Pendulum debugging

Pendulum holder

So, the next step for the pendulum was to try and hold it in one spot and let it go in a reproducible way. To do this, Sarah and I decided to use some 1/4″ Loc-Line coolant tubing. This stuff is great and will become a staple in my tool arsenal from now on. The concept is not new and was repurposed from an Instructables post that I came across a while ago. We purchased ours from McMaster-Carr part number 10095K11. The point of using the Loc-Line tubing is to release the pendulum in a reproducible manner. We want to know what forces the pendulum will hit the target when it is let go at a specified angle and using the tubing is an attempt to reduce error when letting go of the pendulum.

The first thing to do was to modify one of the adapters that came with the package from McMaster-Carr to allow it to be mounted to the Thorlabs breadboard. To do this, I sawed the adapter shown in Figure 1 in half.

Figure 1: Loc-Line Adapter. In order to attach the Loc-Line coolant system to the breadboard, I had to saw one of the adapters in half. I used the one that a 1/4-20 screw could fit through.

Figure 1 shows one of the two adapters that came in the package. I sawed it in half and stuck a 1/4-20 through it and mounted it on the breadboard, Figure 2.

Loc-Line Breadboard Adapter
Figure 2: Loc-Line Breadboard Adapter. Loc-Line adapter attached to the breadboard.

Once the Loc-Line Breadboard Adapter is positioned where one wants it, it’s just a matter of sticking the tubing on the adapter. This is no small feat of strength but, it can be done. Figure 3 shows the Loc-Line Pendulum Holder in action. It basically just holds the pendulum up from the bottom. When you want to let it go, you just move the tubing out of the way. This configuration was found to be the most stable.

Pendulum Holder
Figure 3: Loc-Line Adapter. The Loc-Line pendulum holder in all it’s glory. It works quite well. To release the pendulum, one just moves the Loc-Line system out of the way.

Magnet holder

The magnet holder was another complicated issue that needed addressing. When the pendulum came to the bottom of its swing, it would impact the magnet. The impact of the pendulum on the magnet is not whimpy and would cause the magnet to shatter. This was a problem as I’d like to prevent the device from destroying itself. To prevent this from occurring, a suitable spacer was needed. This came in the form of a hard drive compression gasket seen in Figure 4. Ahh computer junk, the source of so many DIY devices.

Magnet spacer
Figure 4: HD gasket.. The wonky looking thing is a hard drive compression gasket used to keep the HD from spinning out of its box. There is a magnet epoxied to the other side.

Figure 5 shows the pendulum spacer, magnet, and the pendulum together. The spacer prevents the pendulum from coming in contact with the magnet thus preserving it. I’m not sure what the material is of the HD gasket. I think it’s titanium as it is nonferrous and tougher than aluminum.

Pendulum spacer
Figure 5: Magnet spacer. The pendulum ball fits nicely in the center hole of the HD gasket. The pendulum does not impact the magnet directly thus preventing it from shattering.

Sample holder

As much fun as it is to get to build something, this bugger is becoming irritating due to the fact that we have been having to epoxy things together. While epoxy is good for somethings, I dislike the stuff and would much rather use mechanical fasteners when ever I have the chance. Glue is good for wood but, not so much so for metal. Que sera…

The point of this device is to be able to measure quantities of chemicals coming off of different chemical substrates. To do this, we have to collect the chemicals being flung off of the substrate. We decided on using scintillation vials as the way to collect materials being de-adhered from a substrate due to the impact of the pendulum. Hmm, I like that sentence. In order to mount the vial in the pendulum, the optics holder was used, see Figure 6.

4th version of the pendulum
Figure 6: 4th iteration. Fourth version of the pendulum. The scintillation vial is being held by the optics holder. It is prevented from moving backwards with the optical posts due to an impact.

The amount of force being imparted to the scintillation vial is enough to cause the holder’s grip on the vial to slip. This is why a set of optical posts were used to prevent the vial from moving backwards, seen as the metal posts opposite to the pendulum weight. The cap of the vial is attached to the magnet holder which creates a mechanical connection to the vial. Thus, when the pendulum hits the magnet, its impact force will be imparted to the objects of interest attached to the cap.

I’ll show a movie of the pendulum in action once the epoxy on the cap dries. Sarah and I already tried it before the epoxy had time to cure and it just destroyed the epoxy on the cap. So, rather than have to re-epoxy it again, I’m not going to try it now. I’ve done a couple of dry run whacks with the pendulum and a blank cap and so far, it looks great.

I’ll admit that I’m irritated with myself about how long it took to debug this thing. Although, I’m super excited that the problems that did arise were solved easily through communication. Be it with open notebook science, talking with the students in the lab, or Hugh, the pendulum progressed into a solid device in a relatively short period of time. The only two possible kinks left are force calibration and the epoxied cap that holds the substrates.

open notebook science logo

  1. #1 by Virgil Thompson on December 26, 2011 - 7:57 pm

    Andy,my experience with the coolant nozzle material in machine tool environment for the past 15 years is that it must be constantly readjusted. What you will find is that it will change its height over time–it will need monitoring to a standard or it will throw off your data. Two simple yet reliable methods of maintaining constant release altitude come to mind:
    1) Mount a flat plate between 2 collars on an upright post creating a shutter of constant height for the mass to be released from.
    2) If you desire greater accuracy, mount a linear bearing on a post such that a Thomson rod may be extended through the bearing along the plane of travel to become a rest for the mass, and add a solenoid release for consistent results.
    With a surface stone and height gauge, calibration will be quick and simple.

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