Two limit switches, one button

[nickb705]

After talking to my mentor, I had miss understood the intent of the limit switch, we DO want to want to have the motor run as long as the button is held, or the limit switch is hit, whichever comes first. I have figured out how to run the motor (up with button 1 and down with button 2) I could just use some help figuring out how to incorporate the two limit switches to turn the motor off. Any example would be highly appreciated!

[cgmv123]

Originally Posted by nickb705

After talking to my mentor, I had miss understood the intent of the limit switch, we DO want to want to have the motor run as long as the button is held, or the limit switch is hit, whichever comes first. I have figured out how to run the motor (up with button 1 and down with button 2) I could just use some help figuring out how to incorporate the two limit switches to turn the motor off. Any example would be highly appreciated!

There are 6 possible states for a button and 2 limit switches that can't be true at the same time. What do you want the motor to do for each of them?

• Button not pressed, Switch 1 false, Switch 2 false
• Button pressed, Switch 1 false, Switch 2 false
• Button not pressed, Switch 1 true, Switch 2 false
• Button pressed, Switch 1 true, Switch 2 false
• Button not pressed, Switch 1 false, Switch 2 true
• Button pressed, Switch 1 false, Switch 2 true

[nickb705]

To tell you the truth, I'm not sure. We want the limit switches as a safety to stop the motor from from hitting the sprockets. I'm not sure what needs to be true and what needs to be false. I hope that helps?

[Mark McLeod]

I suggest that when you test this you do it at some minimal power, e.g., .4, and hold each limit switch down at the start to verify that the switch matches the button direction.

The limit switch can be wired either Normal-Closed or Normally-Open and the boolean generated might be reversed from what you expect, i.e., it may be True when you expected False.

[Alan Anderson]

Originally Posted by nickb705

After talking to my mentor, I had miss understood the intent of the limit switch, we DO want to want to have the motor run as long as the button is held, or the limit switch is hit, whichever comes first. I have figured out how to run the motor (up with button 1 and down with button 2) I could just use some help figuring out how to incorporate the two limit switches to turn the motor off. Any example would be highly appreciated!

Here's a strong hint: use a boolean AND function to combine the "up" button value with the "top" limit switch value, so that the result is true only when the button is pressed and the limit switch is not activated. Do the same thing with the "down" button and "bottom" switch.

Now you have two boolean values, one saying "move up" and the other saying "move down". If you can't figure out how to use those values to control your lift motor, ask for more help.

[tcjinaz]

Originally Posted by Alan Anderson

Here's a strong hint: use a boolean AND function to combine the "up" button value with the "top" limit switch value, so that the result is true only when the button is pressed and the limit switch is not activated. Do the same thing with the "down" button and "bottom" switch.

Now you have two boolean values, one saying "move up" and the other saying "move down". If you can't figure out how to use those values to control your lift motor, ask for more help.

So when the mechanical system overshoots due to lag in the control system, and the limit switch releases, up becomes a valid command again?

We are scraping for documentation on the new Switch VI's. Are these implemented using the FPGA hardware, so it is sampling quick enough that any mostly full range edge is caught? Does the counter reset on system reset? Can we reset the counter? Any debouncing built in?

Inquiring minds,
Tim

[Greg McKaskle]

The limit switch is a simple wrapper over the DIO functions that were already available. They were added primarily for simulation. In order for a simulated robot to be able to identify how a digital line should toggle when something moves, it is useful to identify the switch, tell the physics engine where it is, how it is wired, etc. This is also true of the potentiometer. It is a simple wrapper over AI.

So the FPGA is doing the low level read and filter of the switch, but it is the RT side that is responsible for modifying a motor or other actuator in response. There is no new way to "tie things together" in the FPGA.

Greg McKaskle

[tcjinaz]

Originally Posted by Greg McKaskle

The limit switch is a simple wrapper over the DIO functions that were already available. They were added primarily for simulation. In order for a simulated robot to be able to identify how a digital line should toggle when something moves, it is useful to identify the switch, tell the physics engine where it is, how it is wired, etc. This is also true of the potentiometer. It is a simple wrapper over AI.

So the FPGA is doing the low level read and filter of the switch, but it is the RT side that is responsible for modifying a motor or other actuator in response. There is no new way to "tie things together" in the FPGA.

Greg McKaskle

Thanks. More "I needed to know that"

[tcjinaz]

Originally Posted by tcjinaz

Thanks. More "I needed to know that"

So, given "Normally Open" port true on open (and wiring GND to Signal on the DIO through the switch) and a 2.0V ish to 0.8 ish transition holding below 0.8V for more than 20nS, we will see WPI_SwitchGetLimitValueReturn true, and WPI_SwitchGetCount will increment.

When does WPI_SwitchGetLimitValueReturn reset? On a transition from -0.8V to 2.0V longer than 20nS?

The counter resets to zero on the call to Begin? And between autonomous and teleop?

I keep getting challenged (good questions, mostly) on stuff like this, so the ducks need to line up

TIA,

Tim

[Alan Anderson]

Originally Posted by tcjinaz

So when the mechanical system overshoots due to lag in the control system, and the limit switch releases, up becomes a valid command again?

If the hardware is built so that the limit switch is not active when the mechanism is at the end of its travel, then what you describe is possible. I wouldn't suggest designing it that way.

[tcjinaz]

Originally Posted by Alan Anderson

If the hardware is built so that the limit switch is not active when the mechanism is at the end of its travel, then what you describe is possible. I wouldn't suggest designing it that way.

The system in question is pretty quick, and I'm not sure I'll get the nice long lever necessary. I'm thinking some latching & resetting by other actions may be in order. I was looking for a state machine this year...

Thank you,
Tim

[mshafer1]

There have been some good comments posted here that I would recommend reading them - especially the one by cgmv123.

For those who are looking for more of a tutorial of one way of accomplishing this task - currently up while button 1 and not top limit switch else down while button 2 and not bottom limit switch -
Please refer to this tutorial that I have made, Click here for tutorial It is under limit switch tutorial, although the state machine tutorial is a similar scenario.

[nickb705]

Thanks so much to everyone who gave their impute! They work like a charm!