Texas Instruments Analog Design Contest Oregon State University Group 23 DL Paul Filithkin, Kevin Kemper, Mohsen Nasroullahi 1. Written desription of the projet Imagine a situation where a roboti limb is needed to perform a omplex task in a plae that does not allow human presene. The limb has to be self-learning and has to have a feedbak system to measure its performane. Now imagine one of the nodes seizing up and hindering the entire systems operation. The system has to be intelligent enough to refous its resoures and still solve the task. eonfigurable Dynami oboti Limb (DL) is a researh oriented projet aimed at reating an intelligent roboti platform to solve these hallenges. The platform offers a unique approah beause it is designed out of idential nodes that an be reonfigured to suit a partiular appliation. Due to its ability to reonfigure, DL an be used in a wide array of appliations in the industrial, military, and spae fields. Figure 1: Prototype system nodes (left) Artists rendition of a 10 node system (right) A key advantage of building the system out of idential nodes is the ost redution of manufaturing the limb. Instead of building eah segment out of unique parts, a single design an be mass produed and onfigured to perform a speifi task. If the partiular geometri onfiguration does not meet the intended goal the system is simply hanged by adding, removing, and rotating segments. The platform is generi, but it still offers a method for interfaing a wide range of appliation speifi hardware with the system via a unique appliation speifi node alled the tool. These key traits save time and money in an environment where, for example, a manufaturer needs to quikly reonfigure a small sale assembly line to build a new produt. The limbs intelligent, antonymous operation is also well suited for aerospae appliations where there is signifiant time delay in the ommuniation between the system and the ontrol enter. Eah node is equipped with a miroproessor running a Linux based operating system whih provides loal intelligene and manages the nodes positions, urrent onsumption, and goal progress. Communiation between eah node is wireless and allows eah node to be remotely onfigured and monitored. The wireless ommuniation also adds redundany to the system and ensures that the limb
an still operate if one node in the hain is disabled during operation. The only onnetions required by eah node are power, ground and two wired ommuniation lines for node identifiation. This simple and robust layout allows the limb to operate reliably in harsh onditions where system uptime is ritial. While eah node is idential in terms of hardware, the system implements a hierarhial ontrol struture that governs how the goal algorithm is exeuted. The system uses the onept of a head node, a single node that is responsible for interpreting the goal and keeping trak of the systems progress. Thus, instead of using the onventional approah where eah node is sent a sequene of oordinates, the head node translates the high level goal into lower level ommands speifi to eah slave node. The head node is attahed diretly to the tool so it is able to obtain world feedbak to self orret the rest of the system. As a result slave nodes do not need to have any awareness of the outside world or the overall progress of the system. This abstrated approah is similar to the nervous struture of the human body where the brain generates high level ommands that are routed and interpreted by the nervous system. Similarly, the brain senses world inputs and is able to self orret the bodies ations through a dynami feedbak mehanism. DL makes use of this neural approah and offers a unique system for testing the next generation of intelligent ontrol algorithms. robotis has an indiret and inreasingly diret impat on our daily lives with the potential of improving prodution failities, exploring distant worlds, and improving health are. DL believes that roboti researh like this is ruial to understanding and solving the problems that fae omplex roboti systems of the future. 2. An explanation of EACH TI analog hip used desribing how the hip benefited the design 2.1 Tool signal onditioning design The tool is a ritial part of demonstrating the suess of this roboti platform as it is responsible for providing the head node sensor feedbak data. As a proof of onept the tool is designed to read an infrared signal from four phototransistors with the goal of pinpointing the soure of a modulated signal. This approah demonstrates the limbs antonymous, intelligent operation with the use of a ontrolled signal. The tool is designed to trak two different emitter frequenies and distinguishes eah by their unique frequenies. This is done by passing the signal through a variable band pass filter that is tuned by a miroontroller. The design also uses a variable gain amplifier to alibrate the sensitivity for different lighting onditions. Phototransistor array Mux ontrol Center frequeny tuner Gain ontrol ADC sampling 4:1 Analog Mux Variable bandpass filter Variable gain amplifier Miroontroller Figure 2: Simplified blok diagram of the DL tool design
To save spae, pins, and eletrial omponents eah phototransistor signal is multiplexed through Texas Instrument s 74HC4051E analog multiplexer. This design benefits greatly from this hip beause of spae onstraints and pin limitations, and it offers an elegant solution that requires only a single opy of the filter and amplifiation iruit. The multiplexer input is seleted by a loal miroontroller and the input then passes through a variable band pass filter that an be tuned by the miroontroller. The variable band pass filter is arefully plaed upstream of the amplifier in order to filter any noise before the amplifiation stage. It also maintains relatively high, onstant input impedane for the phototransistor signal whih ensures the phototransistor outputs an optimal signal in the forward ative region. A multiple feedbak topology is used for the ative band pass filter whih offers independent adjustment of filter quality (Q), gain (Am), and enter frequeny (fm). Extensive analysis and simulations were done to on this part of the iruit to ensure adequate filter quality, gain, and bandwidth. In the SPICE simulation (figure 3) _enter is the resistor value that sets the enter frequeny, and _gain is the resistor that sets the gain. Figure 3: SPICE Shemati of variable bandpass filter and variable gain stage Band pass filter harateristis 1 Center frequeny: f = 2π C + 8 enter enter where C = C2 = C3 Gain at enter frequeny ( f ): A = 8 2 Filter quality: Q =πf 8 C where C = C2 = C3 1 Filter bandwidth: B = π C 2 where C = C2 = C3
Final output harateristis: Output gain at enter frequeny: 8 3 AV = 2 Gain Bandpass filter parameters: = 10kΩ = 200 Center Center Ω f 328Hz 1.kHz A -23.5V / V Q 4.9 B 7Hz A V Final output gain at enter frequeny Gain = 10kΩ Gain = 200Ω 27dB 1dB Figure 4: Magnitude bode plot of tool filter onditioning design Figure 3 shows a bode magnitude plot of four ases of operation of the tool signal onditioning design. The Y-axis represents the gain where 0dB orresponds to the unity gain of the previous gain stage. These ases are the minimum and maximum limits of operating ranges that are used in the design. Plot V1 shows the output at maximum gain with the lowest enter frequeny, plot V2 shows the output at unity gain with the lowest enter frequeny, plot V3 shows the output at maximum gain with the highest enter frequeny, and plot V4 shows the output at unity gain with the highest enter frequeny. It is lear from the plots that the gain and enter frequenies are independently ontrolled by enter and gain. In order to easily interfae this design to the rest of the system the iruit needs to operate on a single +3.3V rail and while this simplifies the digital design, finding a suitable OpAmp was an important task. Both the variable ative band pass filter and the variable gain buffer use Texas Instrument s TLV2372 operational amplifier. The TLV2372 was seleted for this design primarily for its low voltage, single supply, and rail-to-rail operating harateristis. It also offers more than adequate bandwidth and slew rate for the input signal. The design benefited greatly from the TLV2372 beause of the OpAmps
ability to operate with a phantom ground as well as the onvenient spae saving dual OpAmp layout. Figure 5: Final signal onditioning design (note: for omplete shematis please view attahed files) 2.2-1.8 Volt regulator In addition to the tool iruit design, Texas Instruments parts were used in ritial supporting roles for the rest of the system. Eah node uses an AM-9 ore miroontroller that performs loal algorithm omputation, network management, and peripheral ontrol. The miroontroller ore inluding the internal memory, main osillator, and PLL ells require a 1.8V power supply. After areful onsideration Texas Instrument's TPS0500 high effiieny step-down harge pump was hosen for the appliation. Due to the stringent layout onstraints on the board the TPS0500's low parts ount iruit greatly aided the design. It is also provides a more than adequate 250-mA of urrent and effiienies as high as 90%. High effiieny operation redued the risk of overheating on the densely populated layout and dereased urrent requirements on the main power supply. Figure : Final shemati of 1.8V regulator
2.3 - S232 Line driver Eah node relies heavily on UAT ommuniation for addressing other nodes, ommuniating with the ZigBee stak ontroller, and outputting a user debug terminal under Linