APPLICATION NOTE
Controlling a Brush DC Motor with an ST6265 Microcontroller
J. NICOLAI, T. CASTAGNET
INTRODUCTION Variable speed drives have become widespread in the home appliance field. The most classical way to achieve it is to use an AC universal motor with triac and a specific circuit which controls the triac firing angle. We chose rather to use a permanent magnet DC motor driven by a chopper. The chopper is driven by a high frequency PWM signal. Controlling the PWM duty cycle is equivalent to controlling the motor voltage, which in turn adjusts directly the motor speed. Why did we make such a choice? - The main reason is the need of a speed sensor and a closed loop regulation for the universal motor. For the permanent magnet DC motor, the speed is almost proportional to the motor supply voltage. Therefore it is sufficient to control the motor voltage (through duty cycle adjustment) if a 10% speed accuracy is sufficient. This saves the expensive speed sensor. - The DC motor with chopper has higher efficiency, as there are no 50Hz copper and iron losses, due to lower current ripple. - The DC operation allows to reduce the acoustic noise, especially at 100Hz. Figure 1A. Block Diagram
DC Motor
M
ST6265 IGBT
VR01988H
AN414 / 11,93
CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
Figure 1B. Ac Universal Motor Versus Permanent Magnet Dc Motor: The Dc Motor Has A Lower Current Ripple And Therefore Higher Efficiency And Less
Imotor Imotor
t
t
VR01988A
N
M
L M
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
1 PRINCIPLE The drive is supplied from the rectified mains voltage, and consists of a chopper driven by a PWM signal generated by the ST6265 Microcontroller. The motor voltage control is achieved by measuring the rectified mains voltage with the analog-to-digital converter present on the ST6265 MCU and adjusting the PWM signal duty cycle accordingly: the voltage seen by the motor is the product of the supply voltage by the PWM duty cycle. So the motor speed can be stable even if the mains voltage ripple is high, as the MCU corrects the duty cycle according to this ripple. This avoids the use of a speed measurement device and a speed regulation loop. If this voltage measurement / duty adjustment process is carried out often enough, it also results in reducing motor current ripple, and therefore copper and motor iron losses, thus improving motor efficiency. The ripple period being 10 milli-seconds (for 50 Hz mains), the later process must occur at least every milli-second to get a large ripple reduction. In our example, the voltage compensation loop lasts 380 micro-seconds. This drive also implements a power limitation: the motor current is measured by the analog-to-digital converter of the ST6265 MCU, and is used, together with the DC mains voltage, to compute the electrical power taken up by the motor. If it exceeds the maximum programmed power, the PWM signal duty cycle is reduced in real time by the ST6265 to stay below this maximum. This limitation requires only a few discrete components used to measure the current (2 resistors, a diode and a capacitor make up a peak current detector), it is otherwise fully software. Our application features a 300 watts maximum power, but this value can be easily adjusted by modifying a data table in the program, therefore allowing a very short design modification time if the drive must be used with various motors. 2 WHY USE a MICROCONTROLLER? The classical implementation of such a high frequency chopper usually involves an application-specific circuit, and some analog circuitry to implement the additional features such as voltage compensation, power limitation or other. At first look, a microcontroller should reduce the overall cost of the electronics, as a single chip would ideally replace several chips. The real world is not so simple: the ST6265 outputs the PWM signal with a 5 volts level, so a buffer is needed to drive the IGBT with 15 volts, which takes 2 or 3 signal transistors. The MCU must be supplied with a voltage between 3 and 6 volts, and the 15 volts must be generated, so we need a small auxiliary supply to extract 5 and 15 volts from the mains. In addition, the MCU cannot generally replace the fast transistor protection needed for the IGBT, as its response time is in the range of 10 micro-seconds, while less than 1 micro-second is required. In conclusion, the MCU approach comes out at the same price, or slightly more expensive than the analog one, at least if we only account for components costs. We must, however, consider the overall system cost, not only the components cost of one function.
