Using brushed Motors for gripper or hammer weapons

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MySolderIsOlder
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Using brushed Motors for gripper or hammer weapons

Post by MySolderIsOlder » Wed Jan 09, 2019 4:05 pm

Quick question for anyone who's used N20-type motors either for percussive or for gripper/crusher weapons (i.e. where the motor's freedom of movement would be constrained to less than 360 degrees) ... Did you need some kind of slip-clutch, or a clever switch to cut the drive when it reaches the limit of movement - or is it safe to let the motor just strain against the immovable force?
Stuart (Anthony's dad) Our ants;
"Flippin' Ugly" (flipper),
"Other way up" (invertible pusher),
"Ffythiana" (front flipper),
"Spindoza" (basic shover)

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MarkR
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Re: Using brushed Motors for gripper or hammer weapons

Post by MarkR » Wed Jan 09, 2019 4:15 pm

I used one for a flipper weapon, and DerekDudge used one for a hammer weapon (Manticore) you can see in the video from the AWS55 event (It looked great and got a lot of applause but not terribly effective)

I think it depends on the gearbox ratio, but the 100:1 that I used are safe to stall for a while at least at ~ 12V.

Higher gear ratios might strip the teeth off the gearbox if stalled especially at higher volts (>6)

I also over-volted mine on Betsie to 17 volts now. It's probably not a good idea to stall it for too long at that voltage, it would run at >1A and make a lot of heat.
Robot: Betsie - RaspberryPi controlled flipper bot with gyro stablisation - too clever for her own good?

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GeorgeR
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Re: Using brushed Motors for gripper or hammer weapons

Post by GeorgeR » Thu Jan 10, 2019 12:43 pm

I use a pair on my axebot, and haven't had any problems.

They're setup for manual control on the left stick (push left and the motors go forward firing the axe, push right and they go backwards to retract, with no power being applied when the stick is centred). While the motors are stalled for a fraction of a second at the end of their travel, I don't hold the power on for longer than needed, and they don't even get warm, let alone burn out. This is with the Pololu high power ones that pull about 2A stalled.

Not sure if that would still work with a crusher/grabber, as they would need to be on for a longer period of time, but they're definitely OK for an axe.

Here's my axebot if you were interested...
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Team Zero
Zero - rambot - - Axiom - axebot - - Valkyrie - drum spinner
Vampire - horizontal spinner - - RBMK - quad spinner gyro walker

MySolderIsOlder
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Re: Using brushed Motors for gripper or hammer weapons

Post by MySolderIsOlder » Fri Jan 11, 2019 10:04 am

Thank you both for those helpful thoughts.
Mark - good point about the gearbox ratio. I had been thinking about trying it with some 1:298 motors I have sitting around but that might not be sensible... Though since I don't have any other use for them I may just destruction test one to see how long it can last stalled at 6v :evil:
If it can stall for 10 seconds without burning up or stripping the gears then should be fine as a gripper. Otherwise may try will lower ratios.

George - as with many of your creations, that axe-bot is a thing of beauty. On the purely aesthetic level always prefer the non-3d printed antweights, regardless of how effective they are. That arrangement of the motors, using the aluminium hubs to grip a weapon is pretty much what I had in mind and using the self-centering axis on the left stick make a lot of sense.

Having got past the frantic rush of preparing 4 bots plus an arena/case for son's Xmas present, I'm relishing being able to slow down and experiment with ideas a bit more and an axe/gripper bot should stir things up a bit, as well as presenting new challenges. Current very rough plan is an overhead weapon with a gripper for frontal attack and sharpened pick for reverse (interchangeable weapons - for easy repairs and so I can switch between safer or more destructive versions depending on context!)
Stuart (Anthony's dad) Our ants;
"Flippin' Ugly" (flipper),
"Other way up" (invertible pusher),
"Ffythiana" (front flipper),
"Spindoza" (basic shover)

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TeamScrewUp
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Re: Using brushed Motors for gripper or hammer weapons

Post by TeamScrewUp » Sat Jan 12, 2019 7:10 pm

MarkR wrote:
Wed Jan 09, 2019 4:15 pm
DerekDudge used one for a hammer weapon (Manticore) you can see in the video from the AWS55 event (It looked great and got a lot of applause but not terribly effective)
The hammer was very good at helping it get off of wedges.
Daaa
Manta ray (2WD wedge bot)
Poptart tories (Cluster bot)
Krave-lit (Dustpan)
The violarion of our most basic human rights (Horizontal Spinner)
Nommy nommington (Joke)

MySolderIsOlder
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Re: Using brushed Motors for gripper or hammer weapons

Post by MySolderIsOlder » Mon Jan 14, 2019 10:46 am

FWIW, did a very quick test yesterday with one of the 1:298 motors - popped on a large wheel, clamped the motor securely in a vice, wired it directly to a 6v NiMH battery and deliberately stalled it by stopping the wheel turning. To my surprise it had no ill effects - motor didn't get hot (neither did battery) and no gears harmed. When I get a bit more time I'll try it again but with the multimeter connected in series, to measure the stalled current draw - with two motors driving the jaws, don't want the batteries running down too fast.

Either way, a gripper is now well and truly on the drawing board. Working title "Predator-X" (in honour of the slightly larger jawed Pliosaurus funkei)
Stuart (Anthony's dad) Our ants;
"Flippin' Ugly" (flipper),
"Other way up" (invertible pusher),
"Ffythiana" (front flipper),
"Spindoza" (basic shover)

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MarkR
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Re: Using brushed Motors for gripper or hammer weapons

Post by MarkR » Mon Jan 14, 2019 6:32 pm

I don't think you'll run down batteries much with a single N20. In my experiments, the stall current running on a 2S pack is about 800mA (at least on the cheapy motors I have, the polulu high power ones probably much more).

As I run 300mAh packs, 800mA will take longer than a 3 minute fight to have an impact. Even the 180mAh packs could run it stalled for the whole three minutes without problems.

The way to get more power is either use a beefyer motor (like the high power ones), a voltage boost circuit, or run it off a 3S pack (maybe 4S? ). I have Betsie's flipper running on 17v through a voltage boost right now and it's quite good, although I discovered a slight problem with my custom PCB traces being too thin (hmm, nice smell of burning pcb trace / solder mask)
Robot: Betsie - RaspberryPi controlled flipper bot with gyro stablisation - too clever for her own good?

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