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# micrologix 1400 question

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Hi all I have a conveyor with an encoder on it, it is powered by a vfd speed referenced by analog output on a Micrologix 1400. I want to gradually decrease the speed of the conveyor after product Leaves the conveyor (ie as the hsc accumulates) and shut it off completely after a preset limit What is the best instruction to tackle this problem.

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I don't know if I have posted it before, but I did this sort of "poor mans servo" with V/Hz drives in the late 90s with really good success. I used one enable signal and a bipolar output +/-10vdc. I used a timer, dividing the acc by the pre to a float. I used distance for the ramp down, which ended up with a nice smooth deceleration. Same type of math: position_error/decel_distance, clamp between max speed and min speed. Set the drive accel and decel times to the minimum, and always make sure you ramp the value properly in all modes so you don't jerk the hell out of it!

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It's a different application, but I did something sort of similar recently - and anytime you're going to jerk a drive like that (I'm changing speeds drastically with no delay), then the easiest way to avoid having to add an appreciable amount of logic is to simply set the accel and decel times on the drive appropriately. For example, if you're slamming a drive from 15 hertz to 80 hertz or anywhere in between - and doing so as much as 10 times per minute, then simply setting the accel time to "2 seconds" (provided it fits your application) can solve any "jerking" issues. It depends on VFD manufacturer, but the ones I used would employ a scaled 2 second delay across min freq / max freq. For example... VFD hard limit min freq = 0 Hz / VFD hard limit max freq = 120 Hz. Accel = 2 sec. Decel = 2 sec. Therefore... ( 2 seconds ) divided by ( 120 Hz - 0 Hz ) = delay of 17 milliseconds per 1 Hertz step. So when ramping from 0 to 80 Hz, it would take 1.33 seconds. / going from 40 hertz to 60 hertz would smooth out over 333 milliseconds. Etc, etc... It's a nice way to cheat, provided you standardize on VFD types (or at least buy VFD's with comparable feature sets / attributes, and be sure to document what your settings are... I make it a point to put large comment rung in a PLC program where the VFD is referenced and place any relevant / critical VFD parameters in there so that the next guy knows what he needs to do if the VFD blows up and he has to replace it).