Final Design

Flow System Diagram

Physical System

1. Cold Gas Mixing Box

• Provided by Cohu, Inc

• Mix LN2 with either CDA / recirculated cold gas mixture

• 6 separate mixing chambers

• 6 inlets connected to the flow rate control manifold

• 1 main LN2 valve

• 6 individual LN2 zone valves for controlling LN2 input into individual mixing chambers

• All 7 valves are 24V DC solenoid on/off valves.

2. Merger

• Machined out of a 6061 Aluminum block

• Merge separate channels into a single channel

• Six 6.35mm (1/4’’) inlet

• One 12.7mm (1/2’’) outlet

 3. Heater

• Machined out of 6061 Aluminum block

• Emulate a 600-800 W load

• Five 300 W heating rods

• Groove endmilled to enhance heat transfer 

• Gasket sheet between lid and bottom for sealing

• Vent hole design for easy heating rod removal

 4. Compressor

• Provided by Cohu, Inc

• Scroll Laboratories, Inc. SCF-200/08 Oil-Free Scroll Compressor

• Provides a maximum of 80 kPa pressure increase across the inlet and outlet

• Sealed by the team using two aluminum plates bolted together using lead screws

5. Sump

• Modified based on an existing piece of equipment

• Stores excess gas

• Relieves system pressure -- 172 kPa (25psi) pressure relief valve installed

• Provides an initial CDA input used to flush the system.

 6. Flow Rate Control Manifold

• Modified based on a sponsor provided cold gas mixing manifold

• Control the flow rate of the single channel input from the sump

• Separate input flow into six channels

• Output to the cold gas mixing manifold

  7. Temperature Acquisition

• Resistance Temperature Detectors: Platinum 1000

• Adhered to copper tubing via thermal paste and heat shrink

• Adafruit MAX31865 amplifying board

  • 4 wire measurement

  • Arduino compatible

   8. Pressure Acquisition

• Pressure transducers (-40℃ continuous operation, inexpensive)

• T junction mounting for low temperature usage of sensors not rated for low enough temperature

  • Small amount of gas trapped in the junction

  • Junction not insulated to warm up the trapped gas

  • Warm gas close to the sensor

  • Unaltered cold gas flows unimpededly in the main channel

  • Significantly saved budget

 9. Controller Overview

• Primary Arduino

  • Data acquisition

  • Negative feedback loop

    • For valve duty control

    • Use MOSFET for 24 VDC valve control

  • Logic controller for safety operation

• Secondary Arduino

  • Heater Controller

    • For heater wattage control

    • Use Solid State Relay (SSR) for AC heating rod control 

• Designated button for safe stopping sequence

  10. PID Feedback Controller

• Loop time 1000 ms, or frequency 1 Hz

• Valve duty cycle: 0.0 to 3.0 

  • Duty cycle of 3.0 indicates 6 valves opening together for 500 ms (50% of loop time);

  • LN2 valves and recirculated gas valve do not open at same time

• The controller consists of a offset duty cycle fine tuned by a PID controller

• Kp = 0.015; Ki = 0.004 ; Kd = 0.0005;

• Non-Linear Logical Modifications to the PID controller

  • Dynamic offset duty cycle

    • Take the average of the duty cycle if temperature is continuously ± 0.25℃ (±1 Ω) of reference and use as the offset value

    • More robust against system changes (heating power etc)

  • Modified integral term

    • Zero out integral term when temperature reaches ± 0.13℃ (±0.5 Ω) of reference

    • Limit fluctuation and integral wind-up