Technical and Calibration Details
As part of developing the design for the Palm Pilot Robot, we measured the performance of the various components. This page documents the specifications of each component and the results of our calibration experiments.Serial Servo Controller (SSC) Board:
Pontech’s SV203 board was used to connect the Palm Pilot to servomotors and
infrared rangefinders. The board can control up to 8 servos simultaneously and
has 5 analog-to-digital inputs. All communication is done by sending and
receiving short strings. For example:
"SV1" selects a servo number 1, "M128" - moves
selected servo to position 128. Sampling of sensors is done by sending a request
to sample a desired sensor ("AD1\r"), and then receiving a value of
the sensor followed by ASCII 10 and ASCII 13.
Infrared Rangefinders:
Sharp GP2D12 infrared triangulation sensors were used to measure distance from
the robot to an obstacle. Since these devices use linear position sensitive
devices (PSD), their measurements are accurate to approximately 1cm. Please
refer to the section below title Test of GP2D12 Infrared Rangefinder for details
of the sensor’s performance and accuracy.
The SV203 converts the analog sensor outputs into an integer reading as a value
ranging from 0 to 255. Distance can be approximated by using a power regression:
d(s) = 2141.72055*(s^-1.078867)
where d is distance in cm and s is sensor’s value from 0 to 255.
Outputs of these sensors are connected to SSC’s analog input pins, however
their power is connected to unregulated 6V (use Gnd and +V pins of unused
servos), instead of regulated 5V, because of the amount of power required by IR
sensors.
Servos:
Servomotors were used for driving, because they are inexpensive and have
proportional control of speed via PWM. However, one of the disadvantages of such
use is that servomotors are not designed for speed-control and therefore their
speed in not proportional to the 8-bit value (0-255) sent to the board. The
graph below shows a relation between sent values and actual speed.
The curve through the points on the graph can be approximated to a reasonable degree by a cubic regression:
s(v) = 39.8053*v3 - 12.6083*v2 + 22.1197*v + 128.4262
where s is ‘servo value’ from 0 to 255 and v is servos’ velocity in revolution per second.Test of GP2D12 Infrared Rangefinder (using
AnalogDevices 16bit A/D converter)
Illumination angle test:
Varied angle between a cardboard and line of sight of a sensor. All measurements
were taken at 40cm distance from the range finder.
|
Angle |
Sensor Value |
 |
|
0 |
40 |
|
|
20 |
39 |
|
|
40 |
39 |
|
|
60 |
40 |
Cone-shape test:
Sensitive region measured at variable distance.
|
Distance |
Sensitive region |
 |
|
10 |
2.0 |
|
|
20 |
2.5 |
|
|
30 |
2.6 |
|
|
40 |
2.4 |
|
|
50 |
2.5 |
Sensor value test:
Values of a rangefinder were measured at various distances.
|
Distance |
Sensor Value |
 |
|
10 |
143 |
|
|
20 |
77 |
|
|
30 |
52 |
|
|
40 |
39 |
|
|
50 |
31 |
Material-sensitivity test:
Several materials were used to measure sensor’s value at 30cm distance.
|
Material |
Sensor Value |
 |
|
Brown Cardboard |
52 |
|
|
Blue Plastic |
51 |
|
|
Black Leather |
50 |
|
|
White Paper |
51 |