Download erii18_servos - Cornerstone Robotics

Cornerstone Electronics Technology and Robotics II
“Hacking” Servos
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Administration:
o Prayer
Quiz:
Servos Review:
o A servo motor is a motor that can be controlled to turn to a specified
position. Typically, a servo is used in hobby applications such as
controlling steering for an RC (radio-controlled) car or the rudder for an
RC airplane.
o Most servos (servo motors) have a position range of 180 degrees
(check your servos). If the input position does not match position of
the servo, the motor will rotate until the position matches the input
position. EXPLAIN
o Servos are controlled using pulse width modulation (PWM). PWM is
when you turn something on and off very quickly.
Typical Servo Signal Pulse
o Oscilloscope demonstration for motor control.
o When used as a servo control signal, PWM pulse length is critical. The
waveform varies from 920 µs to 2120 µs. See the following diagrams.
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Pulse width for full counterclockwise position:
0.9 msec Pulse Width
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Pulse width for center position:
1.5 msec Pulse Width
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Pulse width for full clockwise position:
2.0 msec Pulse Width
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Continuous Rotation of a Servo:
o By making a couple of modifications (hacking), we can fool the servo
into continuous rotation in either direction. The steps to modify a
Futaba S148 servo are as follows:
 Remove the four screws from the back of the servo.
 Remove the back panel of the servo; then, gently remove the
front panel, making sure none of the gears fall out.
 Remove three of the gears from the front of the panel, exposing
two small screws which hold the motor to the servo package.
 Cut the stop from the potentiometer gear using a small back
saw.
 Remove the two motor mount screws.
 Gently pull the servo control board out of the servo package. It
may be helpful to push from the lower shaft, which is the
potentiometer.
 Carefully desolder the potentiometer from the servo control
board.
 Measure the resistances of several ¼ watt 2.2 K resistors and
pick two resistors with close values.
 The 2.2K resistors mimic the center position of the
potentiometer
 Install the two 2.2K resistors in place of the potentiometer.
Each resistor goes from an outer hole to the center hole of the
three potentiometer holes, so the center hole has a lead from
each of the two resistors inserted.
 Resemble the servo.
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o Assemble the following circuit on a breadboard.
o Now, use servo1.pbp to experiment with the 16F84A chip on your
breadboard. Test values between 100 and 200. Find the setting
around B0 = 150 that stops the servo from rotating.
Quiz Frogbot Week 10
1. What does micro represent both in decimal and fractional form?
2. What is the unit for capacitance?
3. What is the formula for time constant and what does each symbol represent?
4. What is the units of measure for time constant?
5. In general, does alternating current pass through a capacitor?
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6. In Frogbot, are we using a capacitor for power supply filtering?
7. In an RC circuit, what is the time constant if the resistance is 10,000 ohms
and the capacitor is 10 micro-Farads?
8. What is the symbol for a fixed resistor?
9. What is the unit of measure for electrical current?
10. When chemically etching a circuit board, the process is in what order? Put a
number 1 by the first step, 2 by the second step, etc.
Etch your board.
Expose your main control board.
Cut your board.
Develop your board.
Extra Credit:
11. Write Ohms Law, listing the variables.
12. If R1 = 100 ohms, R2 = 250 ohms and R3 = 500 ohms, what is the total
resistance if R1, R2, and R3 are placed in series?
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