DSP-Based Implementation of Permanent Magnet Synchronous Motor ...

DSP-Based Implementation of Permanent Magnet Synchronous Motor Drives for EV/HEV Applications Abdelsalam Ahmed1, 2, An Quntao1 and Sun Li1 1

School of Electrical Engineering & Automation, Harbin Institute of Technology, Harbin, China 2 Dep. of Electrical Power and Machines, Faculty of Engineering, Tanta University, Egypt [email protected]

Abstract - Four-quadrant operation and extending driving over the base speed of motor drives are from the most crucial features of drivetrain of Electric and Hybrid Electric Vehicles (EVs/HEVs) system. This paper presents an experimental implementation of a high performance speed control strategy for a Permanent Magnet Synchronous Machine (PMSM) for EVs/HEVs applications. The control strategy guarantees a robust matching for the command speed within the constraints of the drive system. The control and drive system is implemented by a TMS320F2812 Digital Signal Processor (DSP). To confirm the effectiveness of the proposed control system, an experimental system included by PMSM, DSP control board, IPM inverter module and interface circuits have been set up. The presented control strategy is validated by the experimental results that depict the precisely operation of the PMSMS in the fourquadrant circumstances and also in constant power operation mode. Index Terms – PMSM, field-weakening vector control, DSP

I.

INTRODUCTION

Electric motor/motion drive and control unit is the heart of Electric Vehicle (EV) and Hybrid Electric Vehicle (HEV). That unit affects directly on the dynamic performance of the vehicle during the whole driving trip. From this side of view, many aspects can be studied and researched. The research in the field of electric machines in vehicle propulsion has been intense over the past few years. Different machine types have been introduced, and their performances have been compared. According to [1] and [2], the most popular electric machine type studied for vehicle propulsion is the Permanent-Magnet Synchronous Machine (PMSM). Owing to the advantages of the superior power density, high performance in motion control - fast positioning and better accuracy, PMSMs have gradually used in many automation control fields as actuators [3] and [4]. Its electrical efficiency and torque density are highest of the present-day electrical machines. Direct-driven PMSMs for a full electric 4×4 sports car was presented in [5]. With the rapid development in microprocessors, the high performance TMS320F28x Digital Signal Processor (DSP) chip becomes a popular research on digital control [6-7]. The DSP is used for ac drives due to their high-speed performance, simple circuitry, and on-chip peripherals of a micro-controller into a single chip solution. Therefore, in this paper, a TMS320F2812 DSP embedded with the software of current vector control, Space Vector Pulse Width Modulation

(SVPWM) scheme and PI controllers have been developed for a high performance speed control for PMSM drives. With the excellent characteristics of the used DSP [8], it will make drives of PMSM more programmable, robust and easy implementation. Power electronics integration technologies have been used to build the driving circuits for the PMSM using the TMS320F2812 DSP as in [9]. System hardware design which includes DSP control circuits, power driver circuit, signals detection circuit and protection circuit have been introduced in [10]. Some application problems of PMSM vector control system have been discussed in [11]. Also, [12] proposed the main hardware structure part for the PMSM servo system. It is required and desirable to produce the rated power with the highest attainable speed for many applications such as EVs, people carriers in airport lobbies, forklifts, machine tool spindle drives, etc. [13]. Some Field-Weakening Control (FWC) methods for surface-mounted PMSM drives have been presented in [14-16] and for an advanced HEV that driven by a double-rotor PMSM was presented in [17]. Field-Oriented Control (FOC) algorithm is common in motor/motion drive systems, manufacturing machinery, and industrial automation. This paper considers the operation of the PMSM drives when they are constrained to be within the permissible envelope of the maximum inverter voltage and current to produce the rated power and to provide this with the highest attainable rotor speed. This paper presents a speed controller with field-weakening control for surface-mounted PMSM. The proposed method can determine either the constant torque limit control mode or the field-weakening control mode which should be applied to the drive under different operation conditions. Also, operation of the PMSM in the four quadrants is discussed. Laboratory set up is built and the control strategy is implemented using the TMS320F2812 DSP and finally the dynamic behavior of the system is validated by the experimental results. II.

