Governance by those who do the work.

Monday, August 25, 2014

Convection Instrumentation

My first design for the plate heater involved building a 100+ Volt power supply.  Since then I have realized that by stacking two laptop power supplies, a 40.V supply could be had inexpensively.  Each supply is rated for 90.W, so they have ample power for the heater.  The maximum current the heater will draw is 2 Amps; in order to account for wire losses, the voltage across the heater will be sensed by another pair of wires.

The first design had a digital-to-analog converter driving the gate of a SD220 n-channel MOSFET, and its output driving the gate of an IRF9520 p-channel power MOSFET.  The forward gain of the circuit was over 1000.  With such high gain, only a few of the DAC codes would cover the range of planned drive levels for the heater.  So I have ballasted both MOSFET source terminals with resistors to lower their gains.  The SD220 guarantees its threshold only to be less than 3.V, but the STM32F303VCT6 microcontroller DAC output only drives up to 2.8.V, so an op-amp is needed to servo the source resistor to the DAC output voltage.  Servoing the IRF9520 current to be proportional to the SD220 current means that, within its operating range, the current is set by the DAC.  Multiplying that current by the voltage sensed across the heater measures the power delivered to the heater.

The design includes electronic temperature, humidity, and air pressure sensors.  The only relevant condition not being sensed is windspeed in the chamber.  I investigated many windspeed sensing technologies, but techniques for measuring below 4.m/s require laminar flow, affect the flow, are too large, are inaccurate or are prohibitively expensive.

With a variable transformer I measured consistent windspeeds in the tunnel from 4.m/s down to 1.m/s.  The windspeed varies smoothly with the rotation rate of the fan.  Instead of trying to servo the fan's rotation rate, I will measure the rate of the fan blades interrupting an infrared beam between a LED and photo-transistor on opposite sides of the fan.  Calibrating the relation between windspeed (as measured by an impeller anemometer) and the fan's rotation rate will enable tracking of windspeed over small variations in rotation rate.

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