A novel technique of controlling DC power using microcontroller based PWM controlled rectifier.

Prasad, S.A. Hari ; Kariyappa, B.S. ; Nagaraj, R. 等

Introduction

Even though maintenance cost of DC motors are quite higher than induction motors, due to excellent speed control characteristics of DC Motors [1], they have been widely used in industry. This in turn has attracted researchers to probe on several new and innovative methods to control these motors.

DC power can be controlled using several methods. One such method is by using PWM controlled rectifiers. The required PWM signals are generated using either Analog or Digital controllers. Analog controllers are prone to external disturbances and the component characteristics changes with temperature and time. Moreover Analog controllers lack reprogrammability, flexibility and they are complex in nature.

On the other hand Microcontroller based digital controllers are immune to disturbances and drift, but their performance is not very good due to limited speed control range. However to minimize throughput delay, digital controllers retrieves switching pattern directly from memory so that the calculations can be minimized, but at the same time, this demands more memory space. Larger memory requirements can be overcome if switching patterns are generated using simple control algorithms [2], but in-spite of using simple control algorithms, through put delay may be substantial. With the advent of advanced microcontrollers and Digital Signal Processors (DSP)[3] which have many advanced features like inbuilt PWM generator, event managers, time capture unit, dead time delay generators, watch dog timers along with high frequency clock, the limitation of speed, associated with microcontroller based PWM inverters can be discarded to some extent.

This paper discusses the control of DC power using 8051 microcontroller driven Buck converter. The main aim of this work is to design a simple low cost DC voltage controller used to control DC loads which doesn't demand very high precisions. This paper is organized as follows :

* Review of PWM controlled Rectifiers

* Block diagram of controller.

* Controller implementation (software and hardware).

* Results and Conclusion.

Review of PWM Rectifier

Controlled rectifiers can be broadly classified as Thyristor controlled rectifiers and PWM rectifiers.

In thyristor controlled rectifiers in order to convert Alternating Current (AC) input to a controllable DC output voltage, thyristor firing angle is varied [4]. But the disadvantage associated with thyristor controlled rectifier is, difficulty in removing the low order harmonics. This problem arises because of one pulse per cycle in the input current of the converter and as a result the lowest order harmonic is the third harmonic [5].

In PWM rectifiers, to convert AC input voltage to a controllable DC output voltage, width of the pulse is varied. In order to remove low order harmonics, the converter switches several times during a half cycle (more number of pulses /cycle). This technique produces higher order harmonics which can be easily filtered out.

Controller Block Diagram

The PWM controlled DC converter is as shown in Figure-1. The speed in RPM is entered through the keyboard and corresponding to the key pressed, digital equivalent of that RPM is stored in memory.

Through Infra Red Speed sensors, the DC Motor speed is sensed and the analog output given by the speed sensor is converted to its 8 bit digital equivalent voltage, using 0808, successive approximation Analog to Digital Converter (ADC).

Using 8051 microcontroller ports, the output speed digital data is accepted and is compared with set speed. Using Proportional Control Algorithm an error signal is computed to adjust the PWM controller duty cycle.

The PWM signal, thus generated is used to control the MOSFET switch used in the Buck converter. Depending on the duty cycle of the PWM signal, the average DC output given out by the buck converter changes. User can change the speed at any instant of time in accordance to his requirements.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Controller Design

Controller is designed by using simple low cost components like 8051 microcontroller ([micro]c), 8 bit ADC (0808), 4X4 keypad, MOSFET switch (IRFZ48), speed/Intensity sensor, step-down transformer (12-0-12) and diodes (1N4001).

The implementation can be organized under 4 sections as:

* Keypad interface with 8051 [micro]c .

* A/D converter interface

* Generating PWM signals using 8051 microcontroller.

Keypad Interface

A 4X4 keypad is interfaced with 8051 microcontroller, through which four keys are accepted. The four keys which are accepted is combined to form an external memory address, in which digital equivalent of speed is stored.

