ECE 477 Digital Systems Senior Design Project Rev 8/09

Homework 3: Design Constraint Analysis and Component Selection Rationale

Team Code Name: ______Defender______Group No. ___6___

Team Member Completing This Homework: ______Stephen Wolf______

E-mail Address of Team Member: ____sjwolf______@ 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:

Comments from the grader will be inserted here.

1.0  Introduction

Defender is a turret based defense system for high security installations. A computer base station interfaces with a coilgun turret system and a keypad for identification of approaching personnel. Onboard the turret system is an Atom processor to deal with the requirements of the image processing software for target tracking and identification. The coilgun consists of a custom made barrel and coil, a capacitor bank contained in insulators for protection, a high voltage adjustable power supply, and a solid state discharge circuit. The turret itself consists of two stepper motors controlled with their own IC for control of the azimuth and elevation angles. A microcontroller will act as a master to communicate commands from the Atom board to the various systems and to report back their status. There are also other minor components present such as a speaker for auditory warnings and a keypad for entering PINs (Personal Identification Numbers). An updated block diagram of the system can be found in appendix B.

Updated PSSCs

1. An ability to fire a projectile using magnetic force

2. An ability to aim the projectile by controlling of the azimuth and elevation angles of the barrel

3. An ability to charge a capacitor bank to a variable voltage, up to 400V

4. An ability to gather images and perform target recognition and tracking

5. An ability for a user to enter commands and display the status of the turret through an external interface

2.0  Design Constraint Analysis

There are five major tasks that the Defender system must be able to accomplish; these will be the main focus of this report. Those five are computation, communication, charging, control of motors, and discharging. The computation is discussed in section 2.1. Devices for communication (microcontroller options), charging (DC-DC converter), controlling of the motors (stepper motor IC’s), and discharging (IGBT options) are discussed in section 3.0. There are no particular weight, space, or packaging constraints besides those induced by safety. The system is not currently designed for any particular area or hallway and as such is only a general prototype without any of the size constraints a final physical location would contain. However it is desirable to make the barrel as light as possible to avoid needing more powerful motors. More details on packaging design and constraint will be found in section 2.6.

2.1  Computation Requirements

The Defender system has a significant computation requirement in its image processing. The ultimate goal of the cameras available to the system is to take the image, identify anything in the image that is considered a target, construct a 3-D position of that target, and if necessary move the barrel or assist the user in firing upon this target. All of these must be done in real time on a moving target and communicate to the motors so that they can move quickly enough to the target. The objective for this prototype is for all of this to take place quickly enough to catch targets moving at 1 m/s or less. In order to achieve this level of computation an onboard Atom processor will be utilized. This processor will also serve as the communication hub as described in section 2.2. All other computations are simple tasks, such as tracking motor position based on commanded steps or calculating the voltage required to achieve a certain projectile speed. The microcontroller itself has no real-time computation constraints that any 8 or 16bit microcontroller cannot achieve.

2.2  Interface Requirements

The turret main system has three levels of devices that interface with each other. On the top level is the computer to Atom board system which is done through Ethernet cable on to an existing port on the Atom board. The second level consists of the devices which will be connected to the Atom board itself. The primary communication output of the Atom board is USB; both the cameras and the master microcontroller will interface to the Atom board through the USB inputs available. The microcontroller itself will then be communicating primarily through a multi-device I2C bus, although there may be 1 or 2 peripheral such as the keypad encoder that communicate with a UART. A serial header for BDM will also be present on the board. I2C is a simple 0-5V protocol without any serious current requirements and requires no special level translators. Optical isolation may be used with communication modules to the high voltage charging and discharging for protection. USB requires special regulators to create the 3.3V necessary but all such hardware is contained within the USB module on the microcontroller itself.

2.3  On-Chip Peripheral Requirements

The master microcontroller has two critical requirements: built in USB support and I2C support. Only one USB module is needed to communicate with the Atom board. Optimally the device will have at least two I2C channels due to the large amount of off-chip I2C devices planned. The chip will require at least one external interrupt source and one timer channel for basic functionality, but there is nothing constraining the devices besides the USB and I2C support. Virtually all other functionality will be off loaded from this chip onto external devices as detailed in section 2.4.

2.4  Off-Chip Peripheral Requirements

Defender will have a multitude of off-chip secondary devices that are all controlled either directly through an I2C bus or through I2C shift registers. The capacitor bank requires special ADCs that can handle the high voltages involved to measure and verify operation. Photodiodes and IR diodes will be present in the barrel for monitoring the position and speed of the projectile. A special purpose IC for stepper motor control is required to operate and monitor the stepper motors. A keypad encoder will also be utilized to monitor the keypad for entering PINs. The camera for target tracking is a Canon VC-C50i[1] with its own PTZ stage and requires no support besides the USB interface.

2.5  Power Constraints

Defender will be wall powered with a battery backup. While the basic 12V and 5V rails for component operation are simple and have no particular concern, the charging and discharging of the capacitor bank requires many special components and precautions. The capacitor bank will consist of four 3900 µF U32L capacitors rated to 400V. A specialized DC-DC converter will be required, with a goal of 0-400V adjustable voltage at 100-500W of power. This will allow the capacitors to have charging times of anywhere from 5 to 20 seconds. This capacitor bank will have to be well shielded to prevent against both EMI and accidental discharge. Due to the infrequent nature of the pulsing heat dissipation is not a primary concern in the capacitors. Discharging the capacitors into the coil around the barrel will be done through a solid state switch, specifically an IGBT. This IGBT will have to be very high rated and will be heatsinked using the chassis.

2.6  Packaging Constraints

There are no weight or space constraints on Defender besides those that would make it impossible to eventually ceiling mount it as a defensive turret. Therefore choices should be made to lessen weight were possible but this is not an overriding priority as the maximum weight depends very much on where the turret would be installed. The weight of the barrel and the stage that holds the barrel are however both critical. These weights should be minimized as to reduce the stepper motor size and current required. The primary packaging constraint is then safety. The capacitor bank is to be charged to very high voltages and the barrel will be having large currents pass through as well as heating up. Electromagnetic shielding must also be used so as to not disrupt any nearby devices due to the large pulses of energy associated with firing the device.

2.7  Cost Constraints

While similar systems exist in the realm of remotely operated turrets, such devices do not normally have their prices freely disclosed and also contain much more advanced sensor equipment as well as redundancies when compared to the prototype Defender. Two traditional turret based systems are the TRT-25MM[2] from BAE systems and the WASP[3] from a French company known as Panhard. BAE systems does not give an estimate of how much the TRT-25MM costs but an article[4] from the Defense News of Army Times Publishing Company quotes the WASP at $62,000 per unit, far and above the estimate of $750 our prototype carries. This price difference is the extra cost of R&D, manufacturability, and the reliability that the military requires.

3.0  Component Selection Rationale

There are three microcontrollers that were looked at to act as the primary controller. The MC9S08JM60(S08) from Freescale, the PIC18F65J50(PIC18) from Microchip, and the PIC24FJ64GB004(PIC24) also from Microchip. These three were selected as having both USB and I2C support, the availability in easily soldered and low pin counts such as 44 pin QFP, the availability of other useful peripherals such as ADCs and timer channels, and then selecting those with the highest amounts of Flash and RAM to allow for significant overhead. Also important considerations are the availability of a C programming structure and debugging modes. The PIC24[5] is the device selected to be used in Defender primarily because of its 16 bit architecture where the PIC18 and the S08 are both 8 bit architectures. All three chips have the necessary peripherals but the PIC24 has two I2C channels, more timers and PWM channels, and the most flash and RAM. Instead of a standard USB module the PIC24 also has a USB OTG module which allows for the possibility of the microcontroller to become a USB host if further USB devices were integrated into the design. The PIC24 is available in a compact 28pin SOIC package (24FJ64GB002) but the package that will be used in the design as a 44pin TQFP(24FJ64GB004). The availability of more remappable pins will be useful in the case of any further design changes. The PIC24 and the S08 are both the same price, around $5, with the PIC18 being slightly cheaper at $4. Samples are available for the PIC24 and pricing is not an issue for this prototype.

Locating a DC-DC converter for charging the capacitor bank that satisfied all of our requirements proved to be a difficult task. The only IC that approached the requirements is a block from EMCO[6]. This device only supports up to 300V programmability and only 50mA of current, a total of 15W. The associated 100 second charging time is not very desirable in a turret that should be able to support at least 4 shots per minute. The current design direction is to then custom design a boost converter that will provide the desired functionality and power. We are also in contact with companies such as Linear Technology, EMCO, and Maxim in the hopes of still locating an IC that will provide us with what we need.

Two stepper motor drivers were looked at, the A3988 from Allegro Microsystems and the DRV8821 from Texas Instruments. These chips are both PWM drivers for dual bipolar stepper motors with capacities of up to 1.5A at 35V, more than enough power for our application of moving a barrel. The devices have packaged H-Bridges that allow for the driving of stepper motors at high currents. The DRV8821[7] is chosen for our application due to its simpler control scheme of simple positive edges for steps and a voltage reference for current limiting, the ability to achieve microstepping with 1/8 steps, and the packaging of the heatsinks external to the chip with no pad underneath the IC. Samples are also available from TI.

The IGBT acts as a high voltage high current solid state switch to discharge the capacitor bank into the coil surrounding the barrel. A cheaper option than an IGBT would have been to go with an SCR. An SCR however cannot be turned off until a certain energy limit and requires significantly more calibration than an IGBT module. Three IGBT’s were looked at, the APT200GN60JDQ4 from Microsemi, the APTGT300SK60D3G also from Microsemi and the SEMIX 453GAL12E4S from Semix. The maximum voltage from the capacitors is 400V while the maximum pulsed current through the barrel is estimated at 300-600A. The SEMIX IGBT has the highest ratings with 1200V and up to 1350A pulsed, 685A continuous. This module would be guaranteed to not break but is extremely expensive at $215.48. The APTGT300 module is lower rated at 600V and 600A pulse, 400A continuous. However this module is also expensive at the $155 price range. The reason that those two IGBT’s are so expensive is that they come with massive attached heatsinks. As our application is a pulsed application we are not as concerned with heat dissipation. Therefore we go with the APT200 module[8]. This module is rated even lower than the APTGT300 with 600V and 600A pulse, 283 continuous, but these ratings are still more than sufficient for our application. It comes in an ISOTOP package meant to mount directly to a chassis and is much more reasonably priced at $42 per module.