Measurement and Signal Processing for Electric Drive Control System

Transcription

1 1416 PIERS Proceedings, Marrakesh, MOROCCO, March 2 23, 211 Measurement and Signal Processing for Electric Drive Control System P. Brandstetter, P. Bilek, J. Szotkowski, and P. Vaculik Department of Electronics, VSB-Technical University of Ostrava, Czech Republic Abstract Paper is focused on presentation of possibilities which come with application of digital signal processing into area of electric drives control. In the paper, description of control system, structures for measurement and signal processing of analogue and digital signals are presented. Hardware and software realization of these devices are shown, and also description of control structure which is used for digital signal processing. In conclusion, experimental results of used control system are presented. 1. INTRODUCTION Expansion and miniaturization of microprocessor control systems allows significant reduction of cost, dimensions and also simplify usage of microprocessor control systems into wide area of application. Digital signal processing allows to great improvement of control parameters and increases quality of technological processes. It allows simple remote control of the process and communication with superior control system. The application of control system with digital signal processing will be demonstrated on control system of electric drive. 2. DESCRIPTION OF CONTROL SYSTEM Structure of control system may be divided to three levels. This division is realized according to type of signals which are present at particular level of control system. The basic structure of control system is shown at Figure 1. The highest level is the digital signal processing (DSP) unit itself. A software signal processing and A/D conversion is realized at this level. To connect DSP unit to the drive, second level called interface is necessary. At this level, input signal level conversion is realized to enable connection of sensors to the DSP unit. Next function of interface level is power boosting of output signals which allows connection of DSP unit to the driver of electric drive. The bottom level of control system is drive level. Acquisition of necessary signals from electric drive is provided by measuring elements. 3. MEASUREMENT It is necessary to require measured signals in enough quality to provide good functionality of the whole control structure. To provide this, several demands must be adhered [3]: enough accuracy, high noise resistance, dynamic range, galvanic insulation between measured signal and sensor output. A/D conversion DSP DSP UNIT Signal conversion Output processing INTERFACE Measurement Electric drive DRIVE LEVEL Figure 1: Structure of control system.

2 Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 2 23, Measurement of Voltages and Currents To measure voltages and currents, sensors based on Hall Effect were used. These sensors provide galvanic insulation and enough dynamic range of measurement. The output quantity of sensor is current which is converted to voltage signal on measurement resistor R m. The measurement resistor is situated as close as possible to input of A/D converter. Schematic representation of sensor is shown at Figure Speed Measurement Usually, the incremental encoder is used to measure speed of electric drive. There are two rectangular signals at the output of incremental sensor, shifted by 9 angle. Number of pulses per one round depends on mechanical construction of the incremental encoder. Rotor position change is evaluated by counter, direction of position change is evaluated from mutual shift of signals U A a U B. Output signals of incremental encoder are presented at Figure 3. For very low speed range, incremental encoder with sinusoidal output signals may be used. These signals cannot be directly connected with input of DSP, transformation to rectangular signal is necessary. Realization of this transformation is presented at next chapter. 4. SIGNAL LEVEL CONVERSION Usually, signals from sensors cannot be directly attached to DSP, because level of voltage output differs with possibilities of input gate of DSP. Signal level conversion is necessary in many cases to allow connection between sensor and DSP. Next important function of interface level is over-voltage protection of DSP input gate Processing of Signals from Voltage and Current Sensors Principle of signal conversion is shown at Figure 4. The signal level conversion may be realized by two ways. The first way is application of operational amplifier. This solution requires high quality low noise devices and many circuits are necessary to process larger quantity of signals. All these conditions increases cost of the interface level. Better solution of signal level conversion is application of reference voltage source. Reference voltage merged to measurement resistor provides the same voltage shift as provides operational amplifier, with less circuitry and cost. Schematics of both variants of signal level conversion block are shown at Figure 5. Input and output signals of signal level conversion block are presented at Figure Processing of Signals from Incremental Encoder Application of incremental encoder with TTL signal output requires no signal conversion; interface level provides only protection of DSP. The connection of incremental encoder with sinusoidal output to DSP requires signal conversion of output signals to voltage levels, which can be interpreted by counter of DSP (see Figure 7). Signal processing of signal from incremental encoder with sinusoidal output is shown at Figure 8. 6 U A, U B t [µs] Figure 2: Schematic representation of sensor. Figure 3: Signals of incremental encoder. I abc Sensor R m Offset 1.65 V + + To A/D 1.65 V V U m R 1 R 3 U shift R4 - + R2 D 2 D 1 R U z =3V U s > > Im V R m D2 U shift D 1 R U s U z=3v Figure 4: Principle of signal conversion. Figure 5: Schematics of signal level conversion blocks.

