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  • analog PID control of VTI He vapor pressure

    Abstract

    As written, my draft abstract is too long, but it focuses on the key points of this article (and significantly narrows the scope of the article). Some of the material should move into the main body of the article.

    We present an improved method for temperature control of an ICE-Oxford Variable Temperature Insert (VTI) at temperatures below 4.2 K (down to a base temperature of 1.3 K for our system). The closed cycle VTI is part of a liquid-cryogen-free ICE-OXford cryostat; the sample probe is thermally linked to the VTI through the use of a small amount of He exchange gas, allowing the temperature of the sample probe to be raised above that of the VTI.

    In both the original (ICE-Oxford) and our improved design, the temperature of the VTI is controlled by measuring and controlling the vapor pressure of the VTI 4He bath. A sealed pump continually pumps on the VTI helium space; the liquid helium fill rate for the VTI is adjusted through analog control of an motor-controlled needle valve. A 10 V input to the servo motor controller causes the needle valve to fully open, while a 0 V input causes the needle value to fully close.

    Using the as supplied method, we were able to control the pressure with a precision of \(\pm 0.2\) mbar but with an absolute accuracy of only 2 mbar. With better choice of PID settings, it might be possible to reduce the systematic offset in the pressure stepping, but not overall precision. This is because the as supplied method uses a computer to directly read the low-resolution (\(\pm 0.1\) mbar) digital output of the pressure gauge. The speed with which the software-based PID controller can update the output of the computer D to A board (or, originally, on of the additional analog outputs of a Lake Shore temperature controller) is further limited by the update rate of the digital display.

    Our improved method of temperature control improves the precision and absolute accuracy of the pressure control by an order of magnitude: \(\pm 0.02\) mbar or better in precision and \(\pm 0.2\) mbar in absolute accuracy (compared to the setpoint). To do this, we make two changes the nature of the PID control of the needle valve servo-motor and the manner in which the He vapor pressure is read by the PID controller. First, instead of using a computer to read the and then a low speed software PID control loop to produce the analog voltage input needed by the motor controller, we indirectly measure the higher resolution (\(\pm 0.005\) mbar) analog voltage output of the pressure gauge and, in addition, use an analog PID controller to hold that voltage at the desired setpoint.

    Introduction

    We are collecting energy and entropy data of the superconductivity of organic synthesized molecular superconductors. with the effects of magnetic fields up to 9 Tesla and temperature from 1.5 to 5 Kelvin in order to gain a greater understanding of the materials and their properties.

    Experiment Process

    The experiment will be conducted in a closed system, where the sample will be placed into the Innovatative Cryogenic Engineering (ICE) machine containing liquid helium. Inside the ICE machine, it will be exposed to low temperatures. Ultimately, we are trying to control and maintain the temperature within the Innovatative Cryogenic Engineering (ICE) machine; however, we are unable to measure the temperature directly. As a result, we are measuring the pressure through a pressure sensor in the chamber instead, since we know that there is a direct relationship between pressure and temperature because of the co-existing liquid and gas phases within the chamber. The pressure inside the chamber will be read through the pressure sensor on top of the ICE machine. Any changes in pressure will be noted via the gauge, and will be read either manually or digitally. We will control the internal pressure of the VTI Helium through the amount of voltage we send to the needle valve which controls the inward flow of helium gas: 10V opens the valve fully, increasing the pressure, and 0V closes the valve fully, decreasing the pressure. The pump pumps away helium gas, at a constant rate, into a condenser until we need to pump helium back into the system.

    (NEED A DIAGRAM OF THE SYSTEM)

    Installation of the PID controller

    The PID controller system was installed to digitally analyze and control the pressure instead of the computer program we were using. The implementation of the PID controller remedied two flaws of the previous system. First, we were able to gain more precision and accuracy in pressure value reading with the digital analog output than the computer output reading. Second, with carefully determined PID settings, the pressure in the VTI chamber is much better controlled in both reaction time and precision.

    Analog Output Reading in relation to the Computer Output Reading Resolution Improvement


    Using an analog output reading improved the precision of the reading of the pressure from +/-0.1 mbar to +/- 0.01 mbar. Orginally a computer system was controlling the temperature of the system, by monitoring the pressure; however, the digital signal the computer reading had limited precision, because it only detected variation of 0.1 mbar or larger. The large margin of error for the pressure gave us little control of the temperature inside of the sample chamber. On the other hand, the analog PID receives a continuous reading, thus increasing the response time of the changing pressures inside the ICD machine and its sensitive to pressure changes. This transition from digital to analog allowed us to gain greater precision up to +/-0.01 mbar, which means we have more control of the temperature ranges the sample experiences.


    pressure gauge –> pressure analog digital displayer –> black box(amplifies the current so it can be seen by the motor to control the servo) –> PID(set V0,PID finds difference between set and measured V, use P/I/D settings) –> V that goes to needle valve.