Ted Rogers' "Romberg-Style" Horizontal

Date: Sat, 3 Apr 2004 07:45:12 +1000

I started the project with a simple line drawing to get some idea of size. The base is made up of two 8" (200mm) long 1" (25.4mm) lengths of angle iron with a 1.5" (38.1mm) space between them. The total height is 11" (275mm) and the rear legs 6" (150mm) apart, one of which is adjustable. The Vertical arms are 9" (230mm) and as you may see from the photo the bottom right hand hinge assemble can be move to change the angle of the vertical arms. The first attempt I decided on a 5deg angle for the uprights that meant a 4" (100mm) top plate. With this arrangement the bob fell to one side or the other so I tried a 2" (50mm) plate, this time it worked but the period was too short. I found that moving the lower hinges closer together the period increased but the bob had very little movement before hitting the uprights, so the top plate was made 3" (75mm) - perfect. The new angle is about 10deg but wither this angle would hold good for different model sizes I could not say. I made the right hand level adjusting screw compound - Top part is 32 tpi. The lower part is 40 tpi working in a brass nut assembly, (giving 160 tpi). Even with this arrangement a very slight turn will move the bob quite some distance.

I made the original bob arm and the bracket that holds the feed back coil (50 ohm resistance) of mild steel, but even with the small neo magnet - 1/2" x 1/8" (12mm x 3mm) the attraction was too great so the rod is now brass and the bracket aluminum. I was surprised just how effective such a small magnet could be in this situation. The rod is 1/4" (12.6mm) and threaded 1/4" Whit. Whereas the thread is not important it's tightness to both the bob and the top plate is, any slackness at these two place prevent the pendulum working correctly.

The bob is a 1" (25.4 mm) slice off a scrap bar 2" (50mm) hex. Brass bar. What you can't see is a build up of strips of lead flashing attached underneath. I added the lead after it was built but it does not seem to make a great deal of difference. With a free (no feedback) period of approximately 4.6 Seconds the apparent length of a pendulum rod works out at approximately 828" (21040mm) or nearly 70 feet! Using the formula: L=(375.4/V)Sqr'd. Where: L=Pendulum Rod length and V=Vibrations per minute.

The detector is a phototransistor with a 3-volt pilot globe. The assembly is a piece of aluminum channel drilled to take a piece of aluminum tube (ex TV aerial). The tube length was a gesstimate, I wanted it longish to try to spread the light a bit, the shutter opening was cut first and then the two pieces were aligned with a drill the size of the inside diameter before gluing. The shutter as you can see is tapered instead of a straight vertical cut, this allows for more adjustment and a little temperature correction.

The bottom hanging hinges are 2" (50mm) wide and have two 1/4" (12.5 mm) strips of .002" (.05mm) Brass shim stock at each end. The top hanging hinges are 1" (25.4 mm) wide and are the same material, but just one piece. I would have liked to have used stainless steel shim for the hinges but I haven't been able to locate any locally.

When setting up I adjust the RH hinge slowly to the left until the bob becomes unstable and falls to one side or the other, I then back off until it becomes stable and then a little bit more in case temperature change may effect it. I would get a longer period if wasn't so worried about the temperature effect...
Because of the A/D I use my electronics are all 5 volts. A very small move of the bob gives full voltage swing without any amplification. The present electronics are a modification of Roger Bakers and seems to work quite well.

In an attempt to combat drift problems due to temperature I intend to change the bob rod to aluminum when I can find a rod of correct diameter, also change the tilt adjusting screw to mild steel. Again instead of trying to devise a remotely controlled device for turning the tilt adjust screw I might try to vary the brightness of the light source to hold the output constant, a bit like the AGC in a radio but with a longer time constant, say about one minute. Do you happen to know of any suitable circuits?

In my work area I use a program (Qbasic) I wrote myself. On RH side of the screen is a vertical trace of the ongoing recording, when it reaches the bottom of the screen it is shifted left over writing the previous trace and a new one started on the left. On the LH side of the screen the data as been condense to half hour vertical display columns with minuet markers, there is up to 18 of these traces with the start time of each trace at the top. When all 18 columns have been written the display move one column to the left so that the last 8.5 hours condensed data is displayed. The data is also recorded in two RAM flip-flop buffers. The program contains a trigger that can be set to any level, that when triggered reads from the none active buffer then the active on and writes to a file then picks up from the incoming data for 20 mins. The buffer data holds a time stamp every minuet so an accurate event time can be displayed. An indicator at the bottom of the screen indicates the number of events recorded (0=none).
The program to display the recorded data can use the time records to display parts of the file that may be of interest. The programs them selves are not really of any value to others as they work with the A/D input device that I created, but if you are interested you can have a copy....
I hope you find the above of interest, let me know if I have confused you in any way or if something is not too clear.



A closer view of the detector set-up.

A closer view of the feedback coil and magnet.

A close up of the permanent left hand lower hinge assembly.

A closer view of the moveable RH lower hinge assembly and leveling screw.

Screen shot.

This shows the bob, feedback coil and signal arm.