CONTROL SYSTEM OF PMSM DRIVES

A. Description of the PMSM drive system In this system, the sensored field oriented control of the PMSM using Quadrature Encoder Pulse (QEP) sensor is experimented and explored the performance of speed control. The performance of the FOC system is evaluated by studying the speed responses. The overall system for implementation of the 3-ph PMSM control is depicted in Fig. 1.

IPM power module

Power supply

Power board

L

Current sensors

DC Encoder Transformer

PMSM Rectifier

DC filter

Isolated and driving circuits PWM1...6

Fault protection

TMS320F2812 DSP board

SVPWM

K1PM N_ref

Teref



Teref-new

iq_ref

Vq_ref



d,q

Iq_fdb

N_fdb λmd 0

Speed function program

Kf f(ωbm)

QEP circuit Clarke

a, b, c Parke-1

α,β P/2

id_ref 

0

Field-weakening programing

A, B, Z

ADC

α,β

PI

Vd_ref

Emulator

α,β

θe

PI

Load

Comparator circuit

ib

ia

Vα_ref

Vβ _ref

PI

Low pass filter

JTAG

1-ph AC supply

d,q

Id_fdb

PC with CCS programming

Park Signal conditioning & Speed calculation

Fig.1. Configuration of a fully digital controller of PMSM drives using TMS320F2812 DSP

The PMSM is driven by the conventional voltage-source inverter. The TMS320F2812 DSP is generating six PWM signals by means of space vector PWM technique for six power switching devices in the Integrated Power Module (IPM) inverter. Two input currents of the PMSM ( and ) are measured from the inverter and they are filtered by a lowpass filter and then sent to the DSP control board via analogto-digital converter (ADC). The architecture of the proposed current vector and speed control system for the PMSM are all implemented within a DSP chip. The whole embedded system is composed of speed loop, current loop, speed and rotor position detection via QEP circuit, and fault protection unit. The configuration of the speed loop for a PMSM includes PI controller. The configuration of the current loop includes two PI controllers, Clarke, Park, Park-1 coordinate transformations, SVPWM, and stator current detection. Both of reference torque and flux linkage are functioned using speed function and field-weakening programming modules. B. Mathematical model of PMSM drives The transformations between stationary a-b-c frame, stationary α-β frame and synchronously rotating d-q frame are presented in [3]. Mathematical model of surface-mounted PMSM in the two-phase rotated coordinate can be expressed as follows: The dynamic state voltage equations: ( ) The motor torque equation can be expressed as:

(1)

(2)

where and are the direct axis and quadrature axis voltages, and are the direct axis and quadrature axis currents, is the synchronous inductance, is the resistance of stator winding, is number of poles, is the flux linkage due to the permanent magnet on the rotor, and is the velocity of electrical angle. Considering the load term, the dynamic equation of PMSM can be written as the follows: (3) where , , , and are motor toque, velocity of mechanical angle, inertia, damping ratio and load toque, respectively. C. Speed control with field-weakening strategy The mutual flux linkages reference is generated by the demands of the rotor speed. As long as the line-to-lineinduced Electro Motive Force (EMF) magnitude of does not exceed the dc supply voltage to the inverter, the ratio between the induced EMF and stator frequency remains constant resulting in constant mutual flux. Therefore, as expressed in (4) at control, the EMF is proportional only to the rotor speed . In the flux-weakening control mode, in order to maintain the current control, as well as, to maintain the control over the induced EMF magnitude, the mutual flux is programmed to decrease in inverse proportion to the speed so that the induced EMF is pegged to the level where it corresponds to base speed even though the speed is pushed beyond that. In FWC as shown in (4) when speed exceeds its base value, will be decreased such that is maintained constant. (

)

