Transcript mort11.org

Drives & End Effectors
By Akash Rastogi, Nick Brennan, Sam Mills
& Ethan Miller
FRC 11 - Mt. Olive Robotics Team
Objectives
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Discuss pros and cons of various
drivetrain types
Discuss details of skid steer tank
drives
Learn what an end effector is
Discuss various types of end effectors
used in FRC
Drivetrain Styles
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West Coast Drive - cantilevered drive developed by team 60
Plate - parallel plates support axles on both ends (dead or live axle)
Mecanum - 4 independently driven wheels allow for simple omnidirectional motion
Swerve - complex drive pods allow for superior omni-directional motion
Jump - dual purpose drive which uses both mecanum and high traction
wheels
Butterfly - dual purpose drive which uses both omni and high traction
wheels
West Coast Drive
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Generally constructed with 1/16" or 1/8" wall tubing (1" x 2"), fastened
together through either welds or gussets.
Wheels are cantilevered, so a WCD may require a bearing block for proper
axial support and to tension chain. Bearing blocks can now be purchased
off the shelf. These used to be the limiting factor for most teams when
deciding to make a WCD or not.
WCDs can be relatively simple, produce a low part count, and can be very
light weight, if designed correctly.
WCDs are direct driven, which increases the efficiency of the drive as well
as reliability.
The wheels and gearboxes are easily accessible for required maintenance.
Can be constructed modularly so an entire side can be removed for easy
repairs (see Plate Drives)
West Coast Drive
Continued...
Plate Drives
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Frames are generally made of
parallel 1/8" to 1/4" thick plates of
aluminum and 1/2" tapped or
hollow standoffs, fastened
together with bolts
Standard FRC weight holes
leave extra material around
bearings and the supporting
struts/standoffs
Wheels are supported on both
sides, so there is no need for a
bearing block in the drive.
Shims for bearings can be added
if using thinner plate.
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Plate drives can have a higher
part count,but can still be very
light weight if designed correctly.
Each drive side should be
entirely modular, and should
have the ability be removed
easily for maintenance.
Plate drives are generally direct
driven for improved efficiency
and reliability.
Note: Plates can be substituted for
aluminum tubing
Plate Drives Continued...
Skid Steer: 4, 6, 8 Wheel
Drives
Skid steer drives or "tank drives" (such as parallel plate drives, WCD, etc..) are
made with traction wheels with the center wheel(s) "dropped" so that the
drive turns on the central set of wheels.
4 wheel drives should only be used in a wide frame orientation with a wider
stance than the length of the wheelbase or with omni wheels on one end.
6 wheel drives should use a center "drop" or some may remove the drop and
use omni wheels in the corners. "Rocks" on center wheel (creates two 4
wheel drives).
8 wheel drives use the two center sets dropped below the outer sets in order to
turn properly. Allows for most stable tank drive.
All of these drives must be able to overcome the friction and torque required to
turn in place in order to be successful. These are the most common and,
arguably, the "best" type of drive in FRC. (Just ask the Poofs!)
Mecanum Drive
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The slant of the diagonal wheels must
form an X when viewed from above.
Four gearboxes, one for each wheel, can
cause mecanum drives to have a higher
weight than other drives.
In order to have enough torque to move
sideways instantaneously, a 120 pound
robot must be geared near 8fps
Due to the rollers, it can be very difficult
for a mecanum drive to go up a slope or
withstand being pushed.
Sideways movement can be useful for
getting around defense, but mecanum
drives are also rarely well driven or used
Swerve Drives
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Swerve drive is a drive whose
wheels can change their
orientation within the frame,
wheels can either do this
independently or dependently of
each other.
Allows for omni-directional
motion without sacrificing speed
or pushing power, while
maintaining a constant robot
orientation.
When done well, swerve drive
can be extremely maneuverable
and effective, as seen in Team
1717's 2012 robot.
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Can be extremely complex, both
in terms of the mechanical
design of the drive, and can be
difficult to program as well.
Highly resource intensive
A driver needs many hours of
practice in order to become
skilled at driving a swerve drive
Swerve Drives Continued...
Jump Drive (Octocanum)
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This type of drive uses a pneumatic piston to switch between a mecanum wheel
and a traction wheel.
Another method for achieving omnidirectional motion while not sacrificing pushing
power.
Complicated drive system to design well and requires a large amount of resources.
The traction wheels must be at the pivot point in order to avoid serious issues with
torsion in the module when turning.
Since weight on the mecanum wheels is supported by pistons, no suspension is
needed like on a regular mecanum drive.
Jump Drive Continued...
Butterfly Drive
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Similar setup to a jump drive, except it uses omni wheels instead of mecanum
wheels.
By switching to omni wheels after a period of forward motion, the driver is able to
essentially drift & create smooth turns.
Allows the drive to "float," hence the name butterfly.
Wheel Examples
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Colsons
o Take several competitions to see any significant wear.
o No replacement tread required.
o Very robust & affordable
Mecanum
o Allows sideways motion using 45* rollers
Onmi
o Allows for butterfly motion using 90* rollers
Pneumatic
o High traction and low wear wheels.
o Must be properly inflated and retain air to avoid turning issues.
Treaded Wheels
o High traction
o Treading can be easily replaced during competition using screws or
rivets
Dead Axle vs Live Axle
Dead Axle:
Wheel rides on bearings, and a
sprocket/pulley is mounted directly to
the wheel.
Very robust system that can take hits
and frame warping well.
