proceedings - rochester institute of...

Multi-Disciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P11207

MID-RANGE WIRELESS MODULE

Team Members:Ryan Toukatly - EETrey Minarcin - EEErwan Suteau – EE

Peter Franz - EE

ABSTRACT

The primary goal of this team is to design a mid-range RF module that will serve as the communication link between a base station and a land vehicle for automated control. The module is part of the Wireless Open-Source/Open-Architecture Command and Control System (WOCCS). This module will be interchangeable with another mid-range and long-range RF module and will be used to control the vehicle's movements and bi-directionally send the status of the vehicle in real-time for a range of up to 200m. During the initial stage of the project, much analysis and reference documents were used to design the module and to ensure reliability of the design. Once the module was designed and assembled, intensive programming followed to achieve full functionality of the module. In the content that follows, the design, fabrication, programming, and testing of the module will be described in detail.

NOMENCLATURE

Balun – Impedance Matching ChipBER – Bit Error RateCC1111 – TI System on chip used in PCB designCIMS – Center for Integrated Manufacturing StudiesFCC – Federal Communications CommissionGUI – Graphical User InterfaceIAR Workbench – C code compiler ISM Band – Unlicensed Industrial, Scientific, and Medical communication frequency bandLED – Light Emitting DiodePacket Sniffer – TI test software PC – Personal Computer

PCB – Printed Circuit BoardPCB Artist – PCB Design SoftwareRF – Radio FrequencyRP-SMA – Reverse Polarity SubMiniature version A connector used for RF connectionsRS232 – Recommended Standard 232 serial data protocolSimpliciTI – TI Software Protocol compatible with CC1111Smart RF Studio – TI RF test softwareSMT – Surface Mount Technology LaboratoryTI – Texas InstrumentsUAV - Unmanned Aerial Vehicle USB - Universal Serial BusWOCCS – Wireless Open-Source/Open-Architecture Command and Control System

INTRODUCTION

The use of wireless technology is a vast field in today’s technology. The need for this technology is immense in today’s world. Wireless products may send and receive data across short distances such as in and around the home such as keyboards, routers, garage door openers, and headsets. They may also transmit data over long distances, such as cell phones, radios, GPSs, and satellite television. Wireless data is being used since it increases mobility and thus, opens a wide-range of opportunity.

The WOCCS system can be used in many applications. Its projected use is for is vehicle control, potentially be for the military. The mid-range RF module will be interpreting telemetry data to monitor in real time the various technical aspects of a vehicle’s

Copyright © 2011 Rochester Institute of Technology

Proceedings of the Multi-Disciplinary Senior Design Conference

position and velocity in space. This vehicle could either be a land vehicle or UAV. To accomplish this, there will be two modules, both capable of sending and receiving this telemetry data bi-directionally, creating a wireless link between the control center and the vehicle itself.

The immediate use for the WOCCS system is to educate incoming First Year Enrichment students about types of projects that students design. Many incoming engineering students would like to observe and interact with projects that current and past engineering students have designed. This facilitates the drive of the incoming students to be innovative and creates a positive learning experience for new students.

NEEDS

The goal of the mid-range RF module team was to satisfy the needs that were defined by Dr. Edward Hensel, who will be using the modules to educate incoming FYE students. The students will use this product to control a land vehicle, which will show them sample senior design projects and their capabilities. These needs included in the table below are the basis of this application as well as future uses of the mid-range RF module.

Figure 1: Customer Needs

The module is able to handle real-time telemetry data to control a vehicle and communicate over a medium distance (to be defined later). It uses point to point communication (base station to vehicle) and incorporates multiple channels in case there are multiple units in close proximity to one another. The module coexists with other wireless signals since it must be robust enough to handle RF noise. The unit consumes relatively low power to ensure efficiency. The module is light, compact, long-lasting, and low cost. It also meets environmental regulations to ensure it can pass code and be used in many applications. The

module is easy to use and is user-friendly in its applications. The units are also interchangeable with other WOCCS boards so that multiple boards can interface with the transceiver. These needs are a necessity to a quality transceiver that can be used to interpret telemetry data to control a vehicle.

