Controller based Electronic Speed Controller for MAV Propulsion System

Controller based Electronic Speed Controller for MAV Propulsion System N. Manikanta Babu M. Tech, Power Electronics and Drives VIT University, Vellore, India manikantababu010@gmail.com CM Ananda CSIR National Aerospace Laboratories Bangalore, India ananda_cm@nal.res.in Abstract Electronic Speed Controller (ESC) in Micro Air Vehicles (MAV) is very essential and critical. This ESC is used to interface the propulsion system consisting of Brushless DC Motor (BLDC) and provides required electrical interfaces to the motor. ESC receives the Pulse Width Modulated (PWM) signals from the Autopilot to drive (Throttle) the propulsion system. BLDC motors are mainly used in applications such as Aerospace, Electric Vehicle, trains etc,. Commutation of BLDC motor takes place electronically unlike the dc motor. The design uses threephase bridge to drive the motor as well as to control the speed using Programmable System on Chip (PSoC) controller. 3-phase bridge requires motor rotor position feedback (i.e. hall sensor) for commutation. Hall sensor increases the size and cost of the BLDC motor. Hence BLDC motor can be used with back method as the commutation technique. Reference speed command is given from Autopilot module to the ESC. Based on this command, controller in ESC produces the timed pulses to 3- phase bridge. later sequence S3S2, S3S4, S5S4, S5S6 switches are commutated accordingly. Power Supply Servo PWM RF ZBEE Real Time GPS Interface Controller Electronic Speed Controller (ESC) Pressure 3 Axis 3 Axis Gyro 3 Axis Micro SD Index Terms BLDC Motor, PSoC Controller, Back, 3- Phase Bridge. I. INTRODUCTION From the past few decades aerospace industry is showing interest in development of Micro Aerial Vehicle (MAV). MAV is by definition small aircraft which can fly at relatively low speeds. MAV is used in many applicatons such as Targeting, Reconnaissance,Surveillance, Defence applications, Weather forecast, Wildlife study & photography, Border surveillance, Traffic monitoring, Tracking criminals and illegal activities, Biochemical sensing, inspection of pipes etc,. MAV consists of mainly propulsion system, Autopilot system, Camera payload apart from the basic aircraft structure. Among this propulsion system is the critical subsystem in MAV. It consists of propellers and motors. Autopilot of MAV drives the propulsion system as shown in figure 1. Motor used in propulsion system is sensor-less Brush Less DC Motor (BLDC). BLDC motor has advantages compared to DC motor in terms of efficiency and endurance. BLDC motor needs electronic commutation of phases. Instead of brushes, 3-phase bridge is used to drive the motor as shown in figure 2. Figure 3 shows the switching sequence of a 3-phase bridge. In first mode switch S1 and S6 are ON connecting windings A and B to positive and negative respectively. In second mode S1 and S2 switches are ON and Propulsion System (BLDC Motor) Fig 1 Interface block diagram of systems in MAV This bridge is connected to BLDC motor terminals connecting all three A, B and C windings. Based on switching commutation BLDC motor will operate and commutation takes place based on rotor position of a motor. Rotor position can be detected by using Hall sensor. Hall sensor increases the weight and cost, due to these drawbacks this detection method is not suitable for MAV [1]. Rotor position can be detected by back of the motor. Part of ESC Part of Motor Figure 1: 3-phase bridge

