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Controller Algoritam
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The 1-Wire network protocol was developed by Dallas Semiconductor ( now Maxim ) as a way to interface to range of
small 1-wire devices.Every 1-Wire device contains a unique identifier preset at the factory. The devices are networked
together by a single wire that acts as a physical medium to communicate between a host and a large number of devices
attached along the wire. It is also capable of powering these devices through the same wire. In practice, a second wire
is actually needed to provide a return to ground and complete the circuit.This project uses two 1-wire devices DS18B20
and DS2405.DS18B20 is a digital thermometer with ±0.5°C accuracy over a -10°C to +85°C range.It has an operating
temperature range of -55°C to +125°C.The second device DS2405 Addressable Switch is an open drain N-channel transistor
that can be turned on or off by matching the 64-bit factory-lasered registration number within each part.
The 8-Bit CMOS microcontroller of Microchip PIC16F628A continuously reads the process temperature ( PVar ) from the
DS18B20 sensor and depending on the desired temperature setpoint ( SPoint ) value of the process, manipulates the DS2405
addressable switch,using ON-OFF control algorithm. In this algorithm,the controller will switch the output relay only
when the temperature crosses the setpoint.For heating control,the output relay is on when the temperature is below the
setpoint, and off above setpoint (fig.1).For colling control, the output relay is on when the temperature is above the
setpoint, and off below setpoint (fig.2).
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| Heating control: | | Colling control: |
fig. 1 |
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fig. 2 |
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Since the temperature crosses the setpoint to change
the output state, the process temperature will be cycling continually, going from below setpoint to above, and back below. In cases
where this cycling occurs rapidly,and to prevent damage to relay contacts ,a bandwidth ( hysteresis ) is added to the controller operations.
For this reason, a timer function can also be used.This function sets the relay output to ON when a preset time (timer setting) elapses
after PV has reached the setpoint.
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