(4)

where is a constant that depends on machine parameters and supply frequency. But torque cannot be expected to be maintained at the base level that corresponds to the base speed whose product gives the base air gap power and it can never be exceeded in a machine in steady state as it involves higher losses that will result in thermal run off, leading to machine failure. Further, it requires stator currents exceeding their base values in the machine. These problems are addressed by reducing the torque, when the speed is beyond its base value, by programming it to decrease from its base value so that the air gap power produced is equal to the base power. That will also keep the stator current within its base value. This seemingly complex control can be simply implemented in the following two steps. i. Mutual flux and field-weakening programming A controller, which gives from starting up to base speed, and beyond speed, that gives an output inversely proportional to normalized speed, is created using a speed function generator to delineate the constant torque- and fluxweakening regions of operation of the motor drive. As depicted in Fig. 1, the output of this function is termed ( ) and is proportional to the mutual flux linkages reference . The constant of proportionality is . The )sets the reference for the resultant function generator ( mutual flux linkages, involving the constant . The function generator operating on the speed has the following characteristics: ( )

(9) where and are maximum inverter phase voltage amplitude and maximum inverter line current amplitude, respectively. III.

HARDWARE AND SOFTWARE IMPLEMENTATION

The overall experimental system is depicted in Fig. 2 which includes a TMS320F2812 DSP control board with its different peripherals connectors, a voltage source IGBT-based IPM drives with interface and measurement circuits, 3-ph PMSM with an incremental encoder with integrated commutation signals that used as motor feedback, an electromagnetic brake device, single-phase adjustable output transformer, PC with Code Composer Studio (CCS) installed, and additional instruments such as oscilloscope, digital multimeter, current sensing probe. Specifications of system components on the laboratory are mentioned in Table1. 1 5

2 6

(5) where is the base speed. Then the command of fluxproducing stator current component can be calculated: ((

)

)

(6)

ii. Torque programming in the flux-weakening region This involves the reprogramming of the speed controller produced torque reference as a function of the speed. Just as the mutual flux linkages are programmed to be inversely proportional to the rotor speed, likewise, the output of the flux programmed controller is used to adjust the torque reference to yield a usable torque reference, , in ) as (7). the vector controller by multiplying and ( ( ) (7) For the surface-mounted PMSM, the torque-producing current reference is obtained by dividing the torque reference by the product of the rotor magnet flux linkages and the constant term , that is, 1.5 times the pair of rotor poles for a 3-phase machine as it is proved in (8). (8) For a vector controlled motor drive system, the operating limits are usually expressed in terms of the d- and q-axis current constraint and the d- and q-axis voltage constraint by considering both motor and inverter ratings. These two constraints can be depicted as in (9).

3 4 1. 1. CCS CCS software software program program 2. 2. TMS320F2812 TMS320F2812 DSP DSP and and peripherals peripherals board board 3. 3. Power Power and and interface interface circuits circuits board board 4. 4. Emulator Emulator 5. 5. Power Power supply supply and and adjustable adjustable transformer transformer 6. 6. Digital Digital scope scope 7. 7. PMSM PMSM 8. 8. Load: Load: electromagnetic electromagnetic brake brake unit unit

7

8

Fig.2 Experimental setup for real tests Table1: specifications of the experimental system Device Specification, Value (unit) Rated/Max. Speed, 2000/2500 (rpm); Voltage constant Ke, 80 (V/K rpm); Standstill torque, 3 (Nm); Rotor moment of PMSM inertia, 4.4 *10-4 (Kg.m2); Standstill current, 2.5 (A); Stator resistance, 3.5 (Ω); Torque constant KT, 1.2 (Nm/A); Pole pairs, 3; Inductance, 11.5 (mH). Load Adjustable brake unit: 24 (V), 0.5 (A), 0~10 (Nm), 110 (W) TMS320F2812: 150MHz, 32-bit fixed-point CPU, 12-bit DSP ADC 1kVA, 50Hz, 220V input voltage, 0~250V output voltage, Transformer 4 A rated current Power IPM power module with six IGBT power transistors: board PS21867-AP (600V, 30 A); Hall-based current sensors, 20 A. Incremental 2500 pulses/rev encoder

A. Power drive circuit Power drive circuit using AC/DC/AC voltage inverter circuit are composed by the rectifier circuit, filter circuit and IPM inverter module. Single-phase rectifier circuit is a noncontrolled rectifier bridge. Filter circuit from the DC bus side of the electrolytic capacitors in parallel composition of three of 470µF/450V is used. The used DIP-IPM is an intelligent power module PS21867-AP that integrates power device, driver, and protection circuitry in an ultra-compact dual-inline transfer-mold package for use in driving 3-phase motors. B. DSP control unit The control functions are implemented using the TMS320F2812 DSP which is highly integrated, highperformance solutions for demanding control applications. With the 150 MIPS operating speed and the high-performance 32-bit CPU, it can produce three-phase PWM signals for the IGBTs switching and has all communication interfaces for all peripherals designed to handle all analog feedback and control signals needed to correctly manage the power section of the device. The main performances can be reviewed in [8]. Because the DSP has the properties of fast computation and the complete peripheral circuits for motor drive, a fully digital controller of PMSM drives system, which includes current vector control scheme, SVPWM generation, A/D conversion, coordinate transformation, and QEP detection, is integrated and realized by software within a DSP chip. C. Signal-detecting Unit In order to achieve the PMSM vector control, detection of the motor phase A and phase B currents, speed and the rotor magnetic pole position are needed. System uses the current transducer LA-50P as the current sense element, the output level converted by the input to the ADC port of TMS320F2812 DSP. The current signal is adjusted to 0-3V and acquired by DSP. Speed and rotor magnetic pole position are detected by incremental optical encoder with integrated commutation signals. Optical encoder output is six-way differential signals A±, B±,Z±; its output voltage range is from 0 to 5V. Encoder outputs A and B are two types of squared waves out of phase for 90 electrical degrees. In this paper, the encoder pulse signals are converted into a single output signal by using the differential receiver DS3486 to reduce the Electro Magnetic Interference (EMI) in the input signals. Then, the signal is isolated by the opto-coupler module HCPL2631 and shaped through the Schmitt triggers 74HC14. Finally, the signals are input to the DSP through the QEP module. The resultant A, B and Z signals are acquired by DSP to realize the detection of rotor position and speed. D. Hardware-protecting Unit Fault protection circuit includes a DC bus over-current protection, IPM over current fault and encoder failure. System making a failure signal as an active low signal that will be connected with the (Power Drive Protection Interrupt) PDPINT pin. TMS302F2812 Event Manager (EV) provides an external interrupt PDPINT to achieve the system's hardware protection. PDPINT can be used to inform the monitoring program of motor drive abnormalities such as overvoltage, over-current, and excessive temperature rise.

When the system fails, PDPINT pin goes low. When PDPINT pin is pulled low, it will generate an external interrupt. Onchip Interrupt Service Routine (ISR) curing system automatically stops the 6-channel PWM signal output, and then the controller stops. E. Software programming The PWM switching frequency of inverter is designed with 10 kHz, dead-band is 3µs to prevent three phase legs of inverter from shooting through, and the control sampling frequency of current and speed loop are 10 kHz and 1 kHz, respectively. That means the computational time of the DSP for executing current loop is 100µs and executing PI control algorithm of speed loop is 1ms. Those programs are coded with C language through the CCS programming. IV.

EXPERIMENTAL RESULTS

Several test results will be shown to illustrate the promising features of the proposed scheme in this section. First, the transformer voltage is adjusted at 125V and three tests are conducted to show the operation in the four quadrants and the difference between the control and the proposed field-weakening control. A. Four-quadrant speed-controlled Performance of a four-quadrant speed-controlled with SVPWM current control is discussed in this section. Fourquadrant operation is implemented with a load torque of 3Nm. The results are shown in Fig. 3 and Fig. 4. 1000

Speed (rpm) Current (A)