For example if the keys entered are 4 (04), 6 (06), 8 (09), 9 (09), then they are combined as 4689 (Rotations Per Minute--RPM), which represents External memory address, in which 8 bit digital equivalent of that speed is stored. Data Pointer (DP) H is used to store 46H; Data Pointer (DP) L is used to store 89H. This method doesn't need any program execution to convert the entered speed in RPM into its digital equivalent which saves time. Alternatively digital data equivalent of RPM can be directly entered, provided a conversion chart is available [external look-up table]. This technique will save some more time, since communication with memory can be avoided. The logic of accepting the key is as explained in the flow chart, given in the Figure-3.

[FIGURE 3 OMITTED]

A/D Interfacing

The sensor output (O/P) converts speed of the motor to analog voltage equivalent, from 0 to 5V, (Minimum to Maximum Speed) respectively. This speed equivalent analog voltage is converted to digital data using an 8-bit ADC. This means a minimum of 19.5 mV change in voltage (corresponding change in RPM) is required to change the digital state of ADC. The variation in speed which produces voltage changes within 19.5 mV does not produce any change in the digital O/P of the ADC. This limits the accuracy of the application. The logic of interfacing ADC to [micro]c is explained in the flowchart given in the Figure-4.

[FIGURE 4 OMITTED]

PWM Generation

There is no inbuilt PWM generator in 8051 microcontroller. It is implemented using 'A' Register and any other register (R0-R7) of 8051. The control flow is illustrated in Figure 5.

[FIGURE 5 OMITTED]

A count (ON period time) is loaded onto one of the General Purpose Registers (GPR) which is also the Duty Cycle Register. And accumulator is loaded with zero, then 'A' register is incremented in steps of one and the same is compared with the duty cycle register.

If 'A' Register contents are less in magnitude than duty cycle register contents, a logical high is maintained at P1.1 port line of [micro]c. On the other hand if 'A' Register content is higher than duty cycle register content, a logical low level is maintained on the P1.1port line. The other technique is that, the timer can be used as counter by applying clock pulses externally and comparing the count in counter with 'A' register, but it requires external clock source, since 8051 doesn't have any clock out pin.

The duty cycle of PWM is varied in accordance with the error signal. The error signal is generated by comparing the required speed with accepted digital equivalent speed. If the required speed value is less than the accepted one, duty cycle register value and accepted value is decremented by one and this process is repeated till accepted value is equal to the required speeds digital value. On the other hand when the required speed value is more than the accepted one, duty cycle register values and accepted value is incremented by one and this process is repeated till accepted value is equal to the required speed digital values.

Results and Conclusions

The designed controller card is tested for speed control of a DC motor. The performance of the card, is tested for various duty cycles and for different RPM's. The results shows good accuracy with minimum error. The performance can be improved further by using high resolution ADCs and the delay involved in the software can be overcome by using higher versions of micro-controllers. Figure-6 illustrates DC O/P voltage obtained for various duty cycles. Figure-7 indicates actual motor speed versus set speed.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

References

[1] Sumant. G.Kadwane and others "Converter based DC motor speed control using TMS320LF2407A DSP" IEEE transaction, Industrial electronics, 24-26 may, 2006.

[2] G.S.Buja & P.Fiorini, "Microcomputer control of PWM inverters, "IEEE transaction, Industrial electronics, vol IE-29, pp 212-216, August 1982.

[3] G.S.Buja & Paolo.De.Nardi, "Application of a Signal processor in PWM inverter control, IEEE transaction, Industrial electronics, Vo IE-32, No-1, February 1985.

[4] V.Jagannathan, Introduction to Power electronics, Prenice-Hall of India, Private limited, New-Delhi, 2006.

[5] Muhammad.H. Rashid, power electronics circuits, devices and applications, 3rd edition, Prentice-Hall of India, Private limited, New-Delhi, 2004.

[6] Maksimovic.D and others "Impact of digital control in power electronics, ISPSD-04, May 2004.

S.A. Hari Prasad (1), B.S. Kariyappa (1), Dr R. Nagaraj (1) Commander S.K. Thakur (2)

(1) Department of Electronics & Communication Engineering, R.V. College of Engineering, Bangalore-560059, India

(2) Deputy Director, Naval Research Board, Defence Research Development Organization, New-Delhi, India E-mail: [email protected]