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APPLICATION NOTE
Microcontrollers in Home Appliance: A Soft Revolution
by L.Perier
INTRODUCTION The industrial world is now an arena where many manufacturers produce low cost, high quality products. Cost cutting and outsourcing are no longer sufficient to ensure competitiveness. Creativity and time to market become a must to survive. As a result, traditional electromechanical solutions do not fit many new home appliance requirements. Now microcontrollers (MCUs) offer a strategic advantage for the design of cheap, attractive and environmentally safe products. For example, new MCUs can operate directly from the mains and drive power loads with only few external components. They can reduce the energy consumption, motor size and the cost of the appliance. In the first chapter, this article analyses the forces driving new appliance concepts. In a second part, it describes state of the art solutions. The third chapter presents MCUs well adapted to home appliances and tools for increasing a designer's productivity.
AN674/0694
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1 FROM ELECTROMECHANICS TO MICROCONTROLLERS Home appliance customers are more sensitive to price than performance. Manufacturers are mainly competing by price cutting traditional solutions based on electromechanical components. They also optimize the behaviour and especially the energy consumption of their equipment [1]. Table 1: Equipment price variation 1990/1982 in USA
Air conditioning Refrigerator Washing machine Car Food Medical care -2% +3% +7% +21% +32% +62%
Table 2: Energy consumption variation 1990/1972 in USA
Washing machine Air conditioning Disher Refrigerator Freezer
Source: Appliance Magazine June 1993
-30% -33% -36% -47% -59%
However competition and environmental protection standards are increasing the constraints level. Customers want more comfortable appliances for less money and these objectives are difficult to achieve with traditional solutions. But home appliance is one of the last volume markets with so few electronics. And when electronics exist, they are usually very basic. More than 2/3 of products do not contain electronics and when they do, the semiconductor content is low (usually 1 to 3 US$). So there is a strong potential of improvement with a limited cost impact when embedding more electronics inside home appliances. An MCU is an especially good candidate because with an MCU, speed, volume and flexibility are compatible. One design leads to one product flow, fewer components and more models [2].
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2 INNOVATIVE AND SUCCESSFULL APPLICATIONS Several million MCU based appliances are presently used all over the world. The electronic content is usually not visible but it includes optimized control and protection features. This chapter presents some of these appliances. Then it describes state of the art motor drive techniques. Finally it analyses the cost of these controls and the impact of an MCU on the testing. 2.1 State of the art appliances If a light dimmer works well with discrete components, there is no reason to use an MCU. The MCU is useful if additional features such as soft start, soft stop, touch sensor interface or drive of a 50/60Hz transformer are required [3]. Figure 1: Light dimmer circuit
FUSE LINE
100 1
+5V
USER INTERFACE
RESET PB0 15 4,7M 2x4,7M
+5V BTA 08-600 SW 100K 200K 200K 12V 22K 0V
5V6 1N4148 100F 6V
VDD 19 PA0 18 PA1 17 PA2 16 PA3
TOUCH SENSOR
PB1 14
VERSION
13 PB2 12 PB3
100K
0V
11
PB4
ST6210
5 NMI TEST 6 VSS 20 OSCOUT 4
PUSH BUTTON 0V
+5V
10 PB5 OSCIN
+5V
3 22p
PIEZO
8MHz
22p
POTENTIOMETER
820-1/2W 220nF/400V
0V
0V
0V
0V
NEUTRAL
0V
The majority of European vacuum cleaners have an electronic speed adjustment. Many include an MCU which controls the air flux temperature and the power transmitted to the motor through a triac. Standards make it mandatory that the motor is protected if the air flux is reduced. The air flux temperature is monitored and the power is continuously decreased if the motor gets overheated. This electronic protection avoids oversizing of the motor. Inside vacuum cleaners sold today on the market, parameters are stored in the MCU memory which select different programs, I/O configurations and motor or display parameters. The same MCU is used in a few boards which are mounted in more than ten different models.
