On Mon, Jan 1, 2024 at 07:29 AM, WayBack wrote:
Michael,
Micro step position does vary but I'm not sure it represents a significant issue; this not accumulating during guiding.? The issue appearing like corrected backlash.? And it cannot accumulate in physical position between motor poles (true steps).
I always felt power consumption and the related issue of producing less torque for the power were my issues.? The benefit of being open loop becomes significant only if the weight and power allow for this.
With torque levels at tracking quite small, as you illustrate here, the variation cannot be large, or it shows up in guiding.? I've never seen anyone ask David Partridge?but the RMP of our motor during tracking must be quite low and the ability of our motor to arrive exactly on count at a moment in time also a variant.? Still there too, all just esoteric when compared to the tracking RMS achieved.? Hence the importance placed on achieving Level 6 functionality.
Doug? ?
I did not write the firmware or design these systems but my understanding of the way these system work is that they are quite different.?The way an open loop stepper drive system works vs the closed loop of the Gemini 2 servo motor drive is quite different.
A stepper motor controller like the OnStep (not the stepper driver IC which can be quite sophisticated) blindly sends the step or microstep pulses calculated and has no idea if the stepper motor has actually completed the full step count or lost steps. This is an inherent problem with an open loop stepper drive system especially a marginally powered system for the load. This means that an open loop stepper drive system cannot ever know the mount axes position with certainty. Encoders on the drive motor itself or the movement axes allows the mount controller to know with reasonable certainty the mount's position. Certainly there are encoder equipped stepper motors but they are not being exploited widely yet.?
So open loop stepper motor drive mounts using a worm screw can basically only successfully step and move the mount or lose steps and physical position but the mount controller's model is not aware and has to assume the mount? position is correct which puts the slew off target requiring the mount be recentered. Depending on the how many steps were lost and the application it may or may not matter.?
The closed loop system with encoders on the DC motor drive shaft as used in the Gemini 2 allow the controller to just run the motors and watch the encoder count slowing, then stopping or assuming the selected movement rate for tracking. Positional data for the mount is generally? known and the mount position can be corrected in the case of an under or over shoot of the desired tic count. The Gemini sends a known amount of current to the motors and watches the resulting movement through the tic counts. If the tic count is not accumulating as predicted vs time more or less current is sent as required to match the desired movement rate.?
With the rewritten PIC code and revised and tuned L6 primary controller firmware the Gemini 2 L6 firmware now has a resolution of 0.14 arcseconds. This is getting into the pretty good range IME and is 4x better resolution of the direct stepper drive G11 mount. But if you added a 4:1 reductio with a pulley you would have very similar theoretical resolution though it would still only be a guess being open loop. Adding encoders on the stepper to close the loop and correct any missed microsteps and now you have something.? ?
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Chip Louie Chief Daydreamer Imagination Hardware?
? ?Astrospheric Weather Forecast - South Pasadena, CA?
Chip Louie