ECE 477 Digital Systems Senior Design Project Fall 2007

Homework 5: Theory of Operation and Hardware Design Narrative

Due: Friday, September 28, at NOON

Team Code Name: Led Zeppelin Group No. 1

Team Member Completing This Homework: Danny Ricciotti

e-mail Address of Team Member: dricciot @ purdue.edu

Evaluation:

SCORE

/

DESCRIPTION

10 /

Excellent – among the best papers submitted for this assignment. Very few corrections needed for version submitted in Final Report.

9 /

Very good – all requirements aptly met. Minor additions/corrections needed for version submitted in Final Report.

8 /

Good – all requirements considered and addressed. Several noteworthy additions/corrections needed for version submitted in Final Report.

7 /

Average – all requirements basically met, but some revisions in content should be made for the version submitted in the Final Report.

6 /

Marginal – all requirements met at a nominal level. Significant revisions in content should be made for the version submitted in the Final Report.

* /

Below the passing threshold – major revisions required to meet report requirements at a nominal level. Revise and resubmit.

* Resubmissions are due within one week of the date of return, and will be awarded a score of “6” provided all report requirements have been met at a nominal level.

Comments:

1.0  Introduction

The blimp’s design requires some circuits in order to have all the components function together correctly. Since weight is a major factor in the design, several decisions were made to keep a minimum weight. A voltage regulator will step down the 7.4V battery voltage to run the microprocessor and peripherals. H-bridges will be used to allow the motors to operate bi-directionally. This will allow forward and backward motion from the bottom motors and left/right motion from the tail motor. Several subsystems of the microprocessor and sensor board will be used and is discussed in detail in this report.

2.0  Theory of Operation

The blimp design requires several different voltages, however there will be only two 7.4 V Lithium-ion battery on board. The Stargate board [1] use 5V, the USB webcam must be run at 5V, the MNAV [2] works at a range of voltages, and the servos can be run from 5-6 Volts. The motors are rated to go up to 6 V. Since all of the components can run at 5V we decided that running all of them at this voltage would be appropriate. We will have two voltage regulators on-board. One will be for powering the circuitry of the blimp and the other for powering the motor and servos. Having the separate power supplies will reduce noise from the motors. This has been seen in RC plane and blimp designs on the Internet. Two LTC3812-5 [3] switching voltage regulators from Linear Technology with some external resistors, caps, and diodes will be used to get 5V.

The design decision was made to run the Stargate and USB at 5V. Two jumpers must be positioned correctly if running the Stargate on a Li-Ion battery instead of DC wall power as well as two switches to indicate that the daughter card is being used. The processor is run at 400Mhz and this will provide enough speed for all the computations and video operations that must be done.

The Stargate is connected to the MNAV board via a 51-pin connector and can send the MNAV commands via a built-in RS232 serial connection. The other pins of 51 pin connector are used for sending larger blocks of data while the serial pins are used for commands. The MNAV controls the motors and servos via PWM depending on the commands received from the Stargate. The three motors in the design must be bi-directional to allow for the best blimp control. The two bottom mounted motors will provide up/down rotation as well as forward/backward thrust and the tail motor will provide left/right rotation. Because of this, an H-bridge circuit is added to our design for each motor. Two GPIO pins from unused MNAV servo ports will be used in the circuit to set the direction of each motor; six pins in total. Since the motor’s speed will be controlled via PWM separately from the servo which controls the motor’s rotation, the H-bridge needs to only be able to act in forward and reverse modes; not braking (breaking) mode. A command from the Stargate to the MNAV will say which motors to turn on, the speed of the motor, and the direction. Three Freescale 38866 H-bridge ICs will be used; one for each motor. [4]

3.0  Hardware Design Narrative

The Stargate has several A compact flash port will connect to an 802.11b wireless card through which the blimp can talk to a ground server and a human user. A wireless connection to WiFi networks is configured in software on the Stargate and a jumper must be set indicating which voltage the compact flash is running at. There is a daughter card which connects directly to the Stargate and provides several different peripheral connections including USB and serial. The blimp’s on-board webcam will use the USB port to store video data. No external circuitry is required for the USB and all the configuring is done through software. An RS-232 serial port and controller as well as Ethernet are available however we will not be using them except as debugging and programming tools. The minimal hardware configuring needed in the blimp design makes it very easy to modify the functionality of the blimp if changes were desired down the road. Software scripts to control the motors, webcam, and servo movement are much easier to edit than a physical circuit or even an assembly level microcontroller. Wifi changes to connect to different networks can also be done the same way.

As mentioned before, a MNAV sensor board will be used in the design. The MNAV has the capabilities of 9 PWM channels for servo and motor control. Four channels will be configured for servos and three will be for the blimp’s motors. The blimp design also uses the MNAV’s 16 bit A/D for gathering sensor readings of pressure, acceleration, heading, and temperature. Servos are being used to position the motors and webcam as well as operate the bomb door. Hitec Micro RC servos will be used since they only weigh 8 grams each and provide sufficient torque.

4.0  Summary

There are two important circuits that must be designed; the voltage regulation and motor direction control. The use of 5V throughout the design reduces complexity while not affecting performance or power consumption. A voltage regulator IC will be configured with a small circuit to generate the 5V required in our design. Off-the-shelf H-bridge ICs can be used to control the motors with a handful of external components.

Two major hardware components on the blimp will be the Stargate microprocessor and MNAV sensor board. The microprocessor subsystems give us access to a Flash port for a wireless card, and USB for the webcam. Serial and Ethernet ports are available for debugging as well however will not be used in the final design. The subsystems of the MNAV being used are the 16 bit A/D for sensor inputs of temperature, pressure, heading and altitude. PWM channels will be used to drive the motors and servos of the blimp. Lightweight servos and motors were chosen keep the overall weight low but will still deliver the required performance.


List of References

[1]  Crossbow Technology: Wireless Sensor Networks, “Stargate Developers Guide,” January 2006, http://www.xbow.com/Support/Support_pdf_files/Stargate_Manual.pdf

[2]  Crossbow Technology: Inertial Systems, “MNAV Datasheet,” January 2006, http://www.xbow.com/Products/Product_pdf_files/Inertial_pdf/uNAV_Datasheet.pdf

[3]  Linear Technology, “LTC3812-5 - 60V Current Mode Synchronous Switching Regulator Controller Datasheet,” September 2007, http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1032,C1092,P37683,D25009

[4]  Freescale Semiconductors, “MC33886 H-Bridge Integrated Circuit Datasheet,” September 2007, http://www.freescale.com/files/analog/doc/data_sheet/MC33886.pdf?pspll=1


Appendix A: System Block Diagram

Figure 1.1 – System Block Diagram

Figure 1.2 – MNAV Sensor Board Diagram [2]

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