remove the servo horn from the servo without letting the
snare wire slip through the barrel. So, grab the snare wire
and mini EZ connector tightly before lifting the servo horn off
the snare servo. Tighten the set screw and put the servo horn
back on the snare servo. Before screwing the servo horn back
onto its servo, write test code to exercise the snare servo.
Extend and retract the snare to verify it opens and closes
well. When the snare is closed, it cannot press on the switch.
The retracted servo needs to leave a little space between
itself and the switch button so that only when a target is
captured can the capture switch be pressed. Make additional
adjustments to the snare wire and set screw until the snare
opens and closes properly. Once you’re happy with the
snare’s size, attach the servo horn to the servo with its screw.
Hold onto the ends of the piano wire as you cut off the
excess wire. Hold the wires to make sure they don’t go flying
some place dangerous, like you eyes.
Using the Robotic Arm
Good targets for the robotic arm and its snare have risers
on them, like dowels. You might try lightweight cubes and
glue dowels to one face. Right now, I’m playing with large
plastic jacks. I like their design since no matter which way
they fall, they always have a riser standing straight up.
Now, program your robot for a challenge. Be sure to
raise the robotic arm before moving your robot. That way, the
arm isn’t driven into the wall. Once at its destination, stop the
40 SERVO 05.2011
robot, extend the snare, and lower the robotic arm. If the
snare has lassoed the target, when it retracts it will pull the
target against the capture switch. After retracting the snare,
the robot controller checks to see if input from the capture
switch is low (ground or logical zero). If it is, then the target
has been captured and the arm is ready to lift the target up,
and the robot is ready to drive to its next destination.
If the input from the capture switch is still high (+ 5 volts
or logical 1), the snare has missed the target. In this case, the
robot should open the snare and raise the arm a small
amount. Rock the robot back and forth, and repeatedly lower
the arm and retract the snare. At some point, if the robot
continues to fail, it should back away and reapproach the
destination. Once the robot has driven the target to the
second destination, the snare only needs to extend to release
the target. The release of the target is confirmed by checking
the input from the capture switch to see that it is high (+ 5
volts or logical 1).
Another option is to mount the robotic arm on its side.
In this position, the arm swings left and right to lasso dowels
on the side of its targets. The robot would then have to lift
slightly to raise the target before moving to a new
You can see a short video of an earlier version of this
robotic arm picking up jacks on my website. I am also selling
a complete robot arm kit for roboticists who don’t want to
go to the trouble of locating all the parts for the arm. Find
out more on my website at NearSys.com/catalog. SV
STEER WINNING ROBOTS
Perform proportional speed, direction, and steering with only two Radio/Control channels for vehicles using two
separate brush-type electric motors mounted right and left
with our mixing RDFR dual speed control. Used in many
successful competitive robots. Single joystick operation: up
goes straight ahead, down is reverse. Pure right or left twirls
vehicle as motors turn opposite directions. In between stick
positions completely proportional. Plugs in like a servo to
your Futaba, JR, Hitec, or similar radio. Compatible with gyro
steering stabilization. Various volt and amp sizes available.
The RDFR47E 55V 75A per motor unit pictured above.