Ramp Function

[djhazeleger@mindspring.com]

I am considering building a "Pick and Place" system using Shopbot hardware and software. To be practical, the travels would have to be fast(12"/sec). The transitions in direction would have to be smooth. This leads me to a question concerning the ramp function. Is the acceleration an actual ramp or 1 step in speed before full speed? I suspect it is the later, and this may become a problem at higher speeds. Does anyone foresee any problems or have suggestions running the travels at this speed?

[Ted Hall, ShopBot]

Hi Dirk,

The pick and place project is interesting. We've thought about it from time to time here, but have been too busy to work on it.

The ramps -are- full linear ramps that accelerate from the ramp speed to the move or jog speed, over the number of steps that have been specified for the move and jog ramp size (by default the ramp size is 100 and 150 steps for move and jog, the acceleration rate is a constant, i.e. straight line increase). In general, I think the ramps should serve you well at higher speeds, if you experiment with the parameters that work best for the application. We have noticed recently, as ShopBots have become capable of higher speeds, that there is a little bump in the transition from the speed at the end of the acceleration ramp to the final speed. This happens at speeds of 3in/sec and greater. We're working to get this fixed before the final version 2 release. We are also evaluating adding the option of a smoother, 'S' shaped ramp. This one won't appear in version 2 ... but maybe later.

As to 12/in a sec ... here's my quick and approximate math. Assuming you're using the bigger motors which on the standard tool will produce about 80lbs of linear force and go 3in/sec at 1500 steps/sec. If you change the gearing so that you're tool is capable of 12in/sec speeds, you will only have a linear force of about 20 lbs. By fine tuning and releasing the hold-downs a bit, this may work OK for a pick and place operation. Your other concern will be resolution. You will be going from about 500 steps/in to about 125 steps/in. This may not give you the smoothness in movement operation that you will need. This is probably one of those things that just needs to be tested. Skip the gearbox and put the pinion right on the motor ... this will give you about the gear ration you're looking for and should give you a pretty quick idea of whether the thing will work. If you haven't checked it out, there is some info on step frequencies, speeds and power in the developer section.