Permanent magnet motor has two types of Back s 1. Sinusoidal back and 2. Trapezoidal Back A DC motor with sinusoidal back is termed as Permanent Magnet Synchronous Motor (PMSM) and DC motor with trapezoidal back is termed as BLDC motor [2]. Commutation in 3-phase bridge and speed of a motor can be calculated based on back of a motor [3]. Speed of the motor can be varies by changing the ON time of the switches. Switching pulses are driven by Programmable System on Chip (PSoC) controller in ESC. Back of motor can be given to ADC s of a PSoC controller for position calculation. Fig3: switching Sequence of 3-phase Bridge II. SPEED CONTROLLER FOR SENSORLESS BLDC MOTOR A. Controller Circuit of Sensorless BLDC Motor Figure 4 shows the overall control block diagram of sensor less BLDC motor. Gate drive circuit and voltage divider circuits are shown as block. PSoC controller is used to generate pulses based on commanded speed from the Autopilot in-line with the back of a motor. Back signals are fed to PSoC controller to know the current position of rotor. The speed control changes the duty cycle of pulses for changing the motor speed. Pulses from controller control the switches S1 to S6 [4]. Sequence of a pulses is generated based on Back zero crossing points. B. Zero Crossing Detection Schemes During the operation of 3-phase BLDC motor, two phases are conducting at a given time and other phase is not in conduction at that point of time. The back- of a non-conducting phase is proportional to the speed. During starting of the motor or at low speeds the motor back is too weak to be detected. The zero crossing detection is the one of the simplest way to sense the back of a BLDC motor. This is based on detecting the instant at which the back- of the unexcited phase crosses zero [5]. Different kinds of Back detections schemes are Compare Virtual neutral point with zero voltage Compare terminal voltage with Half DC-bus voltage. Compare Virtual neutral point with zero voltage In this scheme a virtual ground is generated by using 3 resistor networks connected in parallel with the motor windings. The common point of all three resistors is the virtual ground point. Virtual ground point and floating phase are given to comparator through voltage dividers and filters. Voltage divider, divides the voltage level to controller accepted voltage level and filter reduces the ripples in phases of a motor. Comparator output gives the zero crossing point. Compare terminal voltage with Half DC bus voltage In this scheme floating phase is compared with half of the DC terminal voltage. Back signals and half of DC supply is given to comparator. If back signal greater than the half of DC voltage then comparator gives the high signal otherwise it give low signal. Typical scheme is shown in figure 5. Fig4: Control circuit diagram of sensorless BLDC Motor Fig5: Compare terminal voltage with Half DC bus voltage

III. IMPLEMENTATION OF ZERO CROSSING DETECTION SCHEME BY USING PSOC CONTROLLER Zero crossing detection scheme is based on comparing the terminal voltage with half DC-bus voltage [6]. The internal hardware multiplexer, comparator and control register simplifies this procedure. A B TABLE1: Gate logic for switches C S1 S2 S3 S4 S5 S6 0 0 0 0 0 0 0 0 0-1 1 1 0 0 0 0 1 1-1 0 1 1 0 0 0 0 1 0-1 0 1 1 0 0 0 0 1-1 0 0 1 1 0-1 1 0 0 0 0 1 1 0-1 0 1 0 0 0 0 1 1 0 0 Fig6: Back zero crossing detection scheme in PSoC Figure 6 shows the PSoC internal resource configuration diagram as part of the design for back zero crossing detection scheme. Back signals are given to Multiplexer and commutation control register gives the select signal to multiplexer select line. Multiplexer selects one of the back signals and fed to comparator with half of the DC-bus voltage as reference to the comparator. Based on comparator output, sequence of commutation is determined. Table 1 shows the gate logic for switches. In table 1 represents back signal above the half of DC signal and -1 represents back signal below the half of DC signal. Switching sequence is generated [7] based on back. For example B and C are -1 and 1, then S1 and S6 sequence are in ON state. If A and C are 1 and -1, then S2 and S3 sequence are in ON state. Similarly other sequence and accordingly the switch states are switched. Figure 8 shows the terminal voltages in free running mode and terminal voltages in closed loop mode along with zero crossing points. Fig7: Generation of commutation sequence in PSoC5. Figure 7 shows generation of commutation sequence in PSoC controller. Two demultiplexers are used, one generates the pulses for high side switches of the bridge and other one generates the pulses for low side switches of the bridge. PWM block is connected to de-multiplexer and based on control registers signal, demultiplexers selects the PWM signals. Output of the de-multiplexer is given to switches in 3-phase bridge. Based on demanded speed command, de-multiplexer output pulse width is changed and the period of commutation sequence is also changed correspondingly. Fig 8: Back voltages of Motor. IV. HARDWARE IMPLEMENTATION OF SENSORLESS BLDC MOTOR Figure 9 shows the hardware setup for controlling the speed of a sensorless BLDC motor. It consists of PSoC (CY8C5568LTI-114), MOSFET module, gate drive module, mini programmer and battery. PSoC is used to generate the pulses to MOSFET. It receives the back signals from

motor through voltage dividers to know the rotor position. Six pulses from controller drive the six MOSFET s gates through gate driver circuit. Gate drive circuit provides the required amount of voltage to MOSFET to drive the corresponding motor lines. Bridge outputs are connected to 3 phases of a BLDC motor. Mini-Prog is a device used to download the application to the PSoC and also used for realtime debug capability. motor, first motor will run in free running mode. It goes to closed loop mode once sufficient back is generated. In free running mode, 112 ms pulse period is given to motor irrespective of speed command. In closed loop mode, motor speed varies based on speed command. Speed command from autopilot is given to PSoC controller of ESC in the range of 1.2-1.8 ms ON time with a period of 2 ms at a frequency of 50 Hz. Motor speed goes from minimum to maximum for a ON time of 1.1 ms to 1.8 ms respectively. Figure 10 shows the open loop wave forms of A and B phases terminal voltages. Figure 11 shows the speed command of 1.3ms ON time wave form and terminal voltages of motor. Fig 9 Hardware setup for controlling the speed of a Sensorless BLDC motor. Motor parameters shown in table2. Controller components and their ratings are shown in table3. TABLE2: Motor Parameters Parameter Rating Fig10: Open loop terminal voltages of a motor voltage RPM 7.4-14.8V 950KV Max Current 23.2 Max trust 850gm TABLE3: Controller components and their ratings Component Rating MOSFET Battery 60V, 30A 3cell, 2200mAH PSoC Voltage dividers CY8C5568LTI-114 33K&10K V. RESULTS The complete setup was wired along with the required application on the PSoC controller. The experiments were conducted for various scenarios of that of MAV requirements. The results are satisfactory and waveforms of all lines are monitored using digital oscilloscope. When supply is given to Fig11: 1.3ms ON period and terminal voltages of motor VI. CONCLUSION Electronic speed controller for BLDC motor is designed and developed. Back detection is done by using the analog multiplexer and analog comparator. Back waveforms at different speeds are shown in figure 11. Motor speed is controlled by the demanded speed command. The system has undergone initial tests and integration and the results are satisfactory. However this needs detailed testing and validation

with various vehicles and needs endurance tests over couple of hours. The same needs to be repeated over couple of weeks with different environmental conditions as experienced by the MAV during flight. The work is in progress and scheduled to complete shortly. VII. ACKNOWLEDGEMENT Authors would like to thank Mr. Shyam Chetty, Director NAL for continuous support and motivation. Authors also would like to thank Pankaj A, Mr, Sunil prasad, Mr. Pavan Kumar, Ms. Swetha, Mr. Madhu, Mr.Sharath and team of NALMETF for their support in testing and integration of power distribution system. REFERENCES [1] Rubayet Hosen, Khosru M. Salim, Design Implementation and Testing of a Three Phase BLDC Motor Controller, 2nd International Conference on Advances in Electrical Engineering (ICAEE 2013). [2] Zhibin Ren, Xiping Liu, The Design of New Sensor-less BLDCM Control System for Electric Vehicle, Energy and Power Engineering, 2010, 2, 208-211 [3] Jun-Hyuk Choi, Joon Sung Park, Jin-Hong Kim, and In-Soung Jung, Control Scheme for Efficiency Improvement of slim type BLDC Motor, 2014 International Symposium on Power Electronics, Electrical Drives, Automation and Motion, pp 820-824. [4] P. Damodharan and K. Vasudevan, Line Voltage Based Indirect Backemfzero Crossing Detection Of BLDC Motor For Sensorless Operation, International Journal of Power and Energy Systems, Vol. 28, No. 1, 2008 [5] Dino Gu and Jemmey Huang, Sensorless BLDC motor control based on CY8C3866AXI Cypress technical document, 2010. [6] Yedamale, P., Brushless DC Motor Control Using PIC18FXX31 MCUs, Microchip Technology Inc application note, 2004. [7] R. Kandiban, R. Arulmozhiyal Design of Adaptive Fuzzy PID Controller for Speed control of BLDC Motor International Journal of Soft Computing and Engineering,Volume-2, Issue-1, March 2012, pp 386-391. AUTHORS Author 1 N.Manikanta Babu pursuing Masters from VIT University, Vellore and holds Bachelors degree in Electrical and Electronics Engineering. He is currently doing Masters project at National Aerospace Laboratories under Dr. C M Ananda. His areas of interest are propulsion system for Micro Air Vehicle, Power Electronic Converters and Multi-level Inverters. Author 2 Dr. C M Ananda holds PhD (Engineering) in Avionics Embedded systems with Masters degree from BITS, Pilani and Bachelors degree in Electronics and Communication Engineering. He is currently Head of Aerospace Electronics and Systems Division at National Aerospace Laboratories. He has 25+ years of professional experience in the domain. His areas of interest are- Integrated Modular Avionics, Flight Critical System, Embedded distributed system, Re-configurable avionics architectures and Safety critical software, reconfigurable platform, fault tolerance, avionics and systems health.