Future Energy Challenge: Motor Controller | ECE 445, Study Guides, Projects, Research of Electrical and Electronics Engineering

Download Future Energy Challenge: Motor Controller | ECE 445 and more Study Guides, Projects, Research Electrical and Electronics Engineering in PDF only on Docsity! Future Energy Challenge: Motor Controller By Brian H. Green Nestor Castillo ECE 445, SENIOR DESIGN PROJECT FALL 2004 TA: Chad Carlson December 4, 2004 #25 ABSTRACT This paper details the design and implementation of the control system of a single-phase, adjustable speed motor drive for use in the Future Energy Challenge. The design uses a constant volts- per-hertz method with integral feedback control to drive a three-phase induction machine with a PIC microcontroller. First, the intended functions and goals of the project are described. Next, decisions made during the design of the controller are detailed and a final design is presented including the tests used for design verification. Finally, an estimated cost analysis is presented with overall project conclusions. ii 1.3 Subprojects The project was divided in to hardware and software subprojects for the purpose of dividing work among group members.  Hardware : The primary components of the hardware design for the motor controller include a DC bus voltage measurement circuit, a DC bus current measurement circuit, a serial port interface, and a printed circuit board (PCB) layout.  Software : The primary components of the software design include the proper implementation of a volt per hertz control algorithm through the use of an integral feedback controller. Also, an over-current function was implemented in software as well. 2 2. DESIGN PROCEDURE 2.1 Preliminary Design Decisions The first step in designing an induction machine controller was to choose an algorithm. Three algorithms were researched for possible implementation. The first, volts-per-hertz control is a scalar control that provides a constant motor torque until the voltage is limited by the input supply. The other two investigated; direct torque control (DTC) and space-vector modulation control, are vector based control techniques. They provide better dynamic responses at the cost of mathematical complexity. Because the design specifications included no dynamic requirements, the easier to implement volts-per- hertz control was chosen. The second decision, made in parallel with the first, was the platform on which to implement the control. The more complicated algorithms may have required the use of a digital signal processor. The simpler volts-per-hertz control reduced the computational complexity so that a PIC microcontroller was adequate. The microcontroller had both a lower cost and a simpler programming interface. The PIC18F4431 was chosen because it had several features important to motor control. The primary feature is the 6 pulse-width modulated (PWM) outputs. This means that the dead-time required to protect the inverter hardware can be provided in software instead of circuits external to the microcontroller as would be needed if only 3 PWM outputs were available. By implementing dead-time in software, it will be easier to tune and the controller will be more reliable because of the lower component count. Once an algorithm and processor were chosen, decisions had to be made for the supporting hardware. Over-current and over-voltage protection for the system were deemed to be necessary for a robust design. Also an interface to the serial port of a computer was added to the design for debugging support. Each of these peripheral circuits required design decisions. The over-current protection circuit requires that current on the DC bus of the system be measured. The current could be measured with either a shunt resistor or a Hall-effect sensor. The Hall-effect sensor is more expensive, but has the advantage that the signal is isolated. Control ground for the project was chosen to be the negative side of the DC bus, and so isolation was not needed for a shunt resistor. Therefore, the lower cost shunt resistor was chosen. Once the current is measured, the over-current protection can either be implemented by the microcontroller, or by a counter and comparator off-chip. Using an external circuit can provide a faster response time for large current spikes that can harm the inverter. Because the inverter design for this project included protection from over-current spikes, the fast response time was not