Looks interesting Jan, nice work. I think it may be under dampened. The 
standard rule is to pull the boom back a few inches and let it go. The boom 
should loose 30% of its motion on each swing past center and come to rest 
in 3 1/2 swings. My standard non dampened period is 12 - 14 seconds with a 
40-cm boom and 18 - 20 seconds when dampened. I have pasted some table data 
below that will help you determine the angle  (drop) on your boom and the 
natural non dampened period.  In my system, I also use a 5 lb. mass 
however, I needed two of the same magnets you are using to get mine to come 
to rest. They are set in an opposing field design. A 1/4-inch copper plate 
is attached to the boom and passes between  them. I also have another 
thought and it is about wind currents in the room. What kind of cover are 
you using? I don't see one in the photo and wanted to be sure you 
understand, you need to put this unit in an air tight box.

As for noise each site is different. Take a look at Larry's site in Redwood 
City and look at some of the other Lehman style systems. For example, mine 
in Aptos, CA are AT1 and AT2. I live near a busy road and have to deal with 
very high noise.
Regards, Steve Hammond  PSN San Jose -   Aptos, CA

Boom length in CM.	Natural period @ .05 degree	Natural Period @ .1 degree
Natural Period @ .25 degree	Natural Period @ .5 degree
Natural Period @ .75 degree	Natural Period @ 1 degree
Natural Period @ 2 degree
							
100	67.943	48.043	30.385	21.486	17.543	15.193	10.744
99	67.602	47.802	30.233	21.378	17.455	15.117	10.690
98	67.260	47.560	30.080	21.270	17.367	15.040	10.636
97	66.916	47.317	29.926	21.161	17.278	14.963	10.581
96	66.570	47.072	29.771	21.051	17.189	14.886	10.527
95	66.222	46.826	29.616	20.941	17.099	14.808	10.472
94	65.873	46.579	29.459	20.831	17.009	14.730	10.416
93	65.522	46.331	29.302	20.720	16.918	14.651	10.361
92	65.168	46.081	29.144	20.608	16.827	14.572	10.305
91	64.813	45.830	28.985	20.496	16.735	14.493	10.249
90	64.456	45.577	28.826	20.383	16.643	14.413	10.192
89	64.097	45.323	28.665	20.269	16.550	14.333	10.136
88	63.736	45.068	28.504	20.155	16.457	14.252	10.079
87	63.373	44.811	28.341	20.040	16.363	14.171	10.021
86	63.008	44.553	28.178	19.925	16.269	14.089	9.963
85	62.640	44.293	28.014	19.809	16.174	14.007	9.905
84	62.271	44.032	27.848	19.692	16.078	13.924	9.847
83	61.899	43.769	27.682	19.574	15.982	13.841	9.788
82	61.525	43.505	27.515	19.456	15.886	13.758	9.729
81	61.148	43.239	27.346	19.337	15.789	13.674	9.669
80	60.770	42.971	27.177	19.217	15.691	13.589	9.610
79	60.389	42.701	27.007	19.097	15.593	13.504	9.549
78	60.005	42.430	26.835	18.975	15.494	13.418	9.489
77	59.620	42.157	26.663	18.853	15.394	13.332	9.428
76	59.231	41.883	26.489	18.731	15.294	13.245	9.366
75	58.840	41.606	26.314	18.607	15.193	13.157	9.304
74	58.447	41.328	26.138	18.483	15.091	13.069	9.242
73	58.050	41.048	25.961	18.357	14.989	12.981	9.179
72	57.651	40.766	25.783	18.231	14.886	12.892	9.116
71	57.250	40.482	25.603	18.104	14.782	12.802	9.053
70	56.845	40.195	25.422	17.976	14.678	12.711	8.989
69	56.437	39.907	25.240	17.847	14.572	12.620	8.924
68	56.027	39.617	25.056	17.717	14.466	12.528	8.860
67	55.614	39.325	24.871	17.587	14.360	12.436	8.794
66	55.197	39.030	24.685	17.455	14.252	12.343	8.728
65	54.777	38.733	24.497	17.322	14.144	12.249	8.662
64	54.354	38.434	24.308	17.188	14.034	12.154	8.595
63	53.928	38.133	24.117	17.054	13.924	12.059	8.528
62	53.498	37.829	23.925	16.918	13.813	11.963	8.460
61	53.065	37.523	23.731	16.781	13.702	11.866	8.391
60	52.628	37.214	23.536	16.643	13.589	11.768	8.322
59	52.188	36.902	23.339	16.503	13.475	11.670	8.252
58	51.744	36.588	23.141	16.363	13.360	11.571	8.182
57	51.296	36.272	22.940	16.221	13.245	11.470	8.111
56	50.844	35.952	22.738	16.078	13.128	11.369	8.040
55	50.388	35.629	22.534	15.934	13.010	11.267	7.968
54	49.928	35.304	22.328	15.789	12.891	11.164	7.895
53	49.463	34.976	22.121	15.642	12.771	11.061	7.822
52	48.994	34.644	21.911	15.493	12.650	10.956	7.747
51	48.521	34.309	21.699	15.344	12.528	10.850	7.673
50	48.043	33.971	21.485	15.193	12.405	10.743	7.597
49	47.560	33.630	21.269	15.040	12.280	10.635	7.521
48	47.072	33.285	21.051	14.886	12.154	10.526	7.444
47	46.579	32.936	20.831	14.730	12.027	10.416	7.366
46	46.081	32.584	20.608	14.572	11.898	10.304	7.287
45	45.577	32.228	20.383	14.413	11.768	10.192	7.207
44	45.068	31.868	20.155	14.252	11.637	10.078	7.127
43	44.553	31.504	19.925	14.089	11.504	9.963	7.045
42	44.032	31.135	19.692	13.924	11.369	9.846	6.963
41	43.505	30.762	19.456	13.757	11.233	9.728	6.879
40	42.971	30.385	19.217	13.589	11.095	9.609	6.795
39	42.430	30.003	18.975	13.418	10.956	9.488	6.709
38	41.883	29.616	18.731	13.245	10.814	9.366	6.623
37	41.328	29.223	18.482	13.069	10.671	9.241	6.535
36	40.766	28.826	18.231	12.891	10.526	9.116	6.446
35	40.195	28.423	17.976	12.711	10.379	8.988	6.356
34	39.617	28.014	17.717	12.528	10.229	8.859	6.265
33	39.030	27.598	17.455	12.342	10.078	8.728	6.172
32	38.434	27.177	17.188	12.154	9.924	8.594	6.078
31	37.829	26.749	16.918	11.963	9.768	8.459	5.982
30	37.214	26.314	16.643	11.768	9.609	8.321	5.885
29	36.588	25.872	16.363	11.570	9.447	8.182	5.786
28	35.952	25.422	16.078	11.369	9.283	8.039	5.685
27	35.304	24.964	15.788	11.164	9.116	7.894	5.583
26	34.644	24.497	15.493	10.956	8.945	7.747	5.478
25	33.971	24.021	15.192	10.743	8.771	7.596	5.372
24	33.285	23.536	14.886	10.526	8.594	7.443	5.263
23	32.584	23.041	14.572	10.304	8.413	7.286	5.153
22	31.868	22.534	14.252	10.078	8.228	7.126	5.039
21	31.135	22.016	13.924	9.846	8.039	6.962	4.923
20	30.385	21.485	13.589	9.609	7.845	6.794	4.805
19	29.616	20.941	13.245	9.365	7.647	6.622	4.683
18	28.826	20.383	12.891	9.116	7.443	6.446	4.558
17	28.014	19.809	12.528	8.859	7.233	6.264	4.430
16	27.177	19.217	12.154	8.594	7.017	6.077	4.298
15	26.314	18.607	11.768	8.321	6.794	5.884	4.161
14	25.422	17.976	11.369	8.039	6.564	5.685	4.020
13	24.497	17.322	10.955	7.747	6.325	5.478	3.874
12	23.536	16.643	10.526	7.443	6.077	5.263	3.722
11	22.534	15.934	10.078	7.126	5.818	5.039	3.563
10	21.485	15.192	9.609	6.794	5.548	4.804	3.397

-----Original Message-----
From:	Jan D. Marshall [SMTP:jandmarshall@.............
Sent:	Sunday, September 29, 2002 2:52 PM
To:	psn-l@..............
Subject:	RE: New Lehman on line (almost)

see my comments beside your questions below -- Jan M

Jan Marshall
jandmarshall@............
www.cableone.net/jandmarshall
Nampa, ID

  -----Original Message-----
  From: psn-l-request@.............. 
[mailto:psn-l-request@.................
Behalf Of ChrisAtUpw@.......
  Sent: Sunday, September 29, 2002 3:24 PM
  To: psn-l@..............
  Subject: Re: New Lehman on line (almost)


  In a message dated 29/09/02, jandmarshall@............ writes:


    Please visit my web site www.cableone.net/jandmarshall and choose the
Seismic Page to view my project.

    Problems I note right off
       1 - I have a 4-5 hz oscillation in the pendulum.
       2 - I seem to have a lot of environmental noise -- I can not
correlate it with actions in the house
       3 - Am I damped enough?


  Hi Jan,

        The photos are a great help.

        What is the weight of the seismic mass on the end of the arm? 
 right
at 5 pounds

        What undamped oscillation period are you getting?  about 12 - 14 
sec

        Only you can check the damping. Deflect the beam a very small
amount, maybe by putting a screwdriver etc near it, let it go and observe
it. The pendulum should swing back to the zero position, but it should not
go beyond zero and come back, or show any actual oscillations.  OK -- I am
not damped enough -- It oscillates about 4-5 times

        The bearings, general construction etc look fine. Check for
electrical activity in the house / movement of magnetic materials, even 
cars
/ trains outside etc to search for the environmental noise. Does the seis
react when the fridge / cooker / central heating switches on / off? It is
more usual to put both magnets on the baseplate and the coil and damping
plate on the arm. With unscreened magnets on the arm, it can and will pick
up any small changes in the local environmental magnetic field.  I will
consider this

        From the photos, I can't see where the pick up coil is in relation
to it's magnet. The centre line of the coil should be roughly on the end
face of the magnet with your setup.  See the 6th picture down --

        The 5 Hz could be due to the whole arm system nodding up and down 
or
side to side. Try tapping the end post with your finger and see if you get
this sort of frequency response. With your coil system as shown, you will
likely pick up both vertical and horizontal movements of the arm.  I 
lowered
the right end a tiny bit -- the LF oscillations appear to have stopped

        What amplifier / filter / A/D system are you using? What is the
cut-off frequency of the filter?  I am using Larry Cochran's Serial A/D
board and his Filter/Amp both stock as I received them -- I am using his
coil also

        Regards,

        Chris Chapman
 << File: ATT00003.html >> 
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