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In drills and food processors, the load torque can vary greatly, leading to unacceptable speed variations when using a universal motor. As a result, the speed of the motor is often measured with a tachogenerator. This sensor is expensive and difficult to install; using an MCU, the current in the motor is measured and the speed sensor is avoided. Specific control rules are applied in the MCU software to keep the speed almost constant when the load or the input voltage vary [4]. Figure 2: Speed sensorless drill control
BTB 08 600 CW
N 0.2 / 2 W 3K 100 150
M
47 nF 100 uF
Vs SENS GATE OSCOUT
4 MHz
5V6
ST6220 hwd
GND LIM REF OSCIN 0 CROSS
2x33pF
1N4004 20 k 100 k L 2 x 8.2 k / 1W 2M 100 k
MCUs are also used extensively in washing machines. An MCU program controls first the acceleration and the washing speed of the drum. It includes also safety features and washing patterns to reduce the washing time and the power consumption. Using an MCU, modified features can be introduced quickly and without new components. Going away from the analog IC, the parameter tuning is greatly simplified and design improvements such as new washing patterns are protected. The MCU is usually used in association with electromechanical timers which drive power pumps or resistances and interface to the user. A cordless tool requires fast and safe battery chargers. And the faster the charge is, the more accurate the control has to be, especially with new types of batteries such as NiMH. So several parameters are controlled which may vary for different battery types. The charger presented in figure 4 charges NiCd and NiMH batteries in less than one hour. It monitors the variation of the battery voltage (inflexion point and -deltaV), its temperature and the charge time. The method is independent to the battery capacity and charge time. All this is done with a general purpose MCU and as a specific software [5]. Evolutions such as display, charger gauge, simple/double charger, Alkaline or Lithium-Ion charger can be implemented from this basis.
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L4962 1 +Vbatt 7 2 + 5 1nF 10K 3
80uH 1.5A
4 UC3843 7 VIN VREF 22nF 2.2uF 10V 12K 0.33R 1.5W I=1.5A OUT 6 VFB 2 8 15K BYV10-40 BYV10-40 100uF 25V
6
PACK6V BATTERY 1500mAH
+
+
4 4.7uF 10V RT/CT COMP 1 G C/SEN 3 N D 5
2A
11 TO 25V
DC
220uF 25V
-
Figure 3: Fast battery charger (15W - CD/DC)
GND
4.7uF 16V
1 VDD TIMER 680 On 1K Temp 680 Stop OSCIN OSCOUT NMI VPP/TES RESET 8 PB7 PB6 PB5 ST62E10 6.8K 10V 2.2uF 2.2uF 10V PB3 12 PB4 11 9 10 15K 6.8K 15K PB1 14 PB2 13 PA3 16 PB0 15 LED PA1 18 PA2 17 2 3 4 5 6 VSS 20 PA0 19
2MHz
33pF
33pF
27K 6.2K 8 BZX55C3V3 6 7 3 2 1K 514 uA741 9.1K R12 18K 6.8K 2.2uF 10V
MICROCONTROLLERS IN HOME APPLIANCES
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User friendly interfaces can be implemented without MCU oversizing. MCUs such as ST62 include A/D converters and I/O pin functionality reconfigurable by software. These MCUs drive directly an LCD [6], LEDs, a triac, an audiotransducer or a keyboard [7] with a minimum of pin count and components. They enable the design easily and quickly of features attractive for the customers. Figure 4: Direct LCD drive
2 x 8 SEGMENT LCD
VCC R1 470K R3 470K
COM 56
SEGMENTS
2 1109 4 3 8 7
R2 470K
R4 470K
KEYBOARD
15 16 17 18 19 20 21 22 23 24 25 26 27 28
PB4 PB3 PB2 PB1 PB0 PA7 PA6 PA5 PA4 PA3 PA2 PA1 PA0 V SS
PB5 14 PB6 13 PB7 12 RESET 11 TEST 10 PC4 9 PC5 8 PC6 7 PC7 6 NMI 5 OSCout 4 OSCin 3 TIMER 2 V DD 1 ST6215 VCC
VCC D1 1N4148 C3 100nF
C1 4MHZ22pF Q1 C2 22p
In summary, experience suggests many solutions for the design of cheap home appliances, attractive for users and environmentally safer. These appliances are using MCUs which enable optimization of the control and the inclusion of user friendly interfaces for a low cost. 2.2 Improved motor drive The motor and its drive have a high impact on most home appliances features, including cost, size, noise and efficiency. Electronic control is usually necessary when variable speed or energy savings are required. Progressive improvements on material and control techniques lead to the development of "electronic motors", smaller, less noisy and more efficient. These products find applications in freon-free refrigerators, pumps, air conditioning, fans,... but no dramatic breakthroughs are anticipated. The universal motor will be still used if the brushes are accepted. Brushless motors such as the permanent magnet motor for low speed and the reluctance motor for high speed have also a bright future. For whatever motor, low cost MCUs exist today to optimize the control.