needed, and so the microcontroller was used. This had the advantage of reducing the component count and so reducing cost and increasing reliability. Also, by implementing the protection in software, the parameters of the protection are easily tunable without having to make changes in hardware. Both the over-current and over-voltage circuits required the use of op-amps. A smaller design decision was which type of op-amp to use. For both circuits, very high input impedance was desired to maintain the accuracy of the signal. For this reason, J-FET op-amps were used instead of CMOS op-amps. With these preliminary design decisions made, a more detailed design of each of the components could be made. 3 2.2 Voltage Measuring Circuit The voltage on the DC bus is measured to protect the components of the design from harmfully high voltages. To measure the voltage, a voltage divider is used from the high to low side of the DC bus as shown in Figure 2 below. Ra Rb Voltage Sense +Vdc -Vdc Figure 2 Voltage Measuring Circuit Preliminary Design The values of the resistors are chosen so that the nominal voltage (200V) corresponds to a 3V input on the microcontroller, according to a simple voltage divider equation shown in equation (1). The 3V value is used so that the maximum resolution can be used on the 0-5V input signal to the microcontroller. 3 187 200 3 2 1 21 2   R R V V RR R (1) Equation (1) constrains the ratio of the resistors, but their values are not constrained. A lower value of resistance would result in more current flow, and so more loss in these sense resistors. Raising the resistance values, on the other hand, causes the error due to the current drawn at the signal to increase. Resistance values must be chosen so that they do not dissipate an unreasonable amount of energy, but so that the signal they provide is reasonably accurate. We chose resistance values of 680k Ω and 10k Ω 2.3 Current Measuring Circuit The first decision to be made in designing the current measuring circuit is the value of the current sensing resistor. A larger value will correspond to a larger signal and so less gain required on the op- amp circuit that gains the signal into the readable range for the microcontroller. A larger value will also correspond to larger power consumption in the sense resistor. A balance must be struck therefore between too large a gain and too large a power consumption. The small signal provided by the current sensing resistor must be processed so that it is readable for the microcontroller. An op-amp circuit is used to provide this gain as shown in Figure 3 below. 4 Causing the timer overflow to occur more frequently would cause the output PWM voltage to trace out a sine wave faster. This method of controlling the output voltage frequency was used to control the speed of the motor. Because induction machines run slower than synchronous speed by a load-dependent value called slip, a feedback control was necessary to keep the speed regulation within the required 5%. This was accomplished by using a photo-interrupter and an 8-toothed gear shaped feedback device mounted on the shaft and shown in Figure 7. This feedback device provides a square wave with frequency proportional to shaft speed. Figure 7 Speed Feedback Device After the feedback device was decided, a method of feedback control in software needed to be designed. Proportional-Integral (P-I) control was considered. The microcontroller manufacturer provided example code for a P-I feedback control, but the complexity of integrating this example code into the motor control software made it an unattractive option. Instead, a new quasi-integral control method was written which was simpler to integrate into the existing code. The control software written compared the speed of the feedback device to the commanded speed to generate an error signal. A control variable was then incremented or decremented based on this error signal. This control variable was added to the timer reload value to cause a faster or slower overflow, and so a higher or lower frequency output voltage. This method of integral control provided a sluggish response, but was accurate in the steady state. Because the requirements for the FEC were only for the steady state, this was deemed an adequate control method. 7 3. DESIGN VERIFICATION The hardware designs were tested separately from the motor control software. Each hardware sub- circuit was tested and the results are described below. 