Autonomous Robot Electrical System

General Notes

All integrated circuits are installed with 100 nF capacitors between power and ground.
At power inputs from the TIVA, 100 uF and 100 nF capacitors connect power and ground.
Rails on boards that draw significant power also have a 220 uF capacitor between power and ground.
The male connectors on all boards are numbered, as are the female connectors on all wires. This minimizes the risk of incorrect connections when the robot is integrated.

TIVA Connection Board

This board contains all connections between the TIVA and the peripheral boards. The resistor used in the limit switch circuitry is also mounted on this board, as the limit switch does not use a peripheral board.

Tape Sensor

Instead of following the current-carrying wire under the game board using resonant RLC circuitry, our robot followed the black vinyl tape on top of the board. We made this decision for a few reasons:

We did not want to risk that the nearby drive motors would interfere with the circuitry.
We wanted to avoid making two more signal conditioning circuits for the two inductors required to follow the wire.
We did not want to rely on available field space to test - by mounting a small piece of black vinyl tape on a piece of foam core, we were able to test line following at our lab station.

However, before jumping into tape following, there were two major challenges to address. The first is that traditional line following does not output an analog position value to use for closed-loop control. In order to output an analog value corresponding to our position in relation to the tape, we used the Pololu QTR-8A board. This is an array of 6 to 8 IR emitter/detector pairs. By taking a weighted average of the values read on all 6 of the pairs we were using, we could get a pseudo-analog value ranging from 0-500, which corresponded to our position on the line.

The second issue with line following is noise from background IR, which would have made the system unreliable depending on the time of day. To make the system robust in any light condition, we implemented filtration in software - every time the position is sampled, each IR detector is read with no IR LEDs illuminated to determine the background IR level. Then, the LEDs along the array are illuminated, and the detector is read. Finally, the background value is subtracted from the reading after the IR LEDs are activated; this yields the actual amount of reflected IR from the emitter. Few issues were encountered after setup, and the system performed well at a variety of speeds and on sharp corners.

Drive Motor Control

The TLE-5206 H-Bridge was connected using the SPDL-supplied PCB per the manufacturer's specifications, including all capacitors.

Launch Motor Control

A set of coiled wires from the TIVA Distribution Board provided signals for both servos, and power and ground were wired directly from the 6 V voltage regulator.

Team Select Switch and LED Indicators

The RGB LED blocks are 5050 LED Strip packages, which already include current-limiting resistors. The selection switch was placed on the same board as the indicator LEDs to alleviate the need for an additional TIVA output when the data was already available from the switch. It also assists with setup before each match - the LED color indicates which team will be active on the next round, once the TIVA is reset (our software updates the team on TIVA reset, so the color would not be updated immediately were it controlled by a separate GPIO output).

Limit Switch

Two switches are included in parallel, one on each side of the bumper, to ensure that the collision with the resupply depot is detected regardless of which side of the field the robot is using. Because the tape line is not perpenicular to the wall until less than 12" from the wall, the approach angle is steep enough that the robot does not hit the wall head-on. PB5 goes low if either switch is pressed.

Ball Request Subsystem

The top-left IR LED is used to fire the beacon required to request a ball, and the top-right IR LED is mounted across from a phototransistor connected to the circuit illustrated below to count balls as they are received. The red LED was used as a debugging tool - whenever the ball detection IR LED is illuminated and the beam is not broken, the light will be illuminated; if a ball is blocking the beam, the LED turns off. The pull-down resistor was enabled on pin PB3 to ensure current draw was sufficient that a clean signal would be received by the TIVA.

Hall Effect/Magnetic Field Reading

The Hall Effect sensor output was a signal with an average voltage around 1 V and pk-pk of 100 mV. The first op-amp was used as a buffer two reasons - to ensure that sufficient current was provided to the following stages and to isolate the impedance of the sensor from the low-pass filter. The low-pass filter worked to remove noise from, among other sources, the motors. Then, an op-amp centered the signal by removing the DC offset and replacing it with a virtual ground from a filtered, buffered, voltage divider and increased its amplitude such that the Schmitt trigger could detect rising and falling edges.

The circuit needed to be mounted beneath the robot, as inductive noise caused by the motors was a major concern on this circuit. To minimize the effect of the motor on noise, the sensor was not mounted on the center of the robot - instead, it was mounted approximately 4" from the center.

IR Beacon Detection

A photodiode was used for the IR Beacon sensing circuit for its fast response time. A trans-resistive circuit with two amplification stages was included to increase the amplitude of the relatively small signal from the photodiode. A virtual ground was used to provide a buffered reference voltage to the rest of the circuit. The sensor signal went through a high-pass filter to remove any DC offset prior to each amplification stage. The first amplification stage was also a low-pass filter to eliminate noise. Lastly, a comparator was used to generate clean edges for the TIVA input. This board was mounted on the side of the structure of the launcher to minimize the distance from the photodiode to the signal conditioning.

Yiwen Wu

Engineer · Designer · Baker · Traveler