analog PID control of VTI He vapor pressure


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.