Ronak Desai, Cypress Semiconductors

August 10, 2013


Print
Email

Ronak Desai, Cypress Semiconductors

Editor’s note: In addition to discussing home appliance design with microcontrollers, this article describes a more integrated approach using programmable system-on-chip (PSoC) devices that incorporate software-programmable MCUs with hardware-programmable logic arrays.

In many home appliances such as washing machines, air conditioners, microwave ovens, vacuum cleaners, and refrigerators, microcontrollers (MCUs) are used for motor control, analog sensor measurements, front panel keypad control, and LED/LCD displays.

The home appliance industry uses 8-, 16-, and 32-bit microcontroller-based circuitry for motor control and TRIAC/ LED/ LCD drive applications. The MCU controls and manages all the functions and feature of the appliance. When the user presses the start button, the input goes to the microcontroller from the front panel keyboard and the MPU starts the three-phase brushless DC (BLDC) motor/permanent magnet synchronous motor (PMSM). Motor speed will be varied and controlled as per user inputs from the front panel keypad.

The MCU uses either an internal or external serial EEPROM (I2C/SPI based) to store old data. It uses a real-time clock (RTC) for displaying accurate time information. Temperature measurements are done using an onboard resistance temperature detector (RTD)-, thermistor-, or thermocouple-based temperature sensing device.

The MCU uses an external ADC and amplifiers for receiving analog inputs from sensors, temperature, and battery. It uses external signal conditioning, comparators, and gate driver circuitry for driving and controlling a 3-phase BLDC/ PMSM motor. The microcontroller can also receive remote control inputs through an IR receiver (at 38 kHz input).

External buffer driver circuitry is required to drive 7-segment LED/LCD/graphical displays. Typically a 7-segment LED/LCD/graphical display with backlight is used for showing temperature, battery input, speed value, and error/warning messages. The microcontroller also interfaces with onboard peripherals such as I2C/ SPI and external peripherals like UART/USB communications.

MCUs in washing machines
MCUs in washing machines include the following blocks:

Click on image to enlarge.

Figure 1: Washing machine block diagram using an MCU

Click on image to enlarge.

In a washing machine, the MCU receives analog inputs (water level sensor, water hardness sensor, humidity sensor, door open sensor, laundry load sensor, optical sensor, detergent density sensor, load Imbalance sensor, and volume sensor) through an external ADC. Temperature sensing is done by an onboard RTD and an external EEPROM is used to store data, such as customized wash program, memory backup, child lock, and favorites. The microcontroller allows the washing machine to adjust the water and cut power automatically.

The MCU also controls self-diagnostics, including water supply failure, spin failure, drainage failure, child lock, overflow protection, and door lid open. Clocks and timers are used to implement sleep mode and add delays (delay start condition) in operations. A buzzer (PWM-based) generates tones at different frequencies and also provides alert tones during overload conditions. The MCU also automatically turns off the machine after washing is completed, thus saving power.

MCUs in air conditioners
MCUs in air conditioners include the following blocks:

Click on image to enlarge.

Click on image to enlarge.

In air conditioners, the MCU receives analog inputs (sensor input) through an external ADC. Temperature sensing is done by an onboard RTD and thermistor, and an external EEPROM is used to store data (set temperature value). The MCU controls external BLDC motor and fan using PWM and comparators. It receives various filter inputs, which are used for purification.

The MCU controls the motor and compressor based on the temperature set by the user. The MCU also uses a relay driver and TRIAC driver circuitry to switch off power line AC inputs to the system. It uses clock and timers to set up sleep mode, auto switch-off function, and 24-hr on/off timer function. It uses PWM-based buzzer for generating tones at various frequencies. The MCU also controls self-diagnostic features, and includes auto restart and over-current protection. During power failure, it automatically restores the air conditioners to previous settings.

MCUs in microwave ovens
MCUs in microwave ovens include the following blocks:

Click on image to enlarge.

Click on image to enlarge.

In microwave ovens, an MCU receives analog inputs from weight sensor, humidity sensor, volume sensor, CT current sensor through an external ADC. Temperature sensing is done by an onboard RTD and thermistor, and an external EEPROM is used to store data, such as child lock and reprogramable cooking data.

The MCU controls self-diagnostic functions and includes auto restart, auto deodorizer, auto protection, and overflow protection. During power failure, it automatically restores the microwave oven to the previous settings

The MCU uses a clock and timers to implement sleep mode and add delays in operation. It uses a PWM-based buzzer for generating tones at different frequencies. It also automatically turns off the microwave ovens when operation is completed or goes into sleep mode when not in use/not set (by user), thus saving overall power.


Print
Email