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
Three factors at least should be considered: 1- In many cases, the appliance driven by the motor includes some kind of user interface: buttons, potentiometers, LED displays, even LCD displays. If the analog approach is used, more circuitry will be needed to interface them with the motor drive. With the ST6265 MCU approach, the MCU itself can sense the buttons, potentiometers, drive LEDS... In some existing appliances, the analog approach is used in conjunction with a microcontroller, the latter being dedicated to the user interface. This is obviously a waste, as one ST62 MCU could take care of both tasks. If we consider the case of LCD displays, which are becoming more and more popular, a dedicated LCD driver circuit is generally used to interface them to a micro. We have implemented a software LCD drive function in the ST6265 MCU, using its free output pins; the ST6265 MCU can at the same time run the motor, drive the LCD and sense the user speed demand (in our case, two buttons "faster" and "slower"). (Cf. ref. [4] page 12). 2- Time-to-market and design time must be considered: the ST6265 MCU software contains all the characteristics of the application; it is much faster, if the application must be modified, to modify software than hardware especially when OTP parts are used for volume production. We have implemented a software motor power limitation: the motor characteristics are known by the software in the form of a look up table. If the application must accommodate a different motor type, it is very quick to modify the data table in the software. Most modifications of the application can be achieved by simple software modifications. If the ST6265 MCU replaces an ASIC, the time and money saved when it is necessary to modify the initial design goes without saying. The ST62 MCU can also be used to customize the application: to limit the stock inventory, several applications, for example a low-end and high-end vacuum cleaner can use the same electronics board, with the same MCU, but the MCUs have different softwares, or the software is programmed into the ST62T65 MCU (OTP version of ST6265) at the end of the board assembly line. 3- The MCU lets you do things that are not possible otherwise. Simple examples are: - Execute pre-programmed sequences of motor speeds, diagnostic faults and alert the user (with temperature sensors, speed sensors, ... or sometimes without them, by deducing motor speed from current, or temperature from cumulated current ...). - Store in non-volatile memory (EEPROM) user settings which must be saved when power is turned off.
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
3 APPLICATION DESCRIPTION Figure 2. Permanent Magnet Dc Motor Control With St6265 Mcu And Igbt.
M
SUPPLY 15 V 5V 0V Vdd BUFFER IGBT PE AK CURRENT DETECTOR
VR01988B
Ud
USE R INTERFACE
PW M Vs s
MCU
Ic
The power part of the hardware is a bridge rectifier, an input filter capacitor, and the chopper. This converter drives the permanent magnet DC motor. The chopper switch is an IGBT. The control part of the hardware is made of the ST6265 MCU, the user interface and the power interface. The latter is made of a low-power supply (5 volts for the MCU and 15 volts for the IGBT gate), a level shifter to interface MCU and IGBT, a peak current detector and a resistor bridge to measure the loop DC voltage, Ud. The complete schematic with component values is shown in annex page 13 .
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
3.1 What Microcontroller to use? The MCU generates a 5 volts PWM signal which drives the IGBT and permits to control the average voltage across the motor, and consequently the motor speed through duty cycle adjustment. On the MCU are also implemented software functions such as a line voltage compensation and a motor power limitation described later. In order to implement these functions, the MCU should include the following special features: - PWM timer: it must be autoreload to leave the CPU free for other tasks, and fast to obtain high switching frequencies. - Analog-to-digital converter, necessary to measure the DC voltage and the motor current. - Good noise immunity to withstand the chopper and brush sparks proximity. - A watchdog for safe operation We used the ST6260 / ST6265: it is a low-end 8-bits MCU with 20 pins (or 28 pins respectively), depending upon the I/O request of the user interface.
Figure 3. St6260/65 Block Diagram. An Internal Analog-To-Digital Converter And A Fast Pwm Generator Are The
PWM RAM 128 EEPROM 128 TIMER
A/D CONVERTER TIMER
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABUSAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 8 BIT DATA AAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
WATCHDOG
SPI TIMER
AAAAA ROM AAAAA AAAAA 4K AAAAA AAAAA AAAAA AAAAA
8 BIT CPU
PORT A
PORT B
PORT C
20 or 28 pins package
VR01988C
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
3.2 The PWM Control The PWM feature is required to control the average voltage on the motor and therefore its speed. The autoreload timer of the ST6265 is used to achieve the PWM control (Cf. figure 4). - At each end of count, it reloads itself without CPU operation. The CPU is then available for other tasks after it has programmed the frequency and duty cycle of the PWM signal. - The duty cycle can be adjusted by software from 0 to 100%. The maximum frequency is: 31.25 kHz with a resolution of 256 steps (0.4% resolution). For our application, we chose a frequency of 8 kHz.