It was planned that the system was to be presented in the form of a GUI to be run on a two PCs: one side would be the base station, and the other would be the receiver.

SPECIFICATIONS

Each customer need must have at least one engineering specification associated with it in order to satisfy the needs of the customer.

Here, seventeen specifications have been created in Figure 2 to satisfy the fourteen needs given in Figure 1. The included specifications include metrics such as data rate, line of sight range, number of communicating points, and PCB power usage. These all have marginal values that must be met for a successful unit and target values that are ideal characteristics of the ensuing design.

There are also specifications that are Boolean in nature such as operation allowed without a license, use of protocol to allow coexistence, open source software protocol, and interchangeability. All of these specifications were created with the prospective and immediate uses of the module in mind.

Figure 2: Engineering Specifications

CONCEPT SELECTION

The beginning steps of the design commence with the functionality of the unit. The functions of the module will dictate the architecture from an efficiency standpoint. For example, it would not be efficient to transmit data by infrared and attempt to control a unit at 100 meters away. Infrared does not have high data rate, the latency is too high, and it has deficient range.

Project P11207


Thus, a more efficient way a sending data wirelessly at that range would to send the data via RF signals. The content in Figure 3 shows the breakdown of functions that the wireless module must possess.

Figure 3: Functional Decomposition

The main focus of the module’s functional decomposition is to communicate wirelessly. It needs to send commands and receive incoming data since an identical unit will be located at the main base station and at the peripheral side. When transmitting, the module reads in control data from the base station by reading data from the I/O bus. It then sends user data to the peripheral transceiver by use of an RF signal. When receiving, the module reads in digital data from the transceiver by converting the RF signal to a digital signal. The unit also communicates back to the base station by writing digital data to the I/O bus. In event of a receive error, the module requests a retransmission of the incorrect data in order to receive all of the correct data. The module also interprets its own control data, such as low power mode, frequency selectable channels, and it can start and stop data transmission. All of these functions drive the component selection and hardware design decisions.

The basic architecture is shown in Figure 4. This high system level block diagram shows that there are two mid-range modules attached by a wireless link. It also shows them attached to power sources which are enclosed in a housing package. On the peripheral side, there is the system which is to be controlled. On the base station side, there is a link to a user interface. This interface will be a GUI defined by one of the other WOCCS system groups and the housing package will also be defined by a different group within the system. The user will send the unit commands via this GUI.

Figure 4: System Block Diagram

Expanding upon the high system level block diagram, some conceptual decisions were made to progress toward the module block diagram in Figure 5.

An external antenna was chosen due to the range and reliability needed for the system to be successful. The types of antennas that were analyzed were a printed circuit board (PCB) antenna, a wire antenna, and a whip antenna. The PCB antenna does not require an external connector, which makes it compact. However, a PCB antenna can be quite complex to design and the range is heavily reliant on that design. The wire antenna has a vast range, but its orientation can be altered quite easily and thus, it can lose its reliability. Lastly, the whip antenna is rigid and can be very large, but it has medium to large range capability. On the basis of time dedicated to this system, the whip antenna was chosen to be implemented in our system due to the simplicity and reliability of this type of antenna.

There are four connectors that are located on the PCB: the power connector, the data connector, the RF connector, and the programming connector. The power was fed through a power regulator before reaching the transceiver and microcontroller, which will interpret all of the telemetry data. This input power would be decided once the microcontroller and transceiver components were chosen. The use of the data connector is for the user command path, which lies between the transceiver and microcontroller. Thirdly, there is a RF connector, which is reverse polarity subminiature type A (RP-SMA), that will connect the antenna to the PCB. Since the whip antenna was chosen to be implemented, an external RF connection was needed. This connection was routed to impedance matching circuit, necessary to maximize transmission power, before reaching the transceiver and microcontroller. Lastly, a programming connector was needed. In order for the system to be programmable, a programming header would be



needed to implement many of the functions shown in the functional decomposition.

Figure 5: Module Block Diagram

The most important pieces of the design would be the microcontroller and the RF transceiver. The microcontrollers that were observed were the MSP430 and the 8051. Many of the transceivers that were investigated had the 8051 microcontroller embedded into a system on chip that included the microcontroller and RF transceiver in a single chip. The three systems on chips that were analyzed were the CC430, the CC1110 and the CC1111. These components are analyzed in the selection matrix in Figure 6 below. The CC430 and the CC1110 both were built for RS232, which did not have USB pins built in. However, the CC1111 incorporated USB pins into its system. The system on chip was decided once the data type was chosen. All of these systems on chips were designed with the capability to transmit at 900MHz. This was the frequency chosen for the transmission. The feasibility analysis for this frequency and others are discussed in the feasibility assessment section. In the selection matrix, each criterion was given a relative weight and each transceiver was compared to the reference part, the CC430. The part with the highest weighted score, the CC1111, was chosen to be implemented.

Figure 6: Transceiver Selection Matrix

The input data connection was an area for concern due to the scope of the project. The two main types of data that were investigated were RS232 and USB. RS232

has been used for years as a simple data protocol that does not require much programming complexity to send data. This has been used commonly for previous projects for peripheral control units, but it is much less common for new products on the market. Most products today are using USB for data, due to the much improved data rate capabilities. It was a reason of function and future use to implement USB.

There is also a need for visual indicators. There is a need to use external visual indicators for the user to observe if the unit is powered and whether it is transmitting or receiving. A LED bank would be used for this purpose. In addition to external LEDs, it is helpful to have a series of internal LEDs and a push button for the purpose of debugging, while the software is in its developmental stages.

The decision was made to use USB. The CC1111 transceiver from Texas Instruments (TI) was chosen to be implemented into the system as opposed to the CC1110 and the CC430, which only had RS232 communication capabilities. This system on chip has USB pins assigned and would reduce the complexity of USB for the purposes of this product.

FEASIBILITY ASSESSMENT

Before progressing with hardware design, analysis was performed to see if the transmission range of 200 meters is feasible. To obtain the possible transmission range, Friis’ transmission equation must be used [5]. Given two antennas, the ratio of power available at the output of the receiving antenna, Pr, to power input to the transmitting antenna, Pt, is given by:

Pr=Pt ⋅Gt ⋅Gr ⋅ λ2

(4 π ⋅d )α (1)

where Gt and Gr are the gains of the transmitting and receiving antennas, respectively. The symbol λ corresponds to the wavelength of the signal and d is the distance traveled. The inverse of the factor in parentheses, α, is the free-space path loss. The antenna gains are with respect to isotropic, which are not in decibels, and the wavelength and distance units must be the same. When free space is considered, α = 2. This parameter varies depending on the environmental conditions. For example, when propagation in an indoor environment with line of sight visibility, it is common to have α <2. This is due to the reflections that occur indoors which contribute to a better reception. In this case, the analysis will be limited to a free space configuration, thus α = 2 here. Using

Project P11207


decibels and α = 2 in equation (1), Friis’ equation becomes:

PrdBm=P tdBm+GtdB+Gr dB+20 log( λ4 π )−20log (d )

(2)

The wavelength can also be found in both equation (1) and equation (2) by the following relation:

λ= cf

(3)Where c is the speed of light and f is the frequency under analysis.

Here is an overview of the variables in the previous equations:

Pr: Received Power (mW) Pt: Transmitted Power (mW) Gt: Gain of the transmitting antenna (dB) Gr: Gain of the receiving antenna (dB) d : Distance between the transmitter and the

receiver (m) λ: Wavelength (m)

c=3 x 108 ms

: Speed of light

f : Frequency (Hz) α : Environment Parameter

When performing the analysis, there were two parameters that are directly related to the transceiver. The first is the received power, Pr. This will be limited to its minimal value by the sensitivity of the transceiver. Typical transceivers offer a sensitivity of Prmin=−90 dBm for the maximum feasible data rate. The second parameter is the transmitted power, Pt. This is usually configurable in the typical range of -30dBm to 0dBm. The antenna used in the desired application is an omnidirectional antenna, therefore it will have a low gain. The gain of the antenna chosen has 3dBi gain. Therefore, Gr=Gt=3 dBi for this analysis.

There are three frequency bands analyzed here. These are the 433MHz band, the 915MHz band, and the 2.45GHz band. These three bands, also known as ISM bands (Industrial, Scientific, and Medical), are the most common bands used that the Federal Communications Commission (FCC) permits use without a license [4]. For the purposes of this project, the team is limited to these three bands.

The results of the analysis are shown in Figure 7. The most common of the ISM bands, the 2.45GHz has a maximum ideal distance of 314.5 meters while a signal at 915MHz has an ideal distance of 825.1 meters. A 433MHz signal travels the furthest, but the bandwidth of that channel is not ideal for the number of channels that the team requires. Therefore, the 915MHz band was chosen.

Frequency (MHz)

Maximum Distance TX/RX (m)

f 1=433 1739.5

f 2=915 825.1

f 3=2450 314.5Figure 7: Maximum Wireless Range at Various Frequencies

DESIGN OF HARDWARE

Once the components and basic hardware architecture of the system were chosen, the schematic and PCB layout were designed. The CC1111 has many pins that can be assigned to various functions, such as USB data pins, LED pins, and RF pins. These are all previously laid out by TI. Much of the complexity of the schematic is evident in the RF circuitry and noise reduction along the traces.

The most complex area of the module design is the impedance matching circuitry that lies between the CC1111 and the RF SMA connector. Many calculations were performed and numerous reference designs were reviewed to reduce the most risk of this critical circuitry. Without the impedance matched to the antenna, the transmission power would not be optimal and this would greatly reduce the range of transmission. A design note from TI is shown in Figure 8 displays the use of a balun chip (labeled U121) followed by an inductor and two capacitors [1]. The chip shown in the figure is the CC1101, but this design is identical across all CC11xx systems on chips. By following this design exactly, it would significantly decrease the chance of failure from this circuitry.



Figure 8: JTI Reference Design for the CC11xx 868/915 MHz Impedance Matching Circuitry

The other main area of concern was external noise along the circuit traces. To eliminate this noise, many low pass filters would need to be included in the design. From the output of the voltage regulator, a ferrite bead which is tuned to 900MHz was placed along the power trace to eliminate the noise from the RF signal. This ensures that the circuit’s RF function does not interfere with the data interpretation for the CC1111. Also, many low pass filters were implemented close to the system on chip to eliminate most of the noise that would accumulate along the traces. Low-pass filters are implemented here since most of the signals, excluding clock signals from the external crystal, USB data signals, and RF signals, are DC signals. Calculations were performed to determine the capacitance needed to optimally remove the noise from these traces. These values were then confirmed with reference designs from TI [2], [3]. The final result of the mid-range module schematic is shown in Figure 9 below.

Figure 9: Hardware Schematic of Module

The main obstacles presented when converting the schematic into a PCB layout were mainly associated with the RF circuitry. Since this is the area that is most sensitive to noise, more reference designs were reviewed to reduce risk of failure. One design demonstrated that the impedance matching circuitry

would require at least four layers. The design of the PCB reflects this, with ground as the second layer and a power plane as the third layer. Having the ground layer directly underneath the RF circuitry effectively eliminates noise. A copper layer was also filled on the top layer surrounding the circuitry to eliminate noise as well. An unusually large trace from the impedance matching circuitry to the SMA connector was evident in the design and was matched accordingly. A grounding clip was also added to the layout to ground the entire PCB. This was done to eliminate the board from electrically floating when the board was enclosed in the housing package. The design was created in PCB software called PCB Artist. Figure 10 shows the final result of the design in software and the implementation of the design in PCB hardware.

Figure 10: PCB Implementation of Hardware Design

Once the PCB has been implemented, it was assembled in the Surface Mount Technology (SMT) Laboratory located in CIMS on RIT campus. Microscopes in the lab were used to place solder paste on the PCB, the parts were then placed manually excluding the CC1111 chip and the connectors. These parts were reflowed by use of the manual reflow station. All of the thru-hole connectors and switches were hand soldered to the PCB. The CC1111 was placed and reflowed via the precision reflow station. The final module is shown in Figure 11 below.

Project P11207

SMA ConnectorProgramming ConnectorGrounding Clip

915MHz AntennaCC1111 System on Chip

External Status LEDs

USB Connector

Power Connector

Internal Status LEDs

Power Switch


Figure 11: Populated PCB

SOFTWARE

Once the boards were assembled and basic electrical tests were performed, the module’s software became the main focus of the project. Almost all of the module’s functionality was implemented within the embedded software on the CC1111 chip.

For clarification, all software elements associated with the project are described below:

Microcontroller Software – Custom-written in the C language in collaboration with other WOCCS wireless teams. This software is downloaded to the on-board microcontroller and handles RF transmission and reception, USB communication, and the LED indicators.

Testbench Software – Written separately by the Test Bench team, this PC software provides a GUI for a user to communicate with the board, send various types of data, and test technical specifications such as measured data rate.

Programming/Debugging – IAR Embedded Workbench was used to write and compile the microcontroller code. Texas Instruments’ Flash Programmer was used to download the program to the CC1111 chip. SmartRF Studio and Packet Sniffer, tools designed by TI, were used to test and debug the RF aspects of the modules. HyperTerminal was used to send messages and text files to the board for transmission.

Other Software – Various custom programs were written throughout the programming process. They were generally written in C or BASIC, and used for testing or demonstrating module functionality.

It is important to note that the wireless module does not simply pass every received byte between the USB and RF links. Some minor data parsing is required to detect and act upon module control commands (such as changing the RF channel, or requesting a data rate measurement). A custom protocol needed to be designed and agreed upon by all collaborating wireless teams including the test bench team.

A protocol was developed, in which module commands would be sent as serial blocks in the following format:

Command Header - 1 byteCommand Code - 1 byteData Length - 2 bytes (if applicable)

Data - variable length (if applicable)

The command header is one of three special ASCII characters, DC1, DC2, or DC3 (0x13, 0x14, 0x15) depending on whether the command is being sent from PC to module, module to module, or module to PC, respectively. Special “non-printable” ASCII characters were chosen to prevent textual data from accidentally appearing as the header of a new module command.

The command code, a single alphanumeric ASCII byte, indicates the command being initiated. For PC to board commands, it can be one of the following:

C - set RF channel numberN - set module ID number (P11207 specific)D - start of a one-way data transferI - start of ‘important’ data transferL - start of a ‘loopback’ data transferR - requests transmit time of last sent packet

For board to PC commands, the code is one of the following:

D - normal data being receivedA - acknowledged (mirrored back) data receivedT - transmit time (microseconds) of the last packet sentE - indicates an error has occurred

Any data command codes are followed by a 16-bit length of the following data (in bytes, up to 64 kB) and the data itself. Normal data (D) is a one-way transfer. Important data (I) is mirrored back from the remote receiving module as an acknowledgement (A) so that the original sender can verify that the correct digital data was received (This could be used, for example, to confirm that a vehicle received the correct coordinates to relocate). Loopback data (L) is a debugging feature, which writes any received USB data back to the USB port, bypassing any RF activity.

The Set Channel command (C) is followed by a single byte value, 0 to 15, indicating which of the 16 available RF channels to operate on. The Set ID Number command (N) is used specifically for the Mid-Range 1 module, due to the use of the SimpliciTI wireless protocol. A byte value of 0 or 1 is sent, to indicate whether the module is the base or remote unit on the current channel pair. All functionality is the same for each; an ID is simply needed to avoid RF overlap. The transmit time request (R) and response (T) are used by the test bench team’s software to calculate the data rate of the previous transmission.

The goal of this protocol was to remain relatively simple and easy-to-understand, so that future projects can interface with the WOCCS modules, via a



standard serial COM port, through their own software, tailored to their specific applications.

Once implemented, the module was ready to be tested for functionality and technical performance.

RESULTS AND TESTING

Test procedures were developed which each verify that the wireless module passes one or more engineering specification previously listed. Together, these formed a full test plan, which was executed in the final stage of the project. All test results were recorded properly, and every specification was found to be confirmed, although some were near the border between passing and failing.

Many of the physical tests were completed early and easily. Dimensions of the assembled board were measured, and were within the specified tolerance of 90 mm long by 70 mm wide and less than 30 mm tall. Total board mass was measured less than 300 grams. Interface specifications (in the mechanical sense) were tested by verifying the placement and orientation of all PCB connectors, and physically checking that the board fit in its enclosure correctly.

One of the module’s specifications was reliable operation across a given temperature range (0 to 65 C). Due to difficulties in creating a stable test environment at these temperatures, the spec was instead confirmed by analysis, by checking that all PCB components were individually rated to operate in this range.

Maximum power consumption was found by measuring supplied voltage and current while the module was continuously transmitting at full power. It was measured well below the specified 330 mW.

A 1.5 GHz spectrum analyzer was used to directly monitor the RF transmission frequencies and power levels. In compliance with FCC regulation, more than 99% of transmitted power fell within our Region 2 ISM band (902 to 928 MHz) and the maximum power across this band was less than 0 dBm (1 mW).

Module cost was calculated as the total cost of module design, assembly, and testing divided by the number of produced modules, and was less than the maximum of $75 per module.

The remaining tests involved the actual functionality of the module. Correct coexistence between the 16 available RF channels was confirmed through demonstration: all adjacent pairs of channels were

operated in a small room, and no channel interfered with another channel’s communication.

The operational range of the module was tested against a maximum bit error rate (BER) at a set distance of 100 meters. RIT’s Gordon Field House was used as the location for all 100 m line-of-sight testing. One hundred packets were sent from one module to the other, and TI’s SmartRF Studio software was used to determine the percentage of errors, which remained below the specification of 0.5% at 100 m.

Transmitted data rate was the final and most difficult specification to meet. The marginal specification was 40 kbps (or 5,000 payload bytes per second). Initial measurements gave unreliable results, depending on the size and type of data being sent to the module. The microcontroller software was revised numerous times to improve stability and maximum data rate. Eventually, a rate of 40.3 kbps was attained, by sending plain text files through HyperTerminal, a standard Windows application. Custom software was used at the receiving end to monitor the data rate.

CONCLUSION

The WOCCS project has been successful in designing and producing a small-scale wireless system.

A full process consisting of concept generation to meet customer needs, technical design, purchasing, assembly, programming, testing, and documentation was completed. The modules can be used in future projects as a portable, medium-range wireless communication link. The module is easy to interface with and adaptable to many diverse remote applications.

RECOMMENDED FUTURE WORK

Given more time allocated to this project, many issues could have been resolved, most specifically the data transmission. The module sends data too quickly to read, resulting in missed packets. This issue could be detected with time and resolved. Also, minor hardware mistakes present both in the schematic and PCB layout could be corrected for a second hardware implementation.

ACKNOWLEDGEMENTS

Special Thanks and acknowledgments go to all of the individuals who have aided in the design of the mid-range RF module. In particular we would like to extend our thanks to Jeffrey Lonneville from the SMT laboratory for his time and aide to train the team members in PCB assembly. The team would like to

Project P11207


also thank Dr. Venkataraman for reviewing the initial PCB design and offering her advice and her equipment for design and testing of the module.

REFERENCES

[1] Richard Wallace., 2009, “Johanson Technology Matched Balun Filter optimized for CC1101 868/915” MHz

[2] Texas Instruments, “CC1111 USB Dongle Reference Design Schematic”, 2009.[3] Texas Instruments, “CC1111 USB Dongle Reference Design Layout”, 2009.[4] AFAR Communications, Inc., “FCC Rules for Unlicensed Wireless Equipment operating in the ISM bands”, 2010.[5] Wikipedia.org, “Friis Transmission Equation”, 2011.


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