Controlling Arm travel - Limit Switch or Mechanical Stop?

Posted by Peter Zobel at 1/31/2001 3:06 PM EST

Engineer on team #557, Alpha C.R.T., from Detroit Cooley and Ford Motor Company.

Rookie team requesting advice…
What is the best way to limit travel of an arm - limit switch? mechanical stop? Both? Neither?

Posted by Joe Ross at 1/31/2001 3:35 PM EST

Engineer on team #330, Beach Bot, from Hope Chapel Academy and NASA/JPL , J&F Machine, and Raytheon.

In Reply to: Controlling Arm travel - Limit Switch or Mechanical Stop?
Posted by Peter Zobel on 1/31/2001 3:06 PM EST:

: What is the best way to limit travel of an arm - limit switch? mechanical stop? Both? Neither?

We generally use a limit switch (or magnetically activated reed switch) along with either a mechanical stop or some way for the drivers to easily tell when to stop the arm.

Posted by control guy at 1/31/2001 5:34 PM EST

Student on team #250 from HVCC.

In Reply to: Re: Controlling Arm travel - Limit Switch or Mechanical Stop?
Posted by Joe Ross on 1/31/2001 3:35 PM EST:

Last year, we had a device on our robot that moved so fast that it would blow right past the limit switches and never turn off. We put good ol mechanical stops in and no more problem.

So my advice would be, use both. Mechanical stop and limit switch or pot. Only thing with pot is, make sure it doesn’t slip.

Posted by Joe Johnson at 1/31/2001 6:54 PM EST

Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

In Reply to: Mechanical stop good
Posted by control guy on 1/31/2001 5:34 PM EST:

Good, solid, mechanical stops are a must. BUT Limits
switches and pots will save you a lot of headaches.

Basically, running full throttle into a hard stop puts
extremely high stresses on the internal mechanism of
you motors as well as on your robot itself.

Even if your robot can survive, many of the motors
cannot. In particular, the Bosch motor can only take a
dozen or so such hard stops before the output of the
motor is severely limit. The Fisher Price
transmissions are not too happy with hard stops either.

One more reason to put in a limit switch or a pot is
that the motors stall against a hard stop. In the heat
of the battle, an excited driver may stall several
motors against their stops. I know of several teams
that popped their 60 amp fuse because an arm driver
stalled the arm motor against the stop during a pushing
match – ouch!

As to the high speed motions blowing by the electical
limits, one strategy
to deal with this is to have the switch make early
enough to stop the motor then go to a “low power” mode
(via programming) that will allow the motors to be
driven to the end of travel but won’t draw too much
current. You may want to have even this low power mode
time out – i.e. have it go to zero as time passes.

Good luck.

Joe J.

Posted by Matt Berube at 1/31/2001 3:46 PM EST

Engineer on team #49, Delphi Knights, from Buena Vista High School and Delphi Automotive.

In Reply to: Controlling Arm travel - Limit Switch or Mechanical Stop?
Posted by Peter Zobel on 1/31/2001 3:06 PM EST:

: Rookie team requesting advice…
: What is the best way to limit travel of an arm - limit switch? mechanical stop? Both? Neither?

The best way to control the arm position, in my opinion, is to put a potentiometer on the axle and write your code so that the arm stops when the pot gets to a certain value. With this method you can have many “pre-set” arm positions, at least until you run out of push-buttons.

Matt Berube
T49
“that OTHER Delphi team”

Posted by Andy Baker at 2/1/2001 7:11 AM EST

Engineer on team #45, TechnoKats, from Kokomo High School and Delphi Automotive Systems.

In Reply to: Best way, (maybe?)
Posted by Matt Berube on 1/31/2001 3:46 PM EST:

I’ll second Matt’s advice here. Potentiometers are harder to set up than limit switches, but you can do more with them. We’ve used them exclusively over the past three years… sometimes with no hard stops (not always).

The only thing bad about potentiometer sensing is that sometimes it is out of “alignment”, and you gotta tweak its position in order to make sure its limits are the limits desired.

Andy B.

Posted by Chris Hibner at 2/2/2001 7:28 AM EST

Coach on team #308, Walled Lake Monster, from Walled Lake Schools and TRW Automotive Electronics.

In Reply to: Pots seconded
Posted by Andy Baker on 2/1/2001 7:11 AM EST:

You can easily tweak your pots at a small cost of a little resolution and one subtraction operation per pot. Set your pots so that they only use about 20-235 A/D counts from limit to limit in normal operation (20-235 is just an example, use your judgement). You can then put a button on your controller to sample each pot (you should actually take at least 5 samples and average them) and write that value to memory to serve as an offset. Therefore, you don’t have to mechanically adjust the pots - just put the mechanisms in a reference position and push a button and they all sync up. It takes a little work in software but is well worth it if a pot drifts a little between rounds in the finals.

-Chris

Posted by Ken Leung at 1/31/2001 10:59 PM EST

Student on team #192, Gunn Robotics Team, from Henry M. Gunn Senior High School.

In Reply to: Controlling Arm travel - Limit Switch or Mechanical Stop?
Posted by Peter Zobel on 1/31/2001 3:06 PM EST:

: Rookie team requesting advice…
: What is the best way to limit travel of an arm - limit switch? Mechanical stop? Both? Neither?

Last year, we had a giant arm (yes… again…), and we use it to reach up to the hanging bar and pull ourselves up. Since we used the Van door motors to power that arm, along with the fact that Van door motors back drive under large amount of force, we had to come up with a mechanical lock. And basically, there is this spring-loaded flap that will hook onto the body structure once the body is raised close enough.

So why did I tell you this? Well, it’s because while designing this arm, a concern about accidents was raised up. What if the arm moves too fast and locks into the body during a match? The driver might accidentally push the arm too close (well I was the driver and that never happened :slight_smile: ), or some other robot might accidentally (or intentionally) lock our arm in. And we concluded that due to the nature of “accidents”, a limit switch might not be quick enough to prevent the locking, or the outside force might be too great for the Van door motors to push against.

So we ended up making a little door on the structure to prevent the flap from sticking into the structure, and everyone was happy. The little door works perfectly and the arm never ever lock in when we didn’t want it to. We probably should’ve attach a limit switch so our arm won’t keep crashing into the door all the time, but I had enough practice to keep the arm just a little bit away from the body.

So I guess another important thing about this issue is that you want your drivers be trained to not destroy your robot, and good enough to take care of it.

Posted by Jessica Boucher at 2/1/2001 6:37 AM EST

Student on team #237, Sie-H2O-Bots, from Watertown High School and Eastern Awning Systems & The Siemon Company.

In Reply to: our big long arm…
Posted by Ken Leung on 1/31/2001 10:59 PM EST:

Reminds me of when we got jealous after the Philly Regional in 99 when we realised how important pulling the puck was. So, we created a double hook that stuck out like a tail for nationals…it now keeps our door to our white board window closed after we found out that not only would it hook to the puck, but to everything else as well…

-Jessica B, #237

: So why did I tell you this? Well, it’s because while designing this arm, a concern about accidents was raised up. What if the arm moves too fast and locks into the body during a match? The driver might accidentally push the arm too close (well I was the driver and that never happened :slight_smile: ), or some other robot might accidentally (or intentionally) lock our arm in. And we concluded that due to the nature of “accidents”, a limit switch might not be quick enough to prevent the locking, or the outside force might be too great for the Van door motors to push against.