CoAutomation Recent Projects
Multi-channel Oven Controller with Thermocouple Inputs
The Problem:
Reduce the cost of controlling a large continuous-feed oven.
The Solution:
Our client, a manufacturer of continuous-feed ovens wanted to replace several thousand dollars
of off-the-shelf industrial controls with a lower-cost, customized design.
After several meetings to determine needs, we settled on a modular design.
Each control board handles one to eight control zones.
Each control zone has one analog thermocouple input, one digital air-pressure input,
one general-purpose industrial input (for miscellaneous switches, contacts, etc.), one
digital heater control output, a digital fan control output and a general-purpose
industrial output (for indicator lights, pneumatic enables, etc.).
An oven, depending upon its size, uses one, two, or four boards.
All industrial I/O is 24 volts, sourcing-type.
Outputs are capable of sourcing at least 500 milliamps and have short-circuit protection.
The oven uses a small industrial PC to provide a graphics output and touch-panel input.
The control boards communicate with the PC to accept settings and provide status.
Each board has both an RS-232 port and an RS-485 port.
The communications ports are electrically isolated from the remainder of the control
board to protect the PC from any type of electrical surge.
Handling the small voltage signals from thermocouples requires extra-special attention.
As is common when using thermocouples in an industrial environment, the board is carefully
laid-out to protect the thermocouple cold junction and to allow monitoring of the cold-junction temperature.
The boards control the electrical heaters using PWM signals into solid-state relays.
The pulse width is adjusted every zero-crossing of the AC power using a PID algorithm with extra
code to minimize both overshoot and "integral wind-up". The results typically show less than one
degree C of overshoot even when making major changes to the temperature settings. Once the heaters
'settled-in', temperature varied by less than three-tenths of a degree C even when disturbed by a changing load.
The fan speed is set by the operator from 30% to 100%.
Standard PWM control of the fans produced a noticeable noise as the fans were switched on and off.
We changed the fan control from a PWM signal to a Delta-Sigma type of pulse output,
which spreads the power more evenly over the control period.
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