Gripper Designs By Pragnay Choksi

INTRODUCTION Hands of robots referred to as Grippers No single design is ideal for all applications

TYPES OF GRIPPERS 6 different types described here –The Clapper –Two-Pincher Gripper Basic Model Advanced Model Number 1 Advanced Model Number 2 –Flexible Finger Grippers –Wrist Rotation

The Clapper Can be built using –Metal –Plastic –Wood This section describes a metal and plastic clapper

The Clapper Parts List 21.5” x 2.5” x (1/16)” thick acrylic plastic sheet 21” x (3/8)” corner angle bracket 11.5” x 1” brass aluminum hinge 1Small 6-vdc or 12-vdc spring-loaded solenoid 8½” x (6/32)” stove bolts, nuts

The Clapper Consists of a wrist joint (assume permanently attached to forearm of robot) Connected to the writ are 2 plastic plates –Bottom plate is secured to the wrist –Top plate is hinged Small spring-loaded solenoid is positioned between the two plates –When solenoid is active, gripper is closed –When solenoid is not active, gripper is open

The Clapper (construction) Cut both acrylic plastic to 1.5” x 2.33” pieces Attach the lower flap to both corner brackets –Place bracket ~ (1/8)” from either side of the flap –Secure the pieces using (6/32)” bolts

The Clapper Choice of solenoid is important –Must fit between the 2 flaps –Must have a flat bottom to facilitate mounting –Must operate within the voltage used in your robot (usually 6V or 12 V) –If solenoid doesn’t have mounting flanges opposite the plunger, mount it in the center of the bottom flap using household cement

Two-Pincher Gripper Two movable fingers Three types: –Basic –Advance model # 1 –Advance model # 2 Basic Model: –Made from extra Erector Set (similar construction kits may be used)

Two-Pincher Gripper Basic Model (parts) 24.5” Erector Set grinder 13.5” length Erector Set grinder 4½” x (6/32)” stove bolts, fender washer, tooth lock washer, nuts Mics.¼” to 16 gauge insulated wire ring lugs, aircraft cable, rubber tabs, ½” x ½” corner angle brackets

Two-Pincher Gripper (Basic Model) construction Connect the 4.5” grinders and the 3.5” grinder using (6/32)” x ½” bolts and nuts to make two pivoting joints –Shown on page 407 Cut two 3” lengths and mount them –Sand off the ends at an angle to prevent them from touching Cut through two or three holes on one end to make a slot

Two-Pincher Gripper (Basic Model) construction (6/32)” x ½” bolts and nuts to make pivoting joints in the fingers You can actuate it in a number of ways –Mount a small eyelet between the two pivot joints on the angle grinder. –Thread two small cables through the eyelet and attach the cable –Connect to other end of the cables to a solenoid or a motor shaft

Two-Pincher Gripper (Advanced Model 1) Use a readily available plastic toy and convert it into a useful two-pincher gripper for your robot arm Toy is a plastic “extension arm” Inexpensive contraption –Usually under $10.00 –Available at many toy stores

Two-Pincher Gripper (Advanced Model 1) construction Chop off the gripper three inches below the wrist Cut through the aluminum cable Cut another 1.5” of tubing –Just the arm not the cable Fashion a 1.5” length of ¾” diameter dowel to fit into the rectangular arm

Two-Pincher Gripper (Advanced Model 1) construction Drill a hole in the dowel for the cable to go through –Cable maybe of center, compensate for that Place the cable through the hole then push the dowel at least ½” into the arm Drill 2 small mounting holes to keep the dowel in place –Use (6/32)” x ¾” bolts and nuts to secure it

Two-Pincher Gripper (Advanced Model 1) construction Use the dowel to mount the gripper on an arm assembly Flatten one end of the dowel and attach it directly to the arm Gripper opens with only (7/16)” pull Attach one end of the cable to a 1/8” round aircraft cable Crimp-on connector designed for 14 to 16 gauge electrical wire to connect them end to end

Two-Pincher Gripper (Advanced Model 1) construction Attach the aircraft cable to a motor or rotary solenoid shaft Activate the motor or solenoid to pull the gripper closed Spring built into the toy arm opens the gripper when power is turned off

Two-Pincher Gripper (Advanced Model 2) Uses a novel worm gear approach –Without requiring a hard-to-find worm gear Worm is a length of ¼” 20 bolt –Gears are standard 1” diameter 64-pitch aluminum spur gears Hobby stores have these for about $1 a piece Turning the bolt opens or closes the fingers of the grippers

Two-Pincher Gripper (Advanced Model 2) Parts 2 3” lengths (41/64)” x ½” x (1/16)” aluminum channel 2 1” diameter 64-pitch plastic or aluminum spur gear 1 2” flat mending “T” 1 1.5” x ½” corner angle iron 1 3.5” x ¼” 20 stove bolts 2 ¼” 20 locking nuts, nuts, washers, tooth lock washers 2 ½” x (8/32)” stove bolts, nuts, washers 1 1” diameter 48 pitch spur gear

Two-Pincher Gripper (Advanced Model 2) construction Cutting two 3” aluminum channel stock Use the 3” flat mending “T” plates as the base Attach the fingers and gears to “T” plates as shown on page 410 –The distance of the holes is critical It depends entirely on the diameter of the gears you use

Two-Pincher Gripper (Advanced Model 2) construction Secure the shaft using a 1.5” x ½” corner angle bracket Mount it to the stem of the “T” using (8/32)” x 1” bolt and nut Add # 10 washer between the “T” and the bracket to increase the height of the bolt shaft Mount a 3.5” long ¼” 20 machine bolt through the bracket

Two-Pincher Gripper (Advanced Model 2) construction Use double nuts or locking nuts to form a free spinning shaft Reduce the play as much as possible without locking the bolt to the bracket Align the finger gears to the bolt so they open and close at the same angle

Two-Pincher Gripper (Advanced Model 2) construction To actuate the fingers –Attach a motor to the base of the bolt shaft –The prototype gripper used a ½” diameter 48- pitch spur gear –a matching 1” 48-pitch spur gear on the drive motor Operate the motor in one direction the fingers open the motor spins in the opposite direction, the fingers close

Flexible Finger Grippers Clapper and two-pincher grippers are not like human fingers They lack a compliant grip –The capacity to contour the grasp to match the object You can approximate the compliant grip by making articulated fingers for your robot At least one toy id available that uses this technique –Opposing thumb is not articulated, but you can make a thumb that moves in a compliant griper of your own design

Flexible Finger Grippers Use the same method as for the two-pincher gripper

Wrist Rotation Human hand has three degrees of freedom: –Twist on the forearm –Rock up and down –Rock side to side Can add all or some of these degrees of freedom to the robotic hand

Wrist Rotation Most arm designs just want to rotate the gripper at the wrist –Usually preformed by a motor attached at the end of the arm or at the base –A cable or chain joins the motor shaft to the wrist The gripper motor shaft are outfitted with mating spur gears –Can also use chains or timing belts to link the gripper to the drive motor

Wrist Rotation You can also use a worm gear on the motor shaft Another approach is to use a rotary solenoid –Have a plate that turns 30 to 50 degrees in one direction when the power is applied –It return to its normal position when the power is removed Mount the solenoid on the arm and attach the plate to the wrist of the gripper

EXPERIMENT WITH GRIPPER DESIGNS