3.1 Hardware Testing A power supply was used to provide a current signal through the current sensing resistor. The current put out by the power supply was compared to the signal voltage put out by the circuit. The results are shown in Figure 8 below. The circuit provided a signal which corresponded linearly to the input current as expected. A small offset voltage was unexpectedly observed, but was accounted for in software. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Current (Amps) S ig n al ( V o lt s) Figure 8 Current Measuring Circuit Test Results A voltage input to the voltage measuring circuit was provided by a power supply. Again, the signal voltage was observed. The results are shown in Figure 9 below. The signal was linear to the input voltage as expected, and the proper gain was observed from the voltage divider network. 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Voltage (Volts) S ig n a l (V o lt s ) Figure 9 Voltage Measuring Circuit Test Results 8 The isolated serial port circuit was tested by connecting it to a computer. A microcontroller was programmed to output data corresponding to an analog input. The ground reference for the microcontroller was changed to +12V with respect to earth ground. When the analog input was varied, the computer displayed the appropriate results, and so the isolated serial port circuit was verified. 3.2 Software Testing The motor control software was tested by using a purchased power supply and inverter to power a laboratory induction machine. The software was tested separately from the hardware components and was not demonstrated on the PCB layout shown in the appendix. Speed control was verified by using a dynamometer to measure speed and provide loading. Various speeds were commanded, and the motor was observed to stay within the required 5% of the commanded speed. 9 REFERENCES [1] Adel S. Sedra and K.C. Smith, Microelectronics Circuits. New York: Oxford University Press, 2003. [2] Jonathan Kimball, “Future Energy Challenge,” class notes for ENG 491FC1, College of Engineering, University of Illinois at Urbana-Champaign, Fall 2004. [3] Microchip, Appl. Note 900, “Controlling 3-Phase AC Induction Motors Using the PIC18F4431,” pp. 1-24. 12 APPENDIX – Schematics and Layout T i t l e S i z e D o c u m e n t N u m b e r R e v D a t e : S h e e t o f S K 0 0 4 6 - 2 S e r i a l P o r t I n t e r f a c e A 1 1F r i d a y , D e c e m b e r 0 3 , 2 0 0 4 1 2 J 8 C O N 2 R 1 0 1 k 3 - V b u s C 1 1 u F C 2 1 u F C 4 1 u F C 3 1 u F C 5 1 u F R 3 1 k + 5 V D C R 1 1 0 o h m + 1 2 V +5V Nestor Castillo & Brian Green R 2 1 k + 5 V D C T X R X C 6 . 1 u F C 7 . 1 u F R 1 1 1 k 5 9 4 8 3 7 2 6 1 P 1 C O N N E C T O R D B 9 N C 1 1 + V F 2 - V F 3 N C 2 4 V C C 8 V E 7 V o 6 G N D 5 O P T O I S O 1 6 N 1 3 7 V O U T 1 G N D 2 G N D 2 3 N C 4 V I N 8 G N D 4 7 G N D 3 6 N C 2 5 R E G U L A T O R 1 L M 7 8 L 0 5 C 1 + 1 V S + 2 C 1 - 3 C 2 + 4 C 2 - 5 V S - 6 T 2 O U T 7 R 2 I N 8 V C C 1 6 G N D 1 5 T 1 O U T 1 4 R 1 I N 1 3 R 1 O U T 1 2 T 1 I N 1 1 T 2 I N 1 0 R 2 O U T 9 D R I V E R / R E C E I V E R 1 M A X 2 3 2 N C 1 1 + V F 2 - V F 3 N C 2 4 V C C 8 V E 7 V o 6 G N D 5 O P T O I S O 2 6 N 1 3 7 13 I N D X D 2 L E D D 3 L E D D 4 L E D D 5 L E D D 6 D I O D E Z E N E R 1 - V b u s D 7 D I O D E Z E N E R 1 D 8 D I O D E Z E N E R 1 T i t l e S i z e D o c u m e n t N u m b e r R e v D a t e : S h e e t o f S K 0 0 4 6 - 1 3 M i c r o C o n t r o l l e r A 1 1F r i d a y , D e c e m b e r 0 3 , 2 0 0 4 S p e e d I n p u t 1 2 3 4 5 6 J 5 H E A D E R 6 Nestor Castillo & Brian Green C 8 . 1 u F + 5 V D C P W M 3 P W M 5 P W M 4 P W M 1 P W M 2 P W M 0 P W M 5 T X R X I _ S e n s e V _ S e n s e P W M 4 P W M 3 P W M 2 P W M 1 P W M 0 - V b u s 1 2 J 4 C O N 2 S W 1 S W P U S H B U T T O N R 1 2 4 7 0 o h m s - V b u s 1 2 O S C 1 O s c i l l a t o r C 9 3 3 p - V b u s + 5 V D C R 1 7 1 0 k C 1 1 3 3 p - V b u s M C L R 1 R A 0 / A N 0 2 R A 1 / A N 1 3 R A 3 / A N 3 / V r e f + 5 R E 0 / A N 6 8 O S C 2 1 4 R B 7 4 0 R B 6 3 9 R C 0 1 5 R D 0 1 9 R D 1 2 0 A V D D 1 1 A V S S 1 2 R D 2 2 1 R C 6 / T X 2 5 R C 7 / R X 2 6 V S S 3 1 V D D 3 2 P W M 0 3 3 P W M 1 3 4 P W M 2 3 5 P W M 3 3 6 P W M 5 3 7 P W M 4 3 8 O S C 1 1 3 R A 2 / A N 2 / V R E F - 4 P I C 1 8 F 4 4 3 1 I N S 8 6 5 2 0 R 1 3 4 7 0 R 1 4 4 7 0 R 1 5 4 7 0 1 2 3 4 5 6 R J _ 1 R J - 1 1 J a c k R 1 6 4 7 0 - V b u s - V b u s - V b u s 1 2 3 J 9 C O N 3 R 1 8 2 k - V b u s + 5 V D C C 1 2 . 1 u 14