Simple setup and design
Torque is not translated into the axles
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Live Axle:
• The drive shaft is on bearings, so a
sprocket or pulley turns the shaft.
Wheel is attached to the axle via key,
hex.
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It is extremely easy to swap out
wheels. You do not have to deal with
tensioning chain or belts.
Other Drivetrain Criteria
Here are a few items to consider when designing or selecting your drivetrain:
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Speed vs Pushing Power
o Design for your strategy, decide which is more preferable.
Wheel size
o The smaller the better!
Number of Wheels
o generally either 6 or 8, but 4 is also good on a wide bot
Wheel Spacing
o Even spacing on a 6 wheel, uneven spacing on an 8 wheel
Obstacle Clearance
o Use of angles and sloped to get over bumps(2012)
Belting versus chain
4 vs 6 Motor Drive (or other motors)
o 6 cims helps you reach higher speeds, but does not improve speed
significantly at lower speeds.
End Effector
= any component of the robot which manipulates or controls the game piece
and interacts with field elements
TYPES OF END EFFECTORS:
Arms
Telescoping Arms
Elevators
Shooters
Intake Systems
Conveyors
Hoppers and Storage
Hangers
Claws
Dumpers
Bridge Manipulators
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Articulating Arms
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PROS
Simple
Lightweight
Longer reach than elevators
Can be combined with other
manipulators (548-2012)
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CONS
Risks damage when outside of
the bumpers
Can be slightly slower than
elevators for lifting
Arm's position changes center of
gravity
Require additional stability
(linkage arms, gas springs, etc)
May require more drive practice if
there are multiple scoring heights
(2011, 2007)
Articulating Arms
Continued...
4 bar linkage arm
AVOID TOO MANY
LINKAGES/PIVOTS
Telescoping Arms
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PROS
Greater range than basic arm
Can take up less initial space
More reliable than adding
multiple pivots to an arm
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CONS
Relatively heavy
May use more motors
Greater risk to break/malfunction
due to more moving parts
Much more complex to design
effectively
Elevators
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PROS
Fast method of lifting
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Takes up very little space
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Can be combined with other
manipulators. (254 -2011, 17182013)
Contained within bumpers
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CONS
Require high precision to avoid
binding
Can be complex to design and
fabricate
Can be difficult to repair (112008)
Elevators Continued...
Pulley + cable system or chain used
Elevator and Arm Tips
NEVER have too many pivot or
articulation points.
Always have sensor feedback control
using encoders, potentiometers,
limit switches, etc...
Integrating sensors into the design
creates ease of use for the drivers
as well as for autonomous.
Always prototype new systems in the
offseason. You don't want to
discover issues during build.
Using a method of counter-balancing
on an arm or elevator greatly
reduces the stress on motors and
gearboxes and leaves the
mechanism "neutrally buoyant"
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Gas springs
Surgical tubing
Springs
o torsion spring
o constant force spring (254
elevators)
Power systems up and down
See Greg Needel's presentation for
more details
Shooters
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Used to launch game pieces
Flywheels vs Elastics/pneumatics
Flywheel should have a high moment of inertia
PROS
Can be added to other
manipulators
Allows for scoring from a distance
Allows for quicker scoring
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CONS
Must be consistent or
aiming is impossible
Camera tracking for best
results
Intake Systems
USE "ACTIVE" INTAKE METHODS
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Every FRC game piece ever, can be best manipulated by high friction
rollers or wheels.
Recommended materials:
o Rubber tire (11-2011)
o Polycord
Only use as much compression on game
piece as necessary
Make the intake quick and make sure it hangs on!
Conveyors
Use urethane belting or "polycord"
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Should not need strong motors but should have enough power to move
balls or game piece through system without jamming
Size out conveyors based on capacity rules.
Wider conveyors are normally better unless they need to funnel into a single
line shooter (2006)
Hoppers and Storage
Things to consider
Is capacity limited by the Rules?
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What will cause jamming and how to
avoid this
o Unevenly spaced walls (19862013)
o Active unload (1717-2012)
o Spacers between game pieces
(33-2006?) (3539-2013)
o Double Belts (233-2012)
Gravity is NOT the fastest way to
empty a hopper, always rely on
active mechanisms
Hangers
Hanging is a very common task in FRC games.
Many robots latch onto hanging targets and use:
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Winch and pulley systems
Pneumatics or gas springs (0 second hang, stored energy)
PTO - Power Take Off: route all drivetrain power to lifting winch
Ratcheting gearboxes
Main goal - raise up your robot as high as necessary and KEEP it there.
Accomplished through pneumatics, high torque gearboxes, latching
mechanisms, locking mechanisms, ratchet & pawl system
2013 was unique in having the ability to use a passive hanging mechanisms
Dumpers
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PROS
High storage
Allow for the possibility of a large
quantity of game pieces to be
scored at one time
Can have a shooter attached to it
CONS
If robot is not correctly lined up,
could lose points and time
Uses time to fill hopper
Ineffective when the game limits
# of game pieces stored in robot
Bridge Manipulators
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Allow robot to tilt bridge
Best ones can perform while robot is in motion
o Robot does not need to stop to lower bridge
Usually achieved using pistons
Can be small and very simple, yet effective
Sources & Links
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Greg Needel's Manipulators Presentation:
http://www.technoguards.org/system/files/Manipulators.pdf
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Images from Chief Delphi & Picasa:
https://plus.google.com/photos/106946949944311837712/albums/5302862347473440881?banner=pwa