0

- 1000

2.22 1.15

0

Fig. 3 Measured speed and current in four-quadrant operation 0.65 0 - 1.0

2.4

1.9

1.0

0 - 1.0 - 2.4

- 1.9

Id (A) Iq (A) Fig. 4 Measured id and iq currents in four-quadrant operation

The machine is at a standstill at the start and, with a positive speed command (1000 rpm), the torque current component reference is driven to a positive increase (2.4A) and phase current also is driven at high value (2.22A) and are maintained there until the rotor speed matches the speed command. When the rotor speed is equal to the command speed, the torque current component reference comes fixed to match the load torque and the friction torque (1.0A) and phase current reaches to its steady state (1.15A). The torque current component reference is driven to negative when the speed reference changes from 1000 to −1000 rpm. The rotor slows down to zero speed. Maintaining the negative torque forces the rotor to reverse direction and catch up with the speed reference of −1000 rpm. B. Control of PMSM without FWC When the motor is driving above its base speed without FWC, the induced EMF will exceed the maximum input voltage, making the flow of current into machine phases impractical, as presented in this section. First, at 50Hz supply frequency, 2Nm load torque and fixed supply voltage at 125V, the maximum allowable speed of 1650 rpm is measured. This speed is considered her a base value. Figure 5 shows the speed and phase current when the commanded speed is stepped from 0 to 1500 rpm then to 2000rpm. Figure6 shows the waveforms of d-axis and q-axis current response below and above the base speed. 2000

As can be seen below the base speed, the PMSM is precisely controlled; whereas the rotor speed cannot catch its reference value above the base speed and then both stator current components ( and ) behave without control. As depicted in Fig. 6, when the speed of motor exceeds1650rpm, the d-axis and q-axis currents oscillate severely. Meanwhile, the motor has the danger of out of control. It can be concluded that, as soon as the frequency exceeds its base value, it results in the speed being driven beyond the base speed and consequently the induced EMF starts exceeding the dc supply voltage magnitude. That makes the control of stator currents very difficult and thereby the control of the torque also. The drive control becomes sluggish in this region. C. Control of PMSM with field-weakening strategy In EV/HEV system, as the battery voltage and charge decrease with the increasing load current, the FWC operation inevitably occurs at higher rotational speeds. FWC operation is shown in the experimental results of the following figures. Figure7 shows the speed and phase current when the speed is stepped from 0 to 1500rpm then to 2000rpm and then back to 0. It is depicted that the speed catches its reference value below and above the base speed, 1500rpm and 2000rpm, respectively. Phase current at different speed levels is measured as depicted in Fig. 7 that shows starting current, steady-state current at 1500rpm and steady-state current at 2000rpm of 1.75A, 1.0A and 4.3A, respectively.

Speed (rpm) Current (A)

Speed (rpm) Current (A)

1750

2000

1500

1500 0

0 1.76

4.3 1.75

1.14

1.0

0

0

Fig. 7 Measured speed and phase current for speed control with FWC

Fig. 5 Measured speed and phase current for speed control without FWC

0.78

0.78

0

0

1.7

1.74

- 4.4 1.12

0.85

0

0

Id (A) Iq (A) Fig. 6 Measured id and iq currents for speed control without FWC

Id (A) Iq (A) Fig. 8 Measured id and iq currents for speed control with FWC

(a) (b) (c) Fig.9 Phase current at: (a) 1000 rpm, (b) 1500 rpm, (c) 2000 rpm

As seen in Fig. 8, both stator current components behave under full control. Below the base speed, is controlled to be zero whereas is controlled to match the total load torques at starting and at steady state of 1.7A and 1.12A, respectively. Above 1650rpm, flux weakening is initiated using the algorithm given earlier. The d-axis current reference is reduced during flux weakening, reaches to 4.4A, resulting in a reduction of air gap flux linkages. During flux weakening, the torque command is slightly reduced to maintain the constant air gap power. Figure 9 shows focused waveforms of the phase current at 1000rpm, 1500rpm, and 2000rpm, Fig.9 (a), Fig.9 (b), and Fig9 (c), respectively. The behavior of the controlled PMSM during the reverse of the rotation of speed is depicted in Fig. 10 and Fig. 11. Speed (rpm) Current (A)

0 - 1500

- 2000 4.3

1.75

1.0

0

Fig. 10 Measured speed and phase current at reverse direction under FWC 0.78 0

- 4.4

- 1.12

0

- 1.7

Id (A) Iq (A) Fig. 11 Measured id and iq currentsat reverse direction under FWC

V.