Figure 4. Pwm Timer Operation. Operation Of The Autoreload Pwm Timer: The Cpu Controls The Period With The Reload Register, And The Duty Cycle With The Compare Register; The
SWITCH ING PERIOD TIMER 255
COMPARE REGISTER
t 000 DUTY CYCLE
PWM OUTPUT
VR01988D
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
3.3 The Voltage Compensation Using such feature on a Permanent magnet DC motor, the speed is kept almost constant by keeping the motor voltage constant even if the rectified mains voltage is strongly swinging across the filter capacitor. So a closed loop speed control is not necessary and the speed sensor is saved. A compensation feature is implemented in order to: - obtain a motor speed unsensitive to mains voltage variations (static performance), - reduce acoustic motor noise and dissipation by reducing the motor current ripple (dynamic performance). The average motor voltage Vmot must be kept constant. The duty cycle d is modulated as a function of the direct voltage Ud around a reference point given by: - user request duty cycle (d0), - nominal direct voltage (Udnom).
Vmot = d x Ud Vmot = constant = d0 x Udnom
To achieve this function, Ud is measured using the A/D converter of the ST6265 and quantized on 32 levels, and d0 is quantized on 16 levels. A duty cycle correction is picked up from a look-up table versus Ud and d0. Then the correction is added to d0 and the sum d is loaded in the PWM timer register. The table could also contain values chosen specifically for the application. In order to reduce motor current ripple, the control loop must be fast, so as to minimize the time between voltage reading and duty cycle updating. Our compensation algorithm takes 380 micro-seconds, so in one Ud ripple cycle (10 milliseconds for a 50Hz line), the correction is made approximately 26 times.
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
Figure 5 shows results obtained for the static performance of the compensation: the motor voltage is measured while the DC input voltage varies from 100 to 400 volts. It is shown that the motor voltage remains within a 5 to 15% range of the voltage demand. This compensation is good enough for most home appliance applications. Figure 5. Voltage Compensation Static Performances. The Motor Voltage (And Therefore, Motor Speed) Remains Constant While The Dc Line Voltage Ud Varies. So The Motor Voltage And The Speed
V motor (V) 250
200
150
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
100 Vmotor < 10% 50 Ud (V) 500
VR01988E
0
100
200
300
400
Figure 6 shows the dynamic performance of the compensation: the 100 Hz ripple in motor current is considerably reduced when using the compensation (lower trace), compared to the ripple without compensation (upper trace). The measurement is made with a loaded motor: Imotor = 2.5 amps (average) / Vmotor = 105 volts. The mains voltage is 230 volts. Figure 6. Voltage Compensation Dynamic Performance: Motor Current Ripple Reduction (1A/Div, 2Ms/Div). The Top Trace Shows The Motor Current Without Compensation. The Bottom Trace Shows A Significant
I = 2 Amp
0 Amps --
I = 1 Amp
2 ms/dv ; 1 A/dv 0 Amps -Imotor av = 2.5 Amps ; Vmotor = 105 V Uac = 230 V ; C = 100 F
VR01988F
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
3.4 The Motor Power Limitation The purpose of the power limitation is to keep the motor within its safe operating area in case of excessive loading (avoid overheating and hard brush switching). The motor current is evaluated every 3 milli-seconds by measuring the peak chopped current in the IGBT emitter. If we call d the duty cycle of the PWM signal, Ud the motor DC supply voltage and Imot the average current in the motor, the electrical power fed to the motor is:
P = Vmot x Imot = d x Ud x Imot
If we want to limit P to a maximum value Pmax (300 W in our example), we must keep the duty cycle below a maximum duty cycle "d_max":
@
d_max
with
d_max = Pmax / (Ud x Imot)
The program repetitively measures Imot and Ud using the A/D converter of the ST6265. Then it extracts d_max from a table addressed by Ud and Imot. It then compares d to d_max and decreases d if it is larger than d_max. The power limitation presents "smooth" transitions between "limited" condition and "non limited" condition, in order to avoid oscillations or jerks. Figure 7 shows the measured performance of the power limitation. To modify the power limit target, the look-up table can easily be modified. Figure 7. Motor Power Limitation Performance. When The Motor Load Changes, The Motor Current Varies. The St6265 Mcu Measures This Current And Limits The Motor Voltage If Necessary. Software-Limited Motor Voltage (Left Scale) And Electrical Power (Right