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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 R7 1K D2 R6 220 R5 T1 BC337 22K 47K R15 8.2K D1 1N4148 2N2222A T2 1N4148 C6 100 1nF STGP10N50 T3 R3 R19 15K C3 100uF R2 270K
C7 47n
C4 200uF
D4 5.1V
C1
D6 4X3A 600V P2
100u380V
D5 STTA806DI
C8
Figure 5: DC Permanent magnet motor control
R18 12K/2W
100nF
R12 3.9K ST6265 SW2 SW1 D7 LED
R13 3.9K
D10
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 PC3 PB2 4 PC2 TST/VPP 3 PC1/TIM1 PB1 2 PC0 PB0 1
R17 12K/2W
BYT11-600
P1 R10 0.47/3W
F1
R11
3A
D8 LED
MICROCONTROLLERS IN HOME APPLIANCES
0.47/2W
VR01988
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2.3 Cost evaluation In most appliances, a motor or a programmable timer is usually more expensive than an electronic board. However even if a MCU is a bit more expensive than a traditional control, the performance/cost ratio is strongly in favor of the MCU approach. The overcost of optional features such as an LCD or a tachogenerator feedback is also limited because it does not lead to an MCU oversizing. Table 3: Typical cost a small appliance motor drive (drill)
Block Active Passive Components IC Triac Supply Snubber Capacitors Resistances Resonator Mechanics Total board Universal motor Grand total Heatsink PCB 3.5$ 10.0$ 13.5$ 1.0$ 0.8$ MCU board 1.7$
Table 4: Typical cost of option examples
Block LCD display Tachogenerator Components Display Zebra strip Sensor Wiring Diodes Transistor Passive Components 1.1$ Cost 1.2$
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2.4 Test for quality The same MCU can also improve the quality of the finished appliance. During the final test, the MCU can be used in a special "test mode". Then the behaviour of the different sensors and actuators is analysed and stored in the MCU. This data is later transmitted to a PC via a serial port and a PC identifies immediately any possible defects. A washing machine manufacturer proclaimes to have decreased his test times by 20% to 30% when including such autotest features in the program. Once connected to the board, the PC can also load parameters in the RAM or in the EEPROM of the MCU to compensate for some variations in sensors or to indicate the reset conditions of the MCU for the target application. In summary, this second part shows that MCUs are used with success in Europe for home appliances. They enable the design of flexible equiment, cheap to manufacture and test. They open the door to improved motor control which reduce the noise, energy consumption and the size of the appliance. In addition, the manufacturing efficiency reduces the system cost.
