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Motor stop switches

Posted by Zeeri 
Motor stop switches
August 09, 2016 01:17PM
Has anyone tried putting in some switches to stop the motors when the reach the end of the screws?

Currently my Handibot gets to the end of the screw and grinds in order to find its position when I put in a new bit. While I am not sure about how this affects wear on the Handibot, I do know it causes me problems if I don't have the bot clamped to the work piece. When it isn't clamped, the slamming into the end of the screw causes the bot to jump on the work and everything gets miss-aligned.

The lack of switches also affects the cutting if my job goes outside the bounds of the Handibot. In this case, the Handibot doesn't detect that it reached the end of the screw, and it assumes it continued, so when it moves back it doesn't account for the distance it did not actually travel. The result of course is the rest of the job is miss-aligned on what ever axis went out of bounds.

I suppose what is needed is 6 switches and six input pins on the hardware, some software changes to check the appropriate pin after each increment of movement, and placement of the switches.

Alternatively we could just use 3 switches and pins, and have a procedure for detecting the position of the motor at the start of each job by moving the motor until the switches are triggered. Thus we would need to track all movement from there on to know our exact position, and we would need to use the position to detect a movement command that takes us outside the bounds of the hardware and prevent the movement and set an offset to prevent the return movement. We would also need to prevent any cutting while out of bounds.

Another alternative is to not use any switches and move the bot into a corner (grinding of course), then track all movement from there to achieve the same affect as with the 3 switch solution.
Re: Motor stop switches
August 09, 2016 05:07PM

The motors that drive the handibot aren't geared--so the grinding that you hear is just the rotor jumping around as the magnetic field of the stator rotates. This doesn't have a negative effect on the mechanical or electrical life of the tool. Though I will agree that it's not always fun to listen to repeatedly.

The macro that controls this routine is marcro #3--the comments are well written in that macro so you should be able to track down the lines that control the speed of the approach during zeroing. We set them at a level that seemed to work well in the shop here--but you could slow them down to avoid displacing the tool when it is homing.

Under machine configuration there are settings for "machine envelope" I'm checking to see if we might add an optional setting that will pause the tool and give an error if you try to move past the machine envelope. That would be helpful for preserving your home position if your toolpath happens to run out of bounds.

The guts of the tool are already set up to use limit switches. There's one embedded in the frame that will be used to detect opening of the hood as a safety feature. The switches we use are available from digikey: [www.digikey.com] You could wire them into the same header that sees inputs from the stop and go buttons, the hood interlock and the z-zero plate. It would require some thoughtful work with your homing routine macro but could definitely be done!

Re: Motor stop switches
August 09, 2016 06:43PM
Hi Zeeri,

Let me add just a tad bit more to Brian's comments ...

The earlier version of Handibot had the switches. We found that in practice, two different types were too easily damaged in the field given the portablility and variety of uses of the tool. Surprisingly, the technique of banging the stops is actually quite accurate, so precision is not an issue.

More generally, we expect to shortly have software limit checking implemented in the software. Limit checking was a feature of the previous ShopBot software, and we found that this was the best approach to keeping the tool within limits. The software can identify moves that would go out of limits at the time a Job File is being loaded. This gives you a lot of options for reconfiguring, repositioning, and restarting your Job. With limit switches on the other hand, you do not encounter the switch until after a Job has been partially cut, just before you would go out of limits. It is virtually impossible to get the part repositioned in a way that you can continue cutting. So ... soon we will have the limit set-up that provides a more elegant handling of limits.

But, as Brian noted, switches are supported for proximity and limits so they can be added. In that case, we prefer to put the sensor on the carriage and then have a target at each end of travel as this reduces the number of switches.

Ted Hall, Handibot/ShopBot
Re: Motor stop switches
August 09, 2016 11:15PM
Thanks Ted & Brian.

Since there are no gears, I won't worry about the grinding. I will fiddle with the approach speed in macro 3 as suggested and see if I can slow it down some. Hopefully I won't need to clamp it down quite so much then.

I did not notice where it tells me that the job goes outside the bounds of machine. I will have to re-examine the user interface. Not surprising since I haven't spent a ton of time with it yet. I am still trying to figure out how to align everything and swap bits without worrying about miss-alignment.
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