CONCLUSION

A high performance speed control strategy for PMSM has been presented. Hardware structures and the embedded software programming for the system have been implemented

via the TMS320F2812 DSP. The dynamic performance of the PMSM has been evaluated through the experimental set up. The implementation involved speed control in four-quadrant operation and working safely and precisely over the base speed. The proposed scheme has a robust speed control feature for surface-mounted PMSM drives over full speed range and load torque demand. Experimental results demonstrate that the rotor speed of PMSM can fast track the prescribed dynamic response well. Smooth starting, perfect command tracking, four-quadrant operation and extended speed range, which are the most important features of any powertrain for EV/HEV system, have been achieved and experimentally validated. REFERENCES [1] Z. Q. Zhu and D. Howe, Electrical Machines and Drives for Electric,Hybrid, and Fuel Cell Vehicles, Proc. IEEE, vol. 95, no. 4, pp. 746–765, Apr. 2007. [2] G. Pellegrino, A. Vagati, P. Guglielmi, and B. Boazzo, Performance Comparison Between Surface-Mounted and Interior PM Motor Drives for Electric Vehicle Application, IEEE Trans. Ind. Electron., vol. 59, no. 2, pp. 803–811, Feb. 2012. [3] B. K. Bose, Power Electronics and Variable Frequency Drives– Technology and Application, IEEE Press, 1997. [4] Abdelsalam Ahmed and Cui Shumei, An Intelligent Power Management and Control Strategy for Hybrid Electric Vehicles Driven by Electric Variable Transmission Based on Permanent Magnet Electric Machines, Fifteenth International Middle East Power Systems Conference (MEPCON'12), Alexandria, Egypt, December 23-25, 2012. [5] Janne Nerg, Marko Rilla, Vesa Ruuskanen, Juha Pyrhönen, and Sami Ruotsalainen, Direct-Driven Interior Magnet Permanent Magnet Synchronous Motors for a Full Electric Sports Car, IEEE Transactions on Industrial Electronics, Vol. 61, NO. 8, August 2014. [6] B. Zhang, Y. Li and Y. Zuo, A DSP-based fully digital PMSM servo drive using on-line self-tuning PI controller, Proc. PIEMC 2000, vol.2, pp. 1012-1017, 2000. [7] A.M. Trzynadlowski, M.P. Kazmierkowski, P.Z. Grabowski, M.M. Bech, Three Examples of DSP Applications in Advanced Induction Motor Drives, American Control Conference, Vol. 3, pp. 2139-2140, 1999. [8] TMS320F2810 and TMS320F2812 Digital Signal Processors, Handbook of Texas Instruments, 2012. [9] Weifeng Zhang, Yuehui Yu, Zhiqiang Chen, DaidaXie, TMS320F2812 DSP Driving System based on Power Electronics Integration Technology, ICSP2006 Proceedings. [10] Zhao Yinyin, QuYongyin, PMSM Vector Control System Design Based on TMS320F2812, World Automation Congress (WAC), 2012 . [11] Tiecheng Sun, Ce Liu, Ningbo Lu, Deyan Gao, Sanling Xu, Design of PMSM Vector Control System Based on TMS320F2812 DSP, 2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia, June 2-5, 2012, Harbin, China. [12] Zhu Jun, Li Wankui, Han Lili, PMSM Control System Based on Digital Signal Processor, Journal of Networks, Vol. 8, No. 4, April 2013. [13] R. Krishnan, Permanent Magnet Synchronous and Brushless DC Motor Drives, CRC Press © 2010 by Taylor and Francis Group, LLC. [14] Adeeb Ahmed, Yilmaz Sozer, Marv Hamdan, Flux weakening Control for Surface Mount Permanent Magnet Synchronous Motor Drives with Rapid Load and Speed Varying Applications, Energy tech, 2013 IEEE. [15] Shinn-Ming Sue, Ching-Tsai Pan, Yuan-Chuen Hwang, A New FieldWeakening Control Scheme for Surface Mounted Permanent-Magnet Synchronous Motor Drives, Second IEEE Conference on Industrial Electronics and Applications, 2007. [16] C. T. Pan and J. H. Liaw, A Robust Field-Weakening Control Strategy for Surface-Mounted Permanent-Magnet Motor Drives, IEEE Trans. Energy Conv., vol. 20, no. 4, pp. 701-709, Dec. 2005. [17] Abdelsalam Ahmed and Cui Shumei, Parametric Design and Robust Control Strategy for HEV Based on Permanent Magnet Electrical Variable Transmission. Research Journal of Applied Sciences, Engineering and Technology (RJASET), 4(15) May, 2012: 2323-2333.