Encoders and Hoses and Cryo, oh my!

We’ve been quite busy the last two weeks. After successfully testing out the winch system we were almost ready to run a true receiver cryogenic test in an operating mount, but we still needed to put a few more things on. First, we attached the encoder tapes and their read-heads. The encoder tape is a long adhesive backed tape with precision spaced magnetic North and South poles. As the mount turns, the read-head can count the number of poles it passes. We can then feed this information into our control software in order to track the orientation of the mount to extremely high accuracy! Below is a picture of the Azimuth encoder’s read-head (left) and a close-up of the encoder tape as viewed through a magnetic field viewer.

We have a second encoder tape and read head up on our theta axis for accurate control of what we refer to as our “Deck” angle. The elevation axis also has an encoder to measure its location though it is self-contained and we did not have to install it. With the encoder information in the system we can now define our “home” point and reference all our rotations as a number of degrees away from that point, which is a critical step in being able to scan the telescope.

 

Before we could run a cryostat up in the mount, we also had to install the Helium lines. Each cryostat uses a Pulse Tube cryocooler in order to get the two largest internal stages down to temperatures of 50K (-369 DegreesF) and 4K  (-452 Degrees F). These pulse tubes use compressed Helium to extract heat from the internal structures and each one has to be hooked up to a compressor that is detached from the rotating parts of the mount. As a result, we have to run a set of Helium lines from each cryostat, through the Theta axis rotary joint, then through the elevation cable carrier, then down into the Azimuth rotary joint, then down to the floor and into the compressor.

 

We have 8 sets of lines lines that run the four cryostats, 2 sets of guard channels that help to control leaks within the rotary union, and one nitrogen set. With so many hoses running up into the structure, it’s imperative that we take care in routing them so that they aren’t in our way. To do that, we decided to run all the hoses underneath the various beams of the mount. When they approach one of the rotary joints, we attach them to a set of custom low-profile rigid sections that bridge the connection between the fixed rotary union and the flexible hoses.

These are a really cool set of fixtures, and each one is unique. We have only manufactured enough for one set of lines so far, but the rest are in production. This area is going to look very neat once the full set is installed.

With the encoder and Helium lines installed, we were ready to unload the dummy receiver and load in a real Bicep Array receiver with a pulse tube. Although we’d installed the dummy receiver using the new winch system, we hadn’t actually installed the real thing so we weren’t sure if there were going to be additional quirks in the procedure that we needed to figure out.

 

In the end, it installed without a hitch. Everything went smoothly, and we were able to secure it to the mount using the harness system in about 2 hours. We then attached a bunch of extra hardware to the front of it which is necessary for the operation of the pulse tube. With everything plugged in, we turned on the pulse tube and were rewarded with it’s characteristic chirping noise. With the cryostat installed and cooling we took the time to shoot some video.

The chirping noise you can here is the pulse tube. High and low pressure Helium gas are alternately pushed through the pulse tube cold head in order to produce the cooling power required to achieve cryogenic temperatures.

 

We also took a moment and got just about everyone to sit on the mount and shoot a quick video of a partial scan.

 

 

Finally, once we had the telescope cooling and scanning…

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We threw an open house for the physics department. Quite a number of people attended and the donuts were quickly claimed. For an hour we chatted with other members of the department and anyone else who happened to walk by the high bay. Each of us got to talk about different aspects of the experiment we worked on and talk about the overall project while the telescope scanned in the background. It was great to see so many people interested!

 

 

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Receiver Loading System

For the last few months we’ve been taking advantage of the overhead gantry crane in order to load the receivers into the mount because it’s quick and convenient. But we won’t have access to such a crane at the South Pole (at least not regularly). This means we had to design a hoisting system that will let us load and unload receivers at-will. This presented a few challenges as the receiver is pretty heavy, we’re very constrained on space, and we are restricted to loading at one Azimuth orientation.

The solution we came up with was to use three separate hand winches connected to the cryostat with wire rope.  The rope for each winch runs up to one of the three pulley stations that surrounds the crysotat envelope, then back down to the ground where the receiver will be when ready to load. The pulleys are affixed to the theta axis which means they will rotate with the cryostats, so we’ll end up with four sets. The winches however, are fixed to elevation instead (and therefore fixed in Azimuth). This means that we can spin our theta axis without moving the winches away from the loading position, allowing us to load in a receiver and rotate it away opening up another slot for the next receiver.

The photos above show the winches attached to the mount and threaded through their respective pulleys. The two wooden platforms give us a flat surface to walk around on during the loading process but don’t interfere with any hardware. One thing you might notice from the photos above is that the winches don’t align with the beams that they sit on. This is intentional, in fact the winches and the pulleys are all twisted away from this alignment. The reason for this is because the wire rope needs to approach the drum of the winch in a very specific way, otherwise it doesn’t want to wind correctly which can cause problems. However, the pulleys also need the wire rope to approach them in a specific way, if the rope ends up out of the plane of the pulley wheel it can ride up on the side of the pulley and begin to abrade which weakens the rope. All the angles are purposely designed to achieve approach angles that are close to ideal.

 

For our first loading test of this system, we went back to using our “Dummy” receiver. This has the same weight as a real receiver but is slightly shorter. We passed the rope coming from each winch through an eyebolt in the top flange, then hooked it around an eyebolt in the bottom flange. We need to lift from such a low point because we have very little space up at the top end, and the ends of the ropes themselves create conflicts right at the end of the loading process if we have them higher up. Lifting from the bottom flange gets around this problem but presents another complication, we have to lift from below the center of mass. If it begins to tip too far it can become unstable and try to flip over. Passing the rope through the upper set of eyebolts fixes this and helps stabilize the receiver for the loading process.

 

When we finally attempted to load the receiver we came across one last complication. With three separate winches all being activated at different times, the receiver began to sway back and forth by quite a bit, enough that it would hit the mount at the points of tightest clearance. In order to fix this we ended up synchronizing the three winches using an electronic metronome. Interestingly with the winches all synchronized we observed very little sway in the receiver as it was being loaded.

The above two photos show the receiver once it has been loaded and is ready to be attached to the mount with the blue harness arms that can be seen next to it. The left photo above shows just how tight of a space we have up at the top which is the reason we pass the rope through the top set of eyebolts and connect them at the bottom.

 

Finally, with mention of the synchronized winching of course we have to provide a video. We haven’t quite figured out what the optimal speed it yet but experimenting with it should be interesting.

VID_20190415_171349 from Mike Crumrine on Vimeo.

3 – Axis Motion

With successful integration of the Theta motors, we quickly moved into installing the elevation motors. Installation itself was relatively simple, and since we’d already laid the groundwork with our control software we were able to get it up and running in only an hour or so.  Once running, we needed to check that the limit switches functioned as expected. Since the elevation axis isn’t capable of 360 degree rotation, we install limit switches on both sides. These switches act as safeties that prevent us from driving the axis too far in either direction. In the photo below, the blue box with “H” and “S” written on it holds the “Hard” and “Soft” limit switches.

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As the mount rotates counter-clockwise in elevation, the screws just to the left of the limit switches get closer and closer to the flippers on the underside of the labelled box. When the “Soft” switch is tripped by these screws, the controller prevents further commanded motion in that direction and the mount slows to a stop. If the mount continues to travel too far past the soft limit, it will trip the “Hard” limit switch. This switch trips all power to the drive amplifiers, causing the motors to break hard and stop the mount essentially in place. A second set of these limit switches is on the other side of the elevation gear and prevents too much motion in the opposite direction.

 

Once we’d confirmed the operation of the limit switches, we went ahead and drove all three axes of the mount simultaneously for the first time.!

 

We next need to fine-tune the control system to give us the precise motion we want on all axes. Now that all the motors are on we can also attach the Azimuth and Theta encoders which enables high precision tracking of the exact position of the mount on each of these axes. We’ll do this using encoder tape and magnetic read heads for these two axes. The elevation axis uses a more compact encoder that is already attached right next to the bearing.

 

 

 

Second Axis Rotation

Although we were able to get Azimuthal rotation under computer control two weeks ago, we weren’t quite ready to jump right into attaching the motors of the other axes. Instead, we spent the last two weeks digging into the controller software and ensuring that we could get the mount to move in a smooth and controlled manner. Establishing this behavior immediately allows us to test out the other axes in a more controlled manner once we finally install the motors on them.

Once we got a reasonably smooth motion, we ran the motor cords for the Theta axis (which we also call Deck). This allows the array to rotate around its collective boresight. The image below shows one of these motors attached to its gear reducer. In vertical order are the drive gear, the gear reducer, and the motor (the smaller box section). Two of these motors drive this axis.

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Of course, once we attached the motors and hooked up their power and data cables, we had to test out the motion. Below are two videos of the mount under Theta (Deck) rotation.

Next up is installation of the elevation motors which control the tilt of the mount forward and backwards.

 

 

 

Receiver loading and Successful rotation

In the last few months since we fully assembled the superstructure, we’ve been working on a lot of the smaller and more detailed aspects of the mount. Much of this has been going through the procedures for loading and unloading various components multiple times so that we can figure out if there are any difficult spots we need to watch out for.

 

Loading and unloading the receivers – for instance – is something we will have to do many times since we can’t open them up or perform much maintenance on them while they are up inside the mount. To that end, we hooked up some eye bolts and shackles to the cryostat and lifted it up into the mount using our gantry crane. While this isn’t the method we’ll use when we’re actually deployed at the South Pole, it did help get a feel for how tight the clearances are and let us practice mounting and un-mounting the cryostat from the four brackets which affix it rigidly to the steel superstructure.

The images above show the cryostat (silver) during the lift. In places, the edge of the cryostat got within one half inch of the blue structure. Though this is partly due to the fact that we didn’t do a perfect job of centering the crane, the whole procedure is very tight. Once it’s up in place, we can attach the so-called receiver harness: a series of 10 adjustable steel arms that connect to the upper and lower flanges of the cryostat via four steel brackets.

 

When they aren’t attached, these brackets swing out of the lifting path. Once the cryostat is at the right height, all we need to do is swing them in and attach them. It’s a bit difficult to see in these pictures, however the ends of the arms have a right handed and left handed  thread on either end, meaning that they can be turned to lengthen or shorten them and adjust the level of the cryostat. They’re designed so that once we adjust them for a single cryostat we can take it in and out again repeatably without having to re-adjust the arms.  When it’s all mounted, it looks like this:

 

These pictures are taken looking across the drum, the central blue bar is not connected to the cryostat, rather it holds the upper half of the Elevation – Deck rotary union.

After loading and unloading it a few times we felt confident enough to sign off on the harness system and start the design of the real loading system we will use when we’re at the South Pole.

 

One other thing that we just accomplished is the completion and mounting of our drive cabinet. This metal cabinet holds all our amplifiers and controllers for the motors which drive the mount’s three axes of rotation. It’s been progressing steadily over the last few months, however its a slow process. Not only did it need to be built up, laid out, and wired; a lot of manual consultation was required to figure out how exactly everything interacted with everything else.

Once we had successfully tested that we were able to command movement out of the motors and made all of the requisite cords to run up through our rotary union we were ready to mount the drive cabinet and the motors so that we could make the mount move for the first time under motor command.

The drive box mounts to the Azimuth structure. Power and communications route upwards through the first (Stationary to Azimuth) rotary union into the drive box then splits back out to the six motors that run the mount. We hooked up two of these motors (the Azimuth ones) to their respective gear reducers and plugged them in, ready to run from our command station sitting down on the floor. The result is the video below:

 

 

This is the first time we’ve moved the mount under computer control! Depending on your reference, it might appear that we’re spinning pretty slowly. However, this is actually about 6 times faster than we rotate during operation. With one axis down, we’re getting ready to install motors on the other three axes and get them up and running. Once that’s complete, it’ll be time for fine-tuning the amp and motor control code to give us the smooth and consistent motion we want.