3 1418 PIERS Proceedings, Marrakesh, MOROCCO, March 2 23, U m t [ms] t [ms] Figure 6: Input signal (left) and output signal (right) at signal level conversion block. 6 U Asin [V*4], U A Figure 7: block. Incremental encoder signal processing t [οs] Figure 8: Signals from input and output of incremental encoder signal processing block. Figure 9: ADC block scheme. 5. DSP SIGNAL PROCESSING 5.1. Analogue Signal Conversion to Digital Form The analog digital converter (ADC) consists of two converters working independently. Each of them include eight analog inputs and the own sample-hold unit (SHU). The basic parameters of ADC are: input analogue voltage from to 3.3 V, the differential inputs are supported, 12 bit resolution (496 levels), maximum speed is 1.7 M samples per second. The signal processing of controlled AC machines it s necessary to convert at least two channels at same time. Two phase currents measuring of delta connected machines is the reason. The worst but possible variant is using two SHU and sequential samples conversion by single ADC. The conversion speed and number of distinguishable levels are the parameters influence the regulation quality. The benefit of the ADC is the minimum and maximum values monitoring, which is allowed to be use like software protection against over current and short-circuit. It is possible to say that used ADC is suitable for this application. The Figure 9 shows how the signal passes through ADC and its processing. For channels connected with common signal ground there is easy relation between analogue and digital values. Arithmetic-logic unit 56F837 allows to process two basic data types: integer 8, 16, 32 bits (signed or unsigned) and fraction 16 or 32 bits (only signed). The both type can be converged for each other. The benefit of fraction type against integer is correct result at multiplying of high numbers. The overflow cannot occur because the product can be less than coefficient only. But on the other side the information loss is expected when the low number are multiplying. The main data type of our digital processing was chosen fraction type, especially owing to the software libraries, because the library s functions were created for fraction type mostly.

4 Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 2 23, Digital Signal Processing The software processing gives very interesting possibilities, how quickly, effectively and casual resolve the problems. We know that there is special software for resolving almost all kind of problems. In our case the facility is digital signal controller (DSC) 56F837 and the software is development environment called Code Warrior Control Structure and the Main Principle The signal processing was designed for the vector control of AC machines. The following Figure 1 shows control structure for permanent magnet synchronous motor (SMPM). After signal conversion to digital form expressed as integer type we have to convert it to fraction type. It is necessary to measure actual rotor position for creating the system of coordinates, which always has same direction like rotor. The position sensor is a source of two signals with 9 degrees phase shift. The signals are connected with timer through the differentiating amplifier. There is an actual rotor angle saved in timer register after its correct setting. The Clarke transformation (marked 3/2 on the figure) transfers measured phase currents into orthogonal coordination system. Using Park transformation (marked αβ/dq on the figure) we obtain rotating orthogonal coordinate system oriented like rotor. The controllers were chosen PI type or I type with limitation their outputs. The last part is a modulation [1, 2] Output Signals The last block of signal processing is PWM modulator. It converts three digital values on also three two-level signal with changing their ratio. The obtained signals have voltage from to 3.3 V and they will not damage by TTL logic if the open collector property is set. The driver need higher voltage (MOS levels) so there is a levels converter for readjusting differ voltages. 6. EXPERIMENTAL RESULTS The control system with the DSC 56F837 provides digital signal processing. The next Figures 11 and 12 show four stages of digital processing. The SMPM ran up, reversed and stopped on the end. Figure 1: Control structure of AC drive with permanent magnet synchronous motor. Figure 11: Analogue signals of phase currents i a, i b on the ADC inputs (1 V = A). Figure 12: Measured stator currents in three different system of coordinates.

5 142 PIERS Proceedings, Marrakesh, MOROCCO, March 2 23, 211 From the top there are measured phases currents at stator system of coordinates, two components and rotor coordinate system. At last one there are supply voltages in other words they are the output from regulators multiplied DC bus voltage. 7. CONCLUSION The paper deals with digital signal processing for applications in electric drives control. It was shown the techniques with analog and digital signals which are verified on practice application of AC drive control. ACKNOWLEDGMENT In the paper there are the results of the project 12/8/755 which was supported by The Czech Science Foundation (GA CR) and project SP/2198 which was supported by Student Grant Competition of VSB-TUO. REFERENCES 1. Brandstetter, P., A.C. Control Drives Modern Control Methods, VSB-Technical University of Ostrava, 1999, ISBN X. 2. Chlebis, P., P. Moravcik, and P. Simonik, New method of direct torque control for threelevel voltage inverter, Proceedings of 13th European Conference on Power Electronics and Applications, Barcelona, Spain, 29, ISBN Osmancik, L., M. Polak, P. Simonik, L. Hrdina, P. Skotnica, and P. Palacky, Digital signal processor TMS32F2812 and its application in electric drives, Proceedings of International Conference on Applied Electronics, , Pilsen, Czech Republic, 26, ISBN