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3 AN MCU SIMPLE AND SECURE This chapter describes MCUs well adapted to home appliances: the ST62 family. These MCUs are safe when operating directly on the mains in a power environment and they include the features required for appliances. 3.1 Hardware features: Home appliances include strong noise sources such as power motors, micromotors, relays, valves and the power supply. An MCU works in sequential logic, so there is a potential risk that it becomes unpredictable at reset and during other disturbances. ST62 operation is safe even in hostile environments. These MCUs include protection on the I/O pins, a hardware watchdog circuit to restart the MCU if the program flow is lost and other cells shown figure 6. These characteristics simplify the circuitry and make the difference between a product which works in the lab or at customers [11]. They avoid also the usage of expensive shields, PCBs and capacitors networks. Figure 6: Noise immunity by design on ST62
SPREAD SPECTRUM TRIGGER
SERIAL BUS
SAFE RESET
H/W WATCHDOG
WIDE VOLTAGE RANGE
FILTERING i PROTECTION
A/D
ST62
OSCILLATOR SAFE GUARD
OSCILLATOR
ANTI CROSSTALK
CONTROLED SLOPE
NOISE REJECTION
The ST62 can be seen as an analog programmable controller which interfaces sensors, stores data, controls feedback loops and drives power loads. Inside an ST62, the Analog-to-Digital Converter (ADC) and the timers measure sensor information. The same timer provides accurate and stable time bases adjustable for long and short durations without additional components. The 8-bit core treats the data which is stored inside the memory RAM or EEPROM. The multifunction I/O pins drive directly a triac, an LED, an LCD or a serial port. This port flexibility
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is a major factor for adaptation of a single product to different models only by software selection. The ST62 are designed in CMOS technology. So they have a low consumption and they can be supplied from the mains through a low cost RC style circuit. Figure 7: Block schematic of ST6265
PWM RAM 128 EEPROM 128 TIMER
A/D CONVERTER TIMER
8 BIT DATA BUS
WATCHDOG
SPI TIMER
ROM 4K
8 BIT CPU
PORT A
PORT B
PORT C
20 or 28 pins package
VR01988C
The One-Time-Programmable versions contain an EPROM memory instead of the ROM. Programmed by the user, they enable the immediate test of any modifications and avoid the cost and delays induced by a ROM mask. The typical overcost versus ROM parts is between 1.5 to 2 for similar quantities. These OTPs are a key element to accelerating the time to market of new designs. Some ST62 include also an EEPROM memory to store parameters in the production flow or during the product life time, even in case of power fail. With MCUs such as ST62, an home appliance designer can implement sophisticated controls for a low cost. He can develop innovative solutions, quickly adaptable to the market evolution. 3.2 Software features The move from electromechanics to MCUs is delicate and a new design is never straightforward. Easy-to-use development tools are mandatory to accelerate training and design. ST62 tools are designed for this purpose. They include application notes, program libraries and low cost demonstration or programming circuits such as a Power Kit and several Starter Kits.
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High level languages can also accelerate the design. Fuzzy logic is used today extensively in Asia and Europe. For instance, it optimizes the charge of a battery, it minimizes the water and the energy consumption in a washing machine, it filters the IR sensor in a passive presence detector and it regulates the temperature in an air conditioner and in different compartments of a refrigerator. The fuzzy logic approach is especially powerful in home appliances to help a designer with an analog background to develop an MCU based control. Using tools such as fuzzyTECH ST6 Explorer Edition, a designer can develop, test and optimize a regulation loop quickly. For instance, the fuzzy logic motor control described in the bibliography [12] has been done in few weeks. Figure 8: Graphical interface of fuzzyTECH ST6 Explorer Edition
In summary, this third part shows that simple MCUs well adapted to power environments such as home appliances are now available. When development tools optimized for these applications are chosen, a designer takes full advantages of these MCUs: he learns fast the product and tests quickly his solutions.
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SUMMARY In the present global market, companies can not rely solely on price and brand recognition to remain competitive. They have to be innovative and quick to market. Practical experience demonstrates that microcontrollers (MCUs) are now mature for home appliances. A company which masters both traditional techniques and MCUs has a major advantage to answer to new market trends. MCUs such as ST62 match well home appliance requirements. They are secure in the power environment and include cells which minimize the board cost. With OTPs, they give the flexibility to develop rapidly low cost appliances adapted to market evolutions. These products are driven more by imagination than material. BIBLIOGRAPHY [1] Appliance magazine June 1993 [2] Power Semiconductors and Micros - K.Rischmuller [3] Microcontroller and Triacs on the 110/240V Mains / AN392 [4] Sensorless Motor Drive with the ST62 MCU + TRIAC / AN416 [5] From NiCd to NiMH Fast Battery Charging / AN417 [6] Direct Software LCD Drive with ST621x and ST626x / AN594 [7] Using ST6 Analog Inputs for Multiple Key Decoding / AN431 [8] Improved Universal Motor Drive / AN422 [9] Controlling a Brush DC Motor with an ST6265 / AN414 [10] Versatile and Cost Effective induction motor drive with digital 3 phase generation / AN424 [11] Designing with Microcontrollers in Noisy Environments / AN435 [12] An Approach to Motor Control with Fuzzy Logic / AN419
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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.
(c) 1994 SGS-THOMSON Microelectronics - All Rights Reserved
Purchase of I2C Components by SGS-THOMSON Microelectronics, conveys a license under the Philips I 2C 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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