coldBchain

INTRODUCTION
Cold storage is basically a term used for refrigerated of frozen storage facilities, used to store food or pharmaceuticals.
Cold storage could be something as small as a walk-in cooler or freezer or a 200000sq ft warehouse.

BASIC DESIGN

WORKING
Cold storage basically works on the ‘vapour compression refrigeration cycle’ to produce cooling. The cold storage is specially designed and built of concrete,stone in order to prevent the leakage. Cold storage plant also works on the ‘vapour compression refrigeration’ cycle.

COMPONENTS
ST microcontroller (STEVAL-IHT001V2)
Temperature sensor
Ambient Temperature sensor
Deforest and Fan unit
The electrically operated expansion device
Condensing unit
Light bulb and buzzer control

ST MICROCONTROLLER (STEVAL-IHT001V2)

MCU’S IN BOARD

  • STM8S003 microcontroller.
  • STM32F103 microcontroller.

STM8S003 MICROCONTROLLER
The MCU used in the thermostat kit is the STM8S003F3P6 belonging to the
STM8S003 MCU family.
It embeds a large number of features at minimum cost. This part number is used in order to minimize the number of pins.
This causes some pins to be shared for different function recovery (LED3/SWIM_Line, LED4/USART_CTS and LED5/USART_TX, ).

STM32F103 MICROCONTROLLER
As for the connection between the PC and the STM8 microcontroller, a USB bus is used. Since the low-cost MCU dedicated to thermostat control does not embed a USB interface, a second MCU has been used to work as a gateway between the PC and the STM8. This MCU is an STM32F103C6, belonging to the STM32F103 family.
It is only used for demonstrative purposes, i.e. for communication in PC GUI driven mode.

BOARD PERFORMANCE
The STEVAL-IHT001V2 kit provides added value in terms of:
 ● Low-cost solution for spark-free thermostat – No need for a sealed version – Low-cost STM8S microcontroller for thermostat control – Low-cost capacitive power supply
● Efficiency – Fridge consumption lowered by adjusting and reducing the hysteresis threshold of the temperature control (not easy with mechanical thermostats) – Improved efficiency by turning on the defrost resistor only when it’s useful and not at each OFF cycle of the compressor (as done in some mechanical thermostats)
● Flexibility – Customization: program setting with PC interface to change firmware variables – Industrialization: end-of-production MCU programming thanks to FLASH, for soft upgrade and efficient MCU stock management

TEMPERATURE CONTROL
For the thermal sensor, an insulated class-2 NTC resistor has been used. The part number is B57020M2502A020 from EPCOS which is commonly used for these applications. The value of this resistor increases when temperature decreases according to an exponential law. In order to linearize this relation, thus allowing an easier measurement, a series resistor has been added. The NTC must be placed on the evaporator of the fridge or the freezer. The controlled evaporator temperature should be in the range: [-40 to 10] °C.

COMPRESSOR CONTROL
The ACST610-8FP device is used to turn on and off the compressor according to the evaporator temperature, sensed through the NTC resistor. Enough room around this AC switch has been left available to add a heatsink if needed.

LIGHT BULB AND BUZZER CONTROL
If the fridge door or the door switch remains in the open position more than one minute, the buzzer PZ1 sounds, and the LEDs D4, D5, D7, D8, D9 flashes. The buzzer can be stopped by pressing the Temp+ or Temp- button once (the LEDs keep on flashing while the door is in the open position). In the PC GUI driven mode the buzzer can be stopped with the GUI software (refer to the “Help” menu of the GUI). When the door is open, the temperature order can only be changed using the GUI software.

DEFROST RESISTOR AND FAN CONTROL
Defrost resistor is controlled only when the thermostat is set to “Defrost” or to “air circulation” versions; while the fan is controlled only in the “air circulation” version. The defrost resistor is driven by the ACS110-7SB2, Q2 on the thermostat board .
This device is turned on following the “Defrost activation delay” parameter time defined in the GUI software. This time is compared to the sum of the compressor ACST ON times. When this sum is higher than the “Defrost activation delay”, defrost resistor ACST is enabled and then switched on as soon as the evaporator temperature is higher . It remains on during the “Defrost duration” time defined in
the GUI software. The conduction starts just before the conduction cycle of the compressor in order to reduce power consumption.

GRAPHIC USER INTERFACE (GUI)
The graphical user interface (GUI) was developed in order to allow the user to set some control parameters, to configure the thermostat for the different versions (basic, defrost, air circulation), and to acquire variables during tests.
The user can also choose to program the STM8 microcontroller with new control parameters. The connection with the PC, using the USB interface, is ensured by the STM32 MCU embedding a USB peripheral.