PID Temperature Controller dsPIC33EV64GM004 Hardware/Firmware
Initial requirements were to control the temperature of an enclosed system, holding it as close as possible to a set temperature during operation. Venting air out of the enclosure using a FAN would quickly bring the temperature inside the enclosure to match the outside temperature which was unacceptable. Using an MCU we can monitor the temperature and vent air using a fan at a controlled rate. PID algorythms are commonly used in such systems - 'A proportional–integral–derivative controller (PID controller or three-term controller) is a control loop mechanism employing feedback that is widely used in industrial control systems and a variety of other applications requiring continuously modulated control.'
dsPIC33EV64GM004 MCU using 20MHz oscllator, configured with 140MHz FOSC ( 70 MIPS ). This device was selected based on pincount, cost and flexibility of its peripherals. It is presented in a surface mount 44pin quad flat pack that is easily hand solderable, along with a respectable 64k of flash and 8k RAM.
- 4 Input Capture devices for interfacing with temp sensors supporting DHT11 and DHT22 protocols
- 3 2-Channel PWM devices running at 25KHz for controlling 4pin PWN fans. Support for 3pin and 2pin fans using PWM on power pin if required
- 1 UART for communication, loggin with a host, configured at 9600 8N1
- 11 GPIOs connected to 2004 LCD (HD44780) driven using 8bit Data, RS, RW and Enable, reading busy-flag to ensure maximum output throughput
- 12 GPIOs
- 2 dedicated to rotary encoder ( GPIO4, GPIO5 )
- 1 dedicated to a button ( GPIO3 )
- 3 Inputs used to read TACH from 4PIN and 3PIN fans to calculate RPM
- 3 Output to FETs used as low-side switches controlling the FAN 12V power in the case of 3pin and 2pin fans and for power off control of 4pin fans
- 3 spare GPIO0, GPIO1, GPIO2 which can be used for notification of ALARM conditions, ie. FAN not spinning or Sensor not returning data.
- 3 Timers
- TMR1 used as system timer used to monitor 10ms tick for scheduling
- TMR3 used by PWM hardware
- TMR4 used as high frequency timer with 7.3us tick
The system runs with a simple co-operative multitasking round-robin scheduler with a 10ms tick rate and ability to use a second high resultion timer with a 7us period. Tasks can be scheduled with or without a period, added and removed during operation. All tasks are non-blocking.
User interaction is through an LCD display current value from the attached temp sensors and fans. A rotary encoder and button can be used to navigate menus and change settings.
The LCD is updated from an in memory buffer at >120 times a second.
Ouput is also sent via UART over USB for logging purposes.
The firmware was developed usng the Microchip tools, MPLABX (v6.10) using XC16 (v2.10) compiler and MCC (v5) used to generate system, clock and initialisation code for hardware peripherals.
The firmware is written to the MCU in-circuit using a Microchip Pickit5. Using the Pickit5 it is also possible to do source level debugging of the MCU while in-circuit.
For test and verification the first version of the device was coded on a dsPIC33EV64GM002 28pin DIP version of the device and soldered onto stripboard, the results for a 40minute session were logged, once the enclosure reached the required set temperature their was little to no deviation seen over the 40minutes.
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