Vmotor (V) 250 Power (W) 500
200
Power
400
150
300
100
200
50
Vmotor
100
0
0 0 1 2 3 Imotor (A) 4 5
VR01988G
6
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
3.5 Some Program Highlights The total program takes 400 bytes of memory space, plus 1024 bytes for the tables. One timer remains free, and 2500 bytes remain available on the ST6260 or ST6265 for additional software or enlargement of the look-up tables. It is possible, for example, to drive an LCD display by software, to execute pre-programmed sequences of motor speeds, current limitations, ... The EEPROM also remains available, for example to store user adjustments which must be saved when the power is off. The line voltage compensation algorithm must be executed at high speed, as already explained, while the power limitation algorithm can be executed less often. Therefore, the complete program executes seven line compensations for each power limitation. The line voltage is compensated every 380 micro-seconds while the power is sensed and limited only every 3 milliseconds. 4 CONCLUSION Microcontrollers are often limited to controlling and monitoring the user interfaces. This paper describes a digital solution based on a general purpose MCU with on-chip PWM generator, the ST6265. Besides the user interface such as an LCD drive, the MCU controls directly a DC motor with few external components and sophisticated control. A simple instruction set and the use of look-up tables allow for a short time-to-market, while avoiding the investment into an ASIC circuit. The demonstrated concept is applicable to home appliances (food processor, washing machine, refrigerator, HVAC ...) and to industrial applications where high speed accuracy is not required (pumps, fans ...).
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
5 REFERENCES [1] - Microcontroller and triac on 120/230v mains A.N 392/A - Ph. RABIER and L. PERIER (SGS-THOMSON Microelectronics) [2] - Design with microcontroillers in noisy environment A.N 435 - L. PERIER (SGS-THOMSON Microelectronics) [3] - Versatile and cost-effective induction motor drive with digital three phase generation A.N 424 - B. MAURICE, JM. BOURGEOIS, B. SABY (SGS-THOMSON Microelectronics) [4] - Direct Software LCD Drive with ST621x and ST626x A.N 594 - T. CASTAGNET, J. NICOLAI, N. MICHEL (SGS-THOMSON Microelectronics)
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P3 P4 D3 13V R20 2.2K T4 BC327 R9 10K C9 27pF X1 8MHz 27pF C10 C5 100nF R4 4.7K R1 6.8K R16 1K D2 R7 R6 220 R5 22K 47K R15 8.2K T1 BC337 D1 1N4148 2N2222A T2 1N4148 C6100 1nF STGP10N50 T3 R3 R19 15K C3 100uF R2 270K
C7 47n
C4 200uF
D4 5.1V
C1 D5 STTA806DI C8 100nF D10 BYT11-600
D6 4X3A 600V P2
100u380V
R18 12K/2W
R12 3.9K ST6265 SW2 SW1 D7 LED
R13 3.9K
R17 12K/2W
15 16 17 18 19 20 21 22 23 24 25 26 27 28
PA3 PA2 14 PA4 PA1 13 PA5 VSS 12 PA6 VCC 11 PA7 PA0 10 XTAL PB7/TIM20 9 EXTAL PB6/TIM21 8 NRES U1 PB5 7 CKOUT PB4 6 PC4 PB3 5 PB2 4 PC3 TST/VPP 3 PC2 PC1/TIM1 PB1 2 PC0 PB0 1
P1 R10 0.47/3W
F1
R11
Annex: Permanent magnet motor drive complete schematic
3A
D8 LED
CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
0.47/2W
VR01988
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CONTROLLING A BRUSH DC MOTOR WITH AN ST6265 MICROCONTROLLER
NOTES :
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsability for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without the express written approval of SGS-THOMSON Microelectronics.
1994 SGS-THOMSON Microelectronics - All Rights Reserved
Purchase of I2C Components by SGS-THOMSON Microelectronics, conveys a license under the Philips I2C Patent. Rights to use these components in an I2C system, is granted provided that the system conforms to the I2C Standard Specifications as defined by Philips. SGS-THOMSON Microelectronics GROUP OF COMPANIES Australia - Brazil - France - Germany - Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco The Netherlands - Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom U.S.A.
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