Digital Temperature Controller

Copyright Charles Baynham 2018

This repository holds a design for a digital temperature controller. The controller is designed for taking high precision temperature measurements via a thermistor and applying low bandwidth PID feedback in response.

This README file gives a brief overview of the device, but a much more detailed description is available in the manual, located in ./Manual.

./Board design contains schematics and PCB layouts for the controller board. This was produced in the open-source software KiCad.

./Code_for_microcontroller contains the C++ code that must run on the on-board microcontroller.

./Code for Labview contains a sample LabView application for controlling the device. See warning below...

Output

Output is by OPA458 high power opamps. Output can be up to 1A continuous, at voltages up to ±15V depending on the power supply.

Input

Error signal acquisition is done by taking a balanced measurement between a thermistor and a set resistor. The board produces an excitation voltage and measures the difference in current between the thermistor and resistor to produce a temperature measurement. For less demanding applications the set resistor can be on-board. For the highest stability, the precision resistor should be placed next to its paired thermistor. This measurement is performed by an INA330 chip, of which there are two present on the board. The entire analogue section is electrically isolated from the rest of the board to prevent noise on the electrical ground affecting the measurement.

Control

The board can be controlled using a (virtual) serial connection over either USB or RS-485. Both of these interfaces are galvanically isolated to prevent electrical interference. The on-board microcontroller permits headless operation of the device, and commands may be issued using a LabView interface.

Code

The code running on the device is written in c++ and has been well tested. The LabView control interface, designed to be run on a monitoring computer, is less well-tested and should be used at your own peril! For integration into an experimental system, control the device by issuing serial commands detail in the supplied manual.

Manual

You should go read it now. It's in ./Manual.

Extensions

The board is designed to break out the important analog (and come digital) connections, so that additional functionality can be enabled by adding a daughter-board to the main board. For example, you might

• Require higher output currents
• Not need current driving capability, but would prefer very low-noise output instead
• Need an extreme level of stability, so use an external, ovenized voltage reference

None of these daughter-boards yet exist, but the interface required to create them exists from V5 onwards.

Initial setup

The microcontroller needs to be flashed using Atmel Studio and an ISP programmer. You should either:

1. Set the fuses as described in fuse_settings.txt
2. Flash the bootloader saved as optiboot_atmega328_custom_NANO.hex
3. Use the Arduino IDE to compile and upload the latest version of the code

OR

1. Flash the complete production .elf file stored in .\Code_for_microcontroller using Atmel Studio, making sure to select "Flash", "Fuses" and "Verify Device ID" in Atmel Studio. This will give you whatever version of the code was stored in this .elf file, you can then update it using the Arduino IDE if you want to make changes.