Designing a Cold Storage room control board

Project Info

Case

The client requested a control board for a cold storage room designed by a Beagle Bone interface that could be supplied over Ethernet. Cold storage or freezing rooms are used for providing a cold space for storing foods. They use a wide variety of procedures that are mainly dependent on the duration of the refrigeration. For example,

  • If the food should be stored for a short amount of time in a temperature range of 0-7 °C
  • If the food is to be stored for long periods of time under three freezing conditions
  1. Freezing – reaching a temperature of -18 °C in more than 4 hours
  2. Deep freezing – reaching a temperature of -18 °C in less than 4 hours
  3. Freeze-drying – drying the product following a forced evaporation of the moisture contained in the product

As requested by the clients, for such applications, the use of Beagle Bone interface is a good option because:

  • It is quite similar to the Raspberry Pi, but again, has a faster CPU and many more I/O pins.
  • While it is not as common as the Raspberry Pi and Arduino, but it was designed to be compatible with both
  • Compatibility for cape or shield technology e to expand its capabilities

Requirements

  1. The board was requested to be designed using a Beagle Bone interface
  2. The design was to be supplied over an Ethernet through a POE
  3. Implementation of protection and controls for voltages and currents both at inputs and outputs, motion sensors and speed controls for the motor

Challenges

The main challenge in this project was the current limitation of POE that is limited to 2.5 A @ 48 volts and all electronic devices and sensors were to be supplied only by POE. The other issue to be considered was the limitation of Beagle Bone’s GPIOs (input and output pins).

Solution

Regarding the power requirement and in order to make an efficient design while at the same time reducing the design’s power consumption, the design team had:

  1. Carefully choosing the components with regards to their power consumption.
  2. Reducing the physical parts and components in the design and using ready-made components with less power consumption.
  3. Working on a design to compensate for their components’ functionalities by making a more complicated firmware.

The final board was designed in ten parts:

  1. Relays circuit

The relays circuit board consisted of 6 relay circuits with each circuit using a 6N139 (high-speed Optocoupler, 100 kBd, Low Input Current, Photodiode Darlington Output) to isolate the relay from the main board. For each relay, a flywheel diode was used to any high voltage induced by the reverse current in the relay coil. Also, a 25A fuse was used to protect relay contacts and the load.

  1. Motor on-off circuit

The motor on-off circuit was a relay connected to the main board through a 6N139 controlled by the Beagle Bone GPIO

  1. Motor speed control

The motor speed control was a PWM control circuit. The PWM signal produced by Beagle Bone was stabilized and noise filtered by a comparator circuit LM358P. However, other than these functionalities, the main goal of using this circuit was to raise the output voltage up to 10 volts as the GPIO of Beagle Bone maximum voltage is only 3.3 volts.

  1. 4-wire motion sensors

The design was made in a way to connect ten 4-wire motion sensors to the main board through a couple of 1N534 Zener Diodes to protect against any high overvoltage and a 1A fuse to protect the circuit from overcurrent. ULN2003 transistor array is to isolate main board from sensors to protect from any overvoltage produced by a short circuit.

  1. Watchdog timer circuit
  2. Door contacts
  3. 0-10 volt analog inputs for analog sensors

This circuit was considered so that it could be possible to connect any analog sensor working in the range 0-10 volts. The circuit consisted of 5 sets of analog sensor inputs with the protection against any overcurrent and overvoltage.

  1. Thermistors circuit

8 thermistors were connected to the board through a CD4051 IC. The CD4051B is a single eight-channel multiplexer that has three binary control inputs (A, B, and C) and an inhibit input. The three binary signals select one of the eight channels to be turned on and connect one of the eight inputs to the output. So only 4 GPIO of Beagle Bone is used to connect 8 thermistors.

  1. POE circuit

An LT8304 was used to convert the 48V voltage input from POE to 12 volts. Also, a transformer was used to isolate the circuit from the line for reducing any noises. Regulated 5 and 3.3 volts were achieved using LM7805 and LM317 voltage regulators.

  1. Beagle Bone base circuit
Description

The work took about 4 months to get completed. During the course of the work, and due to the request of the client for new features for the design, it underwent several changes which extended the original timetable. The final design was quite satisfying for the client and was submitted to the client for the prototyping.

Client:
Jeff A. Behling
Date:
June 24, 2018
Location:
NY, US
Category:
Electrical and Electronic Engineering