a brushless DC motor and sensor FOC are used. 4 Figure 1.1: The complete servo system, in which the inner servo system for controlling the motor (inside the dashed rectangle) is the focus of this project. 1.2 Purposes and goals The purpose of this project is to develop a servo system, which is the inner part of the bigger system shown in Figure 1.1. The maximum position error of the servo system in this project needs to be at most 0:5 degrees, and the system operates at up to ve revolutions per second. The goal of this thesis project is to develop a prototype of such a servo system, which can be further developed into a nal system that can be used in the above mentioned communication system to control the antenna rotation with the requirements fullled. 1.3 Methodology The components used in this project were carefully chosen based on the requirements. The microprocessor TMPM373 from Toshiba was chosen because it has specic on-chip hardware designed for the purpose of Field Oriented Control. A BLDC motor was chosen because of it's high power to weigth and torque to current ratios. Sensor FOC was chosen above sensorless FOC because of the unreliability of sensorless FOC at low speeds if you do not use expensive and heavy current measurement components. The goal when designing the printed circuit board was to reduce electromagnetic interference and current uctuations as much as possible. This was done by creating return paths for the currents used in the motor in a satisfactory way, by linking the upper and lower layers of the board with multiple holes. Several capacitors were also placed on the board with the sole reason of acting as buers for the spiky current consumption of integrated circuits. Finally, separate channels provided current to the CPU, the integrated circuits and the motor current so that any disturbances in one part would not disturb the other parts. The servo system is built upon a brushless direct current motor with an encoder that keeps track of the rotor position with a resolution of 8192 points per revolution.