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:
Pontechís 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 sensorís 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 sensorís value from 0 to 255.
Outputs of these sensors are connected to SSCís 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 Ďservo valueí from 0 to 255 and v is servosí 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 sensorís value at 30cm distance.

Material

Sensor Value

Brown Cardboard

52

Blue Plastic

51

Black Leather

50

White Paper

51


Last updated: 12/18/2000 09:46