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Unformatted text preview: Determining Correct Encoder Resolution
If you purchased your encoder installation from JMI, you
received documentation on the correct resolution for
each axis and, if included in the purchase, the computer
was set to the proper values. For any other installations
you can use the following information to determine the
correct resolution to enter in the computer's Setup mode.
The Goal
The goal of the following exercise is to get the correct
encoder resolution in the MAX computer by matching the
degrees shown in the Encoder mode display with the
actual degrees of movement of your scope. instance, if you can only move 180º the displayed angle
should be multiplied by two.
Here is an example. Your right ascension resolution is
set to 4096 in the computer. After rotating the right
ascension axis 360º while in Encoder mode the display
shows an angle of 12º. Observing the display during
rotation showed that the angle passed 359º one time so
the true angle is 360º + 12º or 372º. Therefore, the
formula would look like this:
new resolution Calculating for Gears and Pulleys
There should be a sticker with a number on the housing
of each encoder (e.g. 2160, 4000, 4096, or 8192). If
you are using gears or pulleys, simply multiply this
number by the ratio of teeth on the telescope shaft to the
number on the encoder shaft.
encoder tics teeth on telescope shaft
X ————————————
teeth on encoder shaft For example, a 4000 tic encoder using a 100tooth gear
on the telescope shaft and a 50tooth gear on the
encoder shaft would yield a total of 8000 tics per
revolution (4000 x 100/50).
Calculating for Friction Drives
If you have a friction drive where calculating the ratio of
telescope shaft movement to encoder shaft movement is
impractical, you can use the following formula to
determine the tics per revolution:
new resolution = 360 x old resolution
——————————
angle Enter an estimated resolution for the axis using the
Setup mode. This accuracy of this initial number is not
important but it should be placed in the above formula as
the old resolution. Using the Encoder mode and starting
with a display of zero degrees, determine what angle is
displayed after exactly one rotation of the axis. Use this
number for angle in the formula. Be sure to add 360 to
the angle displayed for each time the display passes
359º, if any. Replace the initial number you entered into
the computer with the answer (new resolution) and
repeat the above procedure so that the new resolution
now becomes the old resolution. This process should be
repeated until the result stabilizes (usually two or three
iterations). If the telescope cannot be moved through
360º of rotation you will need to adjust the formula. For = 360 x 4096
——————————
372 The new resolution is 3964. This number should be
entered into the computer and into the formula (as the
old resolution) and the process repeated until the
number does not change. At this point 360º of telescope
axis rotation will translate to 360º on the display
(showing 0º after full rotation).
For a more accurate calculation you can rotate the axis
through several complete revolutions and divide the
angular result by that number of turns to determine the
angle to plug into the formula. For example, if five
revolutions shows a result of 2º and the display passed
359º five times, the total degrees would be 1802º (360º x
5 + 2). Dividing 1802 by 5 gives 360.4º which should
then be used for angle in the formula.
Digital Nature of Encoders
It should be noted that pointing error is always greater
than the encoder resolution. The problem stems from
the digital nature of the encoder steps. The encoder
may be positioned on the verge of a step during the star
alignment, then on the opposite edge of a step when the
target star is sighted. With ideal encoders, perfect
mechanical installation and perfect alignments, the
fundamental error can be three times the encoder step
angle. Fortunately, this rarely happens.
Maximum Practical Encoder Resolution
If you wish to increase the resolution, keep in mind that
while the maximum possible encoder resolution is
32767, in most cases the practical limits are much lower.
This is due to the computer's sampling rate and display
resolution. We target 10000 ticsperrevolution for our
systems. To put it simply, higher resolution requires
slower telescope movement because the computer's
sampling rate determines how fast it can read the data
sent by the encoder.
Further Reading
For more information, refer to the document library on
our web site at jmitelescopes.com. JMI Telescopes
© Jim's Mobile, Inc. 8550 W 14th Ave Lakewood, CO 80215 USA 3032335353 Fax 3032335359 jmitelescopes.com ...
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This note was uploaded on 10/23/2011 for the course SDASD 102 taught by Professor Dsfas during the Spring '11 term at Baptist Bible PA.
 Spring '11
 dsfas

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