Title: Tachometer Replacement Analysis
Author: E. Warmbier
Description: This document determines what is required for a tachometer replacement in terms of voltage, speed, etc.
Directions
Fill in
yellow highlighted
regions.
Variables
Fastest movement of a telescope axis (includes margin: 1800+200 arcse/sec).
Tracking rate.
Ratio of motor shaft to telescope axis shaft.
Input gain stage for tachometer on Safety Board.
Peak to peak error velocity goal for TCS.
Conversion of degrees to arcsec.
Conversion of arcsecs to degrees.
Conversion of telescope axis velocity to motor (and tach) rpm.
Conversion of telescope axis velocity to tach voltage.
PMAC servo loop bandwidth (update rate).
Tach filter in use currently for the TCS3.
Analysis section: Basic System Requirements (Mechanics and Tachs)
Determine what the maximum voltage output of the tach needs to be.
However, the first thing that the Safety Board does is to lower the signal.
This greatly reduces the voltage required and potentially allows more common electronics to be used without modification.
Another main specification is the rpm of the tach/motor shaft at tracking and max speed (just above slew).
At maximum speed (including margin) it is:
Take notice of the LOW speed and difference between the slowest and fastest rpm.
The industry standard specification for tachometer generators is in terms of volts per 1000 rpms.
This is a really high value in terms of industry standard, off the shelf tachometer generators.
However, keep in mind that the first thing that occurs is a gain of around 1/5 is applied to the signal.
Finally, determine the minimum resolution.
12 bits is the bare minimum.
Analysis Section: Encoder Basic Requirements
Determine how many pulses per second an encoder (plus quad decode) will need to produce (in terms of pulses/revolution) to match the tachometer as it is currently used in the system.
The tachometer has a low pass filter of:
At tracking, the shaft rate is:
So, finally, the absolute, minimum encoder pulser per revolution should be:
Pulses per revolution.
The PMAC loop is actually operating at:
Therefore the encoder should probably have a rate at least 3-4 times faster.
Analysis Section: Gurley R158 Encoder
The Gurley R158 is an example of an off-the-shelf encoder that can provide 1,000,000 counts per revolution (after quad decode).
At tracking:
(Stripped off some units)
(counts per second)
Now determine the pulses at maximum speed (just above slewing):
(Stripped off some units)
(counts per second)
Consideration #1: Encoder with Converter (US Digital - ETACH2)
US Digital makes an inexpensive and simple quadrature to analog converter.
The resolution should be analyzed.
In terms of full scale percentage error:
Which is less than:
In absolute terms:
So, the US Digital solution is very close to the minimum and most likely does not have adequate resolution.