Irregular Steps - StackShot

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jin
Posts: 93
Joined: Sun Aug 26, 2012 6:35 am
Location: Singapore

Irregular Steps - StackShot

Post by jin »

Hi Guys,

Just received the Stackshot controller with my own stepper motor with the coupling method similar to Rik's ie, hose as coupler between motor and microscope knob.

However the issue I have got here is I noticed the step is very irregular! For example if set the travel step to 1 small marking on the knob (~2um on my Olympus BH) and once I started the sequence I notice the travel between steps is not constant. Usually after the initial step, the next step goes a little more than 1 mark and the subsequent one can be even more and than maybe back to original 1 mark this goes on.

I tried to make very the parameters in Globe Setting such as reducing the Speed/Sec to 100u/ sec, increase the Tramp to >3sec, changed torque but cannot seem to fix the issue.

Since this is the 1st time I use the Stackshot controller can the forum members kindly advise what might be wrong in my application ie, the motor I use or settings etc.

Mnay thanks in advance for your advice,

Regards,
jin

p/s: I thought I will be quicker to get answer from forum members here so I have not checked with Stackshot.

rjlittlefield
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Post by rjlittlefield »

Be sure that the StackShot controller is set to High Precision mode.

If the controller is not in High Precision mode, then there is a problem with severe nonlinearity when microstepping. See HERE, second image, left curve on the graph. What happens is that while actively driving the stepper, the system follows a linear curve like shown on the right, then when it drops into "holding torque", it transitions to a big-steps curve like shown on the left. In other words the motor reaches the intended point, stops, then moves to another point that in some cases can be as far as about 1/2 step away from the intended point. My early observations about the effect were similar to what you describe. They are discussed later in the same thread you mention, HERE.

--Rik

jin
Posts: 93
Joined: Sun Aug 26, 2012 6:35 am
Location: Singapore

Post by jin »

Hi Rik,

Thanks for pointing me to the threads, the travel (tried roughly 1um and 2um) seem to be stable after set the Hi Precision to On, except that I need to have a proper coupler as the hose I used is little soft and will flex sometime.

I also observe the starting point is always overshot (inaccurate) the initial starting point I set, not sure why, but I think must be settings in Global Setting is still not set to give optimal performance.

Regards,
Jin

rjlittlefield
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Post by rjlittlefield »

When using the StackShot rail, it's very common for the first step or even the first few steps to be inaccurate. The problem there is mechanical backlash, and the cure is to set the endpoints and then manually move the rail back behind the start position before starting the automatic sequence. That way the rail has to move forward to reach start, which removes all the backlash so that the rail can continue moving smoothly for subsequent steps.

I have not seen a similar problem with my microscope setup, but from your description I suspect that something similar is happening. Try the same fix and see if it goes away.

By the way, I find it's always a good idea to shoot a few extra frames at the beginning and end of the stack. I can always remove them after the shoot, and it greatly reduces the frustration of discovering after processing a long stack that I messed up and left some bit out of focus.

--Rik

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