Last updated: 18 January 2007
Sent: Friday, December 8, 2006 08:05:28 From: Henrik VAN HOLTHOON (firstname.lastname@example.org) I have looked at your article covering the LNT module. Very interesting I must say. However in my opinion the LNT module has the following sensors; Electronic level Electronic compass These components are not expensive anymore. You state also there is an array of accelerometers incorporated into the LNT ? So my question to you is why do you need accelerometers, an accelerometer measures change in speed dv/dt. I do not understand what would be the function of an accelerometer in the LNT module this in relation to the orientation. The electronic compass does the orientation and the electronic level measures the leveling of the OTA and the tilt of the vertical axis as you can see during the automatic alignment after pointing to north the ETXPE measures the horizontal plane in two directions 90 degrees apart in order to do the leveling and tilt measurement of the horizontal axis. But I did not open the LNT module so you can be right of course. Most regards Henrik
From: email@example.com (firstname.lastname@example.org) Hi Henrik, I'll try to explain my conclusions like this: The LNT does not have an actual electronic compass. It has a sensor that detects when it slews past Magnetic North. When it does, it switches a digital signal off or on. The Autostar detects this signal and saves the position of the drive encoders at that instant. This becomes the North reference. From then on, Autostar uses the drive encoders to determine what direction the scope is pointing. For determining level, The LNT does not use any kind of mechanical level device. It uses the accelerometer array to detect vertical motion with respect to the Earth's gravitational field. As the scope slews up or down, the array tilts with respect to the field so it's acceleration is a vector quantity with both horizontal and vertical components. When it is exactly parallel, there is no horizontal component so that position is saved as level. When determining the Tilt coefficient, the scope slews about 90 degrees in Az while the accelerometer measures any vertical movement.. This allows the tilt of the platform to be calculated by the Autostar. When that part of the Auto Alignment is completed, the Autostar has the encoder position for "0" degrees Alt and Az, and also a correction for any variation of the Alt position as the Az position changes. From that point on, the LNT serves no further purpose. To further refine the positions, Autostar adds a correction for magnetic variation determined from the lat/lon of the observing site, plus corrections measured during the Cal Sensors procedure. All the above is based on my own assumptions and the actual components used in the LNT, which you can see here: http://www.weasner.com/etx/techtips/2005/lnt1.html I hope this is understandable. Regards, Mike Hogan
And some additional information:
Subject: re: LNT summary Sent: Friday, December 8, 2006 20:23:34 From: richard seymour (email@example.com) Part of the possible confusion about the LNT construction arises from Mike Hogan's (recent) choice of words. The "accelerometer array" is a single device (when viewed as parts on the board). If you read the datasheets for the US D2020E chip, one of the suggested uses is as a "level" sensor by detecting the local "vertical" gravity vector. The "compass" is a simple coil of wire which is connected as a "bridge" between two current sources. When the magnetic field of the earth is exactly perpendicular to the centerline of the coil, the bridge is "balanced" and the current flow in both sides of it are equal. When the moving coil is -not- perpendicular, the current flows are not equal, and one of the legs will have different current flow than the other. I've attached a link to a schematic from the LX200gps, helpfully provided by www.telescopeservice.com The left half of the diagram shows the magnetic sensing coil (L1) and associated parts. Please -ignore- the level sensor on that diagram, since the LX200gps uses a rolling steel ball which passes in front of a photosensor for its "level sensor". The LNT method is much "cleaner" (and quieter... many new LX200gps owners are disturbed and puzzled by the "click" sound they hear as the telescope tilts... it's the ball rolling inside its plastic tube hitting the end of its travel). have fun --dickhttp://www.telescopeservice.com/pdf/LX200GPS/LX200GPS%20GPS_Level_Magnetometer%20-%20Sheet%202.pdf
From: Henrik VAN HOLTHOON (firstname.lastname@example.org) As I said I have not opened the LNT module and will not do so if not really necessary but as engineer I am always curious how things work. When reading the summery concerning the LNT module I though accelerometers for leveling that is new for me, obviously it is possible even with such small accelerometers. The basic concept of an accelerometer is a mass on a spring with some device to detect the inertial of the mass i.e. piezoelectric material, servo controlled mass, mass in coil etc.. I still do not know how this array works exactly but I can now see it can work if it works on the gravity force and vertical as the force is then maximal. As long as the rotational speed of the OTA is not constant you have a signal, must weak though. The magnetometer in a bridge of wheatstone that is clear to me, clever design. Thanks for your explanations. Henrik
Thanks Dick, Mike hogan did send me the following ethernet link; www.memsic.com/memsic/data/products/mxd2020e/mxd2020e.pdf So if you want to refresh your knowledge about this clever 2 axis accelerometer go to this site, under technologies you find a more specific explanation of the heat transfer principle. This accelerometer works without a mass as normal accelerometers do i.e. from Endevco (www.endevco.com). In our case, LNT operation, this accelerometer measures the deviation of the horizontal plane (X,Y) tested by ETX in 2 directions 90 degrees apart as we can see during the automatic orientation procedure. I am really in focus now to speak in telscope terms. What surprises me is the incredible resolution and sensitivity of this accelerometer device. Well we are never to old to learn. Most regards Henrik
Subject: LNT operation in relation to accelerometer Sent: Friday, December 15, 2006 01:51:56 From: Henrik VAN HOLTHOON (email@example.com) It is about 30 years ago that I sold Endevco accelerometers and Spectral-Dynamics vibration analyzing equipment so my interest in the use of this small accelerometer was immediately provoked.I studied this 2 axis accelerometer in relation to the LNT orientation system, we do not know exactly of course how this accelerometer is used by Meade, and they will not tell us, but I think it works as follows; What does the ETX PE during the orientation process Position LNT home position (Manuel) LNT level OTA ETX GOTO magnetic North ETX GOTO true North LNT relevel OTA LNT determine tilt vertical axis LNT AZ turn 90 degree LNT level OTA LNT determine tilt vertical axis The remaining process steps have no relation with LNT The 2 axis accelerometer used is an accelerometer where the mass is heated air (gas) when de the LNT is not level the gravity force pulls the gas to one side of the temperature sensor, causing a different heat transfer between heated gas and T sensor. This gives a delta T. If the LNT is in the horizontal plane the heated gas is equally distributed over the sensors and delta T then 0. The accelerometer has sensors in the X axis and Y axis, only in one plane. Lets assume that the accelerometer is oriented in the LNT module so that the X axis is parallel to the OTA and the Y axis perpendicular to the OTA and Z is the vertical axis (ETX fork) of the horizontal plane. I have also determined that the OTA is not driven exactly level this I checked with a spirit level. The OTA is pointing slightly upwards so level signal will not be 0 during orientation process. 1)LNT level OTA accelerometer gives digital signal out proportional to the offset of the horizontal 2)LNT determines tilt Zy by reading digital Y signal (perpendicular to OTA) 3)Drive OTA 90 degree 4 Same as point 1 5)LNT determines tilt Zx by reading digital X signal (perpendicular to OTA) Digital signals accelerometer, offset to horizontal ,tilt Zx and tilt Zy are then used to calculate the offsets to the horizontal plane and the tilt of the vertical axis and used in the GOTO positions. Well that How I think how the LNT works in relation to the 2 axis accelerometer. Most regards Henrik
Subject: Re: LNT Sent: Monday, January 15, 2007 08:12:12 From: richard seymour (firstname.lastname@example.org) Thank you for the report. The LNT's accelerometer is certainly a "more elegant" solution than the rolling ball used by the LX200gps and (i believe) RCX400. Since the Autostar is "simply" trying to determine the -plane- tangent to the earth's "sphere", it does not matter that the Accelerometer's "level" is not true when the OTA is level. (i hope) After the alignment process completes, you can level the OTA and see if the Autostar agrees that Alt=0 degrees (press the [mode] key 3 seconds, release, then scroll through the various displays) have fun --dick
From: Henrik VAN HOLTHOON (email@example.com) Hello Dick, Thanks for your reaction I did what you suggested but Autostar gives me only declination 1degr 50 m which is not very helpful. As I more familiar to theodolite values inclination is more to my liking. So I started Starry Night 6 pro plus and connected the ETX to a laptop when I had the OTA level, measured with a spirit level, I got a altitude of 0.3 minutes, which is not to bad I think. This 2 axis accelerometer used as 2 axis inclinometer is rather good I think. Maybe your ball type unit is even better I do not know. Have fun Henrik
Go back to the Tech Tips page.
Go back to the ETX Home Page.