From ascen@world.std.com Tue Sep 20 12:35:00 1994 Return-Path: Received: from mx3.u.washington.edu by stein2.u.washington.edu (5.65+UW94.4/UW-NDC Revision: 2.30 ) id AA26547; Tue, 20 Sep 94 12:35:00 -0700 Received: from hitl-new.hitl.washington.edu by mx3.u.washington.edu (5.65+UW94.4/UW-NDC Revision: 2.30 ) id AA02827; Tue, 20 Sep 94 12:34:28 -0700 Received: by hitl.hitl.washington.edu; id AA21685; Tue, 20 Sep 1994 12:32:55 -0700 Received: from relay3.UU.NET by rodan.UU.NET with SMTP id QQxife11908; Tue, 20 Sep 1994 15:34:22 -0400 Received: from uucp3.UU.NET by relay3.UU.NET with SMTP id QQxife23432; Tue, 20 Sep 1994 15:34:21 -0400 Received: from world.UUCP by uucp3.UU.NET with UUCP/RMAIL ; Tue, 20 Sep 1994 15:34:26 -0400 Received: by world.std.com (5.65c/Spike-2.0) id AA17542; Tue, 20 Sep 1994 15:32:26 -0400 Newsgroups: sci.virtual-worlds Path: ascen From: ascen@world.std.com (Ascension Technology) Subject: Re: TECH: Magnetic trackers Message-Id: Organization: The World Public Access UNIX, Brookline, MA X-Newsreader: TIN [version 1.2 PL2] References: <35cjd3$b6u@news.u.washington.edu> Date: Tue, 20 Sep 1994 19:32:20 GMT Lines: 81 Apparently-To: uunet!sci-virtual-worlds@uunet.uu.net X-Status: Status: OR Anant Chandra (71052.1323@CompuServe.COM) wrote: : I was wondering whether anyone has any on magnetic interference : with the Polhemus and Ascension trackers. For, example, when : designing the headmount, obviously lots of steel close to the : tracker would be undesirable. But how much is too much, how far : away should steel parts be, and what is the impact of other : metals? Magnetic based trackers have to function around 2 distorting factors when working around metal. They are: 1. Distortions due to eddy-currents. Varying magnetic fields generate electrical currents is conductive materials. These eddy-currents in-turn generate smaller magnetic fields, which in-turn generate secondary currents which generate smaller magnetic fields, etc. The Ascension Technology Corporation's Flock of Birds tracker uses a pulsed DC magnetic field. After a field is generated, the system waits before taking a measurement. Given a small amount of time, all the eddy- current fields drop out, and the original magnetic field remains to be measured. 2. Distortions due to ferrous metallic substances. Ferrous metallic substance bend/distort normal magnetic fields. Generally, all metal has some ferrous metallic substance, which affects magnetic fields to varying degrees. Some metal alloy have little (stainless steal 200 series) and some have a greater distortion effect. Distortions in magnetic fields from our tracker due to ferrous metal is usually not significate. By placing the receiver a small distance from the ferrous metal is enough to eliminate the distortions. Placing our receiver on a plastic mounting block (or wood, fiberglass, non-metal) is usually enough. For huge ferrous metal sources, such as metal floors/walls, we recommend keeping the distance from the receiver to the transmitter less then the distance from the large ferrous metal source to the transmitter. Even large amounts of ferrous metal directly within the working environment is not an insurmountable problem. The area may be "mapped" with our device and distortion-correction algorithm implemented, but this is usually not needed. On a head mounted display you should not run into any trouble as long as you do not mount the receiver directly onto metal (or use metal screws). We are mounted on many HMD on the market, including Virtual Research, Liquid Image, and others. Another concern with CRT based HMD is electrical noise from the CRT burst. We supply a CRT sync pickup with our system to eliminate this as a problem, but good placement on the HMD will prevent the problem from developing. If you are still not sure, or need more detail, please call me (Steve Work, Ascension Technical Support) at (802)860-6440 and I will be happy to help further. Regards, Steve