CCD Controller Board Designs

CCD Controller dewar fill system, EL-3415

The CCD controller dewar system operates to automatically keep the CCD dewar from running out of liquid nitrogen. For the most part, the system hasn't changed but there are some small differences that make it worth while to rework the write-up so that a technician needn't have to flip back and forth between two documents. To this end, I will liberally cut-and-paste from the original document to this document. The material from the original write-up will appear in green text while new or changes text will appear in black. The original write-up is also available at the bottom of this web page (click here).

Dewar fill system:

It is best for the operation of the instrument if the number of times a person enters the Instrument Enclosure is kept to a minimum. Add to this the fact that the instrument is not in the friendliest of environments for people to work. It is a great advantage therefore if the instrument can automatically keep the level of liquid Nitrogen within the Dewar Reservoir at a level sufficient to insure that the CDD will remain cold. The logic described in this drawing does this task.

The main control of the automatic fill is provided by the Utility board. The general I/O bit Dout 7 that originates on the Utility board is wired to the UCO/Lick Utility Support board via a 64-pin ribbon cable. This signal is wire through the Utility Support board and into the HIRES Daughter Board as shown on the left side of the drawing. The Daughter Board is used to optically isolate the signals (including the analog signals) and pass them to the new Power & Miscellaneous Signal Board shown to the right. From there, a DB25 cable passes these signals to the Footlocker Interface Chassis. There are five input signals and one output signal for this portion of the instrument control. Two of the inputs are analog (Ain 7 & Ain 11 on pins 4 & 17). The others are a digital input from the 'Manual Fill' push-button (Din 7 on pin 20), and two digital inputs from the switches mounted onto the transfer valves on the 50L LN2 dewar (Din 1 & Din 2 on pins 21 & 9). The output is a digital signal (Dout 7 on pin 7) that is wired to an EL-1349 Solid State Relay board. This relay controls a two way solenoid valve that introduces about 2.5 PSI into the 50L dewar. This causes liquid Nitrogen to be pumped into the CCD dewar reservoir. Note on the right of sheet 1 that the wiring to the air solenoid valve

Note: This write-up is a verbatim copy of Terry Ricketts' write-up in the original HIRES installation.

It is best for the operation of the instrument if the number of times a person enters the Instrument Enclosure is kept to a minimum. Add to this the fact that the instrument is not in the friendliest of environments for people to work. It is a great advantage therefore if the instrument can automatically keep the level of liquid Nitrogen within the Dewar Reservoir at a level sufficient to insure that the CDD will remain cold. The logic described in this drawing does this task.

The main control of the automatic fill is provided by the Utility board shown on the left side of the drawing. It is cabled to the terminal strip where the various signals become available. There are three input signals and two output signals for this portion of the instrument control. Two of the inputs are analog (Ain 6 & Ain 7 on pins A32 & C31). The other input is a digital from the 'Manual Fill' push-button (Din 7 on pin C2). The two outputs are digital signals (Dout 0 and Dout 1 on pins A9 & A8) that directly drive two Solid State Relays. These relays control the pumping of liquid Nitrogen into the reservoir. Their operation be described later.

On the right side of the drawing, the dewar reservoir and 50L Dewar are represented by rectangular dashed boxes. Within each of them is a special sensor called a tri-axial capacitor sensor. These sensors are constructed from brass tubing with teflon spacers. The other cylinder is a ground shield. The inner cylinders form a capacitor When inserted into the liquid Nitrogen the capacitance of the sensors will change with the amount of change dependant on the level of liquid Nitrogen between the cylinders. By measuring this change the level can be deduced. This measurement is accomplished by the Liquid Nitrogen Level Monitor (EL-2070) to which the sensors are attached. The operation of the Monitor will be described later. These particular sensors were chosen because they are simple, generate no heat within the dewars, and have been in use at Lawrence Berkeley Laboratory for many years.

The Liquid Nitrogen Level Monitors produce an analog signal of 0 - +10V which is proportional to 0 - 100% full. There is a meter mounted on each of the Monitor boxes to give a local reading of the level of liquid Nitrogen in the reservoirs. The analog signal is cabled directly to the two analog inputs of the Utility board. Also mounted on each of the Monitor boxes, near the meter, is a push-button. These two buttons are wired in parallel and cabled to the digital input of the Utility board. It allows the user to override the automatic features of the instrument and locally command the Utility board to begin the pumping.

There are two means of pumping that could be employed in this system. Both are very simple and do not require special cryogenic materials. The primary means of pumping uses the pressurized air provided to the instrument. The air is filtered and the pressure regulated at 2.5PSI. A two way air solenoid is controlled by one of the Solid State Relays mentioned above. This solenoid will normally vent the air pressure within the 50L Dewar to the outside. When power is supplied to it the solenoid will close off the vent, and apply the 2.5PSI air pressure to the Dewar. This pressure is enough to force the liquid Nitrogen to flow through the tube inserted in the liquid at the bottom of the dewar to the Dewar Reservoir. When the reservoir is full, the pumping is stopped by removing power from the solenoid. The pressure will then be removed, and the internal pressure of the dewar will be vented to the outside.

The second method of pumping is kept as a standby should the air vent fail for any reason. It incorporates a Watlow 30W immersion heater. Turning on the heater will cause the liquid Nitrogen to boil. The resulting pressure produced by the boiling will pump the liquid through the tube. It will take longer to pump since it will take a while to build up the pressure necessary. In typical operation it takes about 45 minutes using this method to fill the reservoir. The first method will do the same operation in about 15 minutes.

Mounted on a small panel on the 50L dewar is a manual control switch and meter (top right of drawing). The meter is wired to the level sense signal from the CCD dewar, and will display the level in that dewar. If the switch is moved to 'manual' the air solenoid will be disconnected from the computer control and directly connected to a 110VAC power cord. This power cord needs to be plugged into a local power socket. Switching to 'manual' will force the Liquid Nitrogen (The write-up ends at this point)