Pushy Grabber

June 2017

Meet Pushy Grabber, my new 30# bot designed for open stage sumo competition at Robot Battles events.  The weapon system is basically a pair of counter rotating cylinders that grab and lift the opponent by the sides.   This method proved very successful in my Huggy Bear mini-sumo bot.  The rollers are controlled with my left joystick and can go forward, reverse and left or right.  They are on hinges so they work when the bot is inverted.  The bots "brain" uses an accelerometer to sense when the bot has flipped over so it can automatically reverse the controls.  

Front view of Pushy Grabber

 

 Below is the frame.  The rear panel is 3/8 inch 6061-T6 aluminum.  The other pieces are .090" 7075 aluminum.  You are probably wondering how I bent 7075 ninety degrees without cracking it.  Well, there were a few steps including getting a new furnace to handle the long pieces.  This is the process:

I used 7075-0 stock which is annealed and therefor bendable.  During the CNC cutting operation the bend lines were scored with a 60 degree V cutter .032" deep.  That made it easy to accurately make the bends.   Unfortunately 7075-0 is very soft and will clog end mills.  I had to use lots of mist coolant and take many shallow passes to avoid cutter clog.  

Next I had to harden the 7075.    After the bends were done I placed the pieces in the furnace at 925 degrees F for 20 minutes then plunged in cold water immediately.  After the abrupt cooling they were placed in a toaster oven at 250 degrees for 24 hours.  This is like Viagra for aluminum. What was soft and supple becomes stiff and springy. 

 

Added drive motors, wheels, chain drive and weapons.    I used #25 roller chain.  Drive sprocket has 16 teeth, the wheel sprocket has 30 teeth for a ratio of 1.875:1 .  The gearbox is 26:1 from Banebots .   This spins the 6" wheels at almost 400 rpm max.   Full speed is a little under 10 feet/second.

Banana for scale.

 

Chain guards installed.  Made from .063 6061-T6 aluminum cut with a band saw.  Paper templates were glued on to mark cuts and holes.

 

This is one of the brushless out-runner drive motors  bolted to a Banebots 26:1 gearbox.  Tiny but powerful.

 

 Another view.

 

Here is the weapon drive unit.  It's a 1000KV brushless out-runner geared down 81:1 to drive the roller at about 200 RPM max.   The large homemade UHMW bearings handle the side load  so the gearbox doesn't have to.   They are a little soft so I may need to replace them with Delrin .

 

 

The battery pack.    Six A123 cells (26650 size) make 20 volts at no load.  The top panel has a charging port on the right end and a homemade power switch on the left.  Balancing connections are also on the right.

I used battery holders from AA portable Power as a base for this assembly.  

 

 

Here's a closeup of the power switch. It's two crossed pieces of aluminum separated by polycarbonate.  The lower piece is threaded for a 10-32 brass screw.  The upper piece is drilled .275" for plenty of clearance.  To complete circuit, tighten screw.  Simple.

 

Power switch bottom view.

 

The 2.5 inch diameter weapon rotor tube is  wrapped with material from a Home Depot floor mat.  To help keep it from unwinding I wrapped 3 turns of copper wire around it.  Crude but effective.   Glue alone is not enough.  It's does not adhere to the rubber very well.

 

 Here is the custom made controller.  It can control 4 servo PWM devices and has 2 analog inputs for sensors.  The processor is a Teensy 3.2.  I used an XBee for the radio and put in a gyro and accelerometer.

Not shown are two Afro 30 amp ESCs for the weapon rotors and two 50 amp Ztw Spider ESCs for the drive motors.  They are flashed with SimonK firmware. Each ESC is fused to prevent embarrassing incidents of smoke and fire.

 

Here she is, pretty much complete except for the polycarbonate covers.

 

 

Ready to push and grab because when you're a star you can do anything.

 

 

Here's one fight from Robot Battles 63 at MomoCon 2017.  Fight starts at 0:30 .  We won this one but in general Pushy Grabber has problems with opponents equipped with lifters (There were 3 ).   We did not make it to the final.  There were some minor programming issues with the onboard controller and I was still learning to drive the thing.

Click for more videos from Robot Battles 63 at MomoCon.

 

 

 

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