Intermittent response from motor?

Could someone please explain the behavior we are seeing in this video?

With a motor (both 3-wire and 2-wire tried) on the claw, it refuses to open fully without a pause after closing. After 1 second it will open partially. But 2-3 seconds are needed for it to be able to open fully.

This is the RF Transmitter and Receiver, using the Line Splitter to power 5 motors. The claw is on the digital Channel 6.

The batteries are Eneloop rechargeable, providing 7.6 volts measured this morning. The batteries are plenty strong, as the robot can drive around swiftly with no problems. But this behavior also began before we switched to Eneloop, on regular alkaline batteries.

What is puzzling is we’ve only begun to notice this behavior in recent weeks. Maybe it’s due to the fact that our team is operating the claw more rapidly, but since this is completely reproducible now, I don’t know if this is expected behavior or if something is wrong.

Hello losjackal,

This is a very strange issue.
In order to isolate the problem further, please try a different motor in the claw.

Let me know your results.

-Eli

I can also mention that the problem manifested itself on both of our robots (Junior Varsity and Varsity teams), initially 3-wire motors on VEX Explorer red claws. We have since purchased a VEX green claw and a 2-wire motor as well (both shown in the video), and the problem remained.

But I will swap out the motor again tonight and test. I anticipate the same behavior. If there are other tests you’d like me to try as well, let me know. (So we have 3 RF transmitters, 3 RF receivers, several crystal pairs, a VEX Signal Splitter on one robot and the VEX PIC micro controller on the other…and mostly 3-wire motors.)

I did test the signal transmission with a freestanding motor not connected to the claw, it spins forward and backward continuously, on demand, as expected. So I believe the problem has to do with the motor stalling at the maximum point of claw closure. Furthermore, even though the video doesn’t illustrate it well, it seems that the longer we continue to apply the “close” signal during motor stall (to have a solid grip while the robot goes over bumps), the longer we must wait for it to be able to fully open again…up to 3 seconds.

I did some more testing. When a motor stalls due to resistance, can it get “tired”? That would seem to be the case with the original video above. So in testing some other combinations, I focused a bit on that.

Here is a video with one of our older 3-wire motors operating the same claw as above, without a clutch (to remove one variable). You can see it can barely open the claw once, but it gets a little stronger after a pause. (I was also a bit abusive with it intentionally, to show that with enough torque it would still open.)

Here is a different claw with a 3-wire motor, again no clutch. It behaves like a champ for a while, and then gets “tired” especially after holding the close signal for a tight grip.

[http://www.youtube.com/watch?v=bSRaEjx-omU

I understand these should not be operated without a clutch, but the original video was a 2-wire motor which doesn’t require clutches anymore.

So if we were to describe this common behavior…is there an issue with motors not having as much strength after meeting resistance over and over? Where a pause allows it to somewhat reset? Here I am using a freshly recharged set of Eneloops, powering a Signal Splitter.

I mentioned we also have a VEX PIC micro controller. Should that exhibit different behavior, in terms of available torque? Because there is continuous resistance from the spring too, which also contributes to “tiring” the motor.](http://www.youtube.com/watch?v=bSRaEjx-omU)

losjackal,

Thanks for your detailed response.
Based on your comments & videos, I can see that you are overheating the motor and engaging the thermal breaker inside the motor. When you press and hold the button on your transmitter you are basically stalling the motor and generating heat, after enough heat is generated the motor will shut off and will only engage after it cools off.

The VEX Claw has enough grip to hold a game object without the need to hold the button in your transmitter.

Regards.

-Eli

Thanks, Eli. We are using the robots for the Science Olympiad Robo-cross event, which has some different requirements. For instance, being able to carry a AA battery in the claw over a small bump. Most of the time, that wouldn’t survive without the additional force of the claw close signal.

Could you advise which motor (3-wire or 2-wire) is better suited to this purpose? Perhaps the thermal breakers would behave a little differently. Right now the attached 2-wire is doing fairly well…as you see in the original video we only need to give it a couple of second rest for it to re-open. Also, does the Signal Splitter vs. VEX PIC micro controller make a difference?

I also experimented a bit using P. Mix on the transmitter to only apply partial signal to the appropriate channel on the Signal Splitter…above 20%, it didn’t seem to make much of a difference than sending a 100%.

losjackal,

There are a few modifications that you can make to your robot to alleviate the issue, such as the following:

  • Adding anti-slip mat to the claw of the robot for extra grip

  • Adding a rubber band to the claw for extra pressure on the game object being held.

  • Upgrading your Signal Splitter to one of our programmable microcontrollers, such as the PIC.
    The PIC microcontroller will allow you to write & download a custom program which can alleviate your problem. The PIC is also compatible with the transmitter you currently have.

  • All of our motors feature a thermal breaker; therefore, using a different VEX motor will not alleviate the problem.


Good luck in your Science Olympiad tournament.
Regards.

-Eli

Thank you for those suggestions. One last question if I may: what kind of PIC logic program are you imagining would alleviate my problem? We have a PIC, but I haven’t ventured into Vex programming yet…but I just don’t know what you’re getting at, needing to hold the claw closed without overheating the motor.

losjackal,

The PIC can be programmed with any of the following programming packages:
easyC V2 by Intelitek
ROBOTC 3.62 by CMU

Feel free to download a free trial of each and experiment with the software. You should be able to write a program that allows you to hold the object at 25%-50% power, rather than 100%.
This will delay the generation of heat that engages the thermal breaker.

If you decide to purchase the software, please note that you will need a Hardware Programming Cable and I also recommend running the PIC on a VEX battery pack (rather than AAs).

Please contact our sales team for further details on placing your order.
Regards.

-Eli

Much obliged. I looked into RobotC already and it looks straightforward.

I may just go with your rubber band suggestion, as long as the motor appears strong enough to reliably open!

Last technical question I promise: the Claw motor is in digital Channel 6. Nothing is in Channel 5 in the Signal Splitter. If I use the Transmitter P. Mix on Channel 5 to also send 25-50% signal to Channel 6, would this functionally be the same thing as programming the PIC? (I’ve tried it out and it does seem to work that way, so I’m asking if it’s identical to the result that programming would have.)

Hi losjackal,

Thanks for your question.
No, the P. Mix function allows you to program one axis “mix” into your controls; therefore, moving a single joystick axis can control more than one output channel.

See page 18 on this guide for further details.
Regards.

-Eli