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.


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.



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.

First Rotations

With all the superstructure built up, there’s much less visual progress being made on the mount. We’re ordering lots of parts and waiting for their delivery so that we can begin work on other components like the drive system and the routing of gas hoses. In the meantime, we got a few videos of the mount in action.

The first video shows the elevation stage moving. As the post about its installation described, this stage moves in a nodding-like motion with a full range of 55 degrees (including 10 degrees past vertical).

The second video shows one of our rotary unions in action. This union is attached between our stationary structure and the Azimuth ring (rotating in the above video). As intended, the bottom portion of the joint remains fixed during this rotation which gives us a stationary interface to which we can attach our pressurized hoses. The upper half of the joint then rotates along with the Azimuth structure, allowing the hoses to move without getting twisted up.

Progress on the mount is slowing down significantly now that we have the steel superstructure assembled, however there’s plenty more happening at Minnesota that will soon make its way online.

Installation of the Theta stage and Rotary Unions

After we installed the Elevation stage early in the week we were ready to install the final ring which we refer to as “Theta”. Unlike the Azimuth and Elevation stages which change the direction that the entire array points, the Theta stage rotates all four receivers about their common center known as the “bore sight” (the red dashed line in the below image).BA_Assembly_IsoSection_20180417.JPG

Bicep Array observes polarized light, and its detectors are sensitive to either light that is polarized “vertically” or “horizontally”.  Rotating about the bore sight changes the angle of our detectors (redefining “horizontal” and “vertical”) without changing where they are pointing on the sky. Observing at a few select rotations of this third axis helps suppress systematic errors due to the shape of our beams or differences in response from detectors.

Unlike Azimuth and Elevation the Theta stage didn’t need to be built up at all before it was installed, so it went up pretty quickly. As evident in the right hand image above, this ring actually slides downwards into the elevation stage, so we had to align things very carefully. The middle flange of this ring attaches to the other half of the second ring bearing and will be driven by the two gears visible on the top of the elevation stage in that same image.

After the ring was installed, we attached the top frame. The central piece of this frame has four openings which provide holes for the receivers to sit in, and hoist points where we can mount the pulleys that will hoist the receivers up into the mount. In the right hand image above, extensions to this frame are being attached. The outside of these extensions provide the second attachment point for the environmental boot (transparent teal in the first image of this post) and a base on which to mount the top panel that closes out the internal mount space.


With just about all the large pieces attached the mount is impressively tall, though it still looks a bit bare without everything that goes inside it. Next up was installing two of these components, the rotary unions.

As mentioned in a previous post, these rotary unions provide channels for pressurized Helium and Nitrogen, along with electrical and data connections. Since the upper and lower halves rotate separately, we can use these connect our rotating stages. Using these eliminates the need for a cable wrap that winds and unwinds as the mount rotates and gives us continuous rotation about the Azimuth and Theta axes. In the above image, the red caps indicate the locations of the channels for pressurized gas, while the electrical and data connections are indicated by wires poking out the top and bottom ends.

With the rotary unions installed we began testing the motion of the mount, driving it a bit in each axis and adjusting the gears as needed. This doesn’t produce much of a noticeable change when rotating in Azimuth or Theta but everything looks a bit different when we rotate in Elevation. The entire top of the mount tips forwards and it gets a bit taller. The internal structure changes pretty significantly as well, and the rotary unions move fairly far apart as seen in the right hand image above.

With the steel structure assembled we can now turn our attention to the other components of the mount: like the drive assembly and controllers, cable and hose routing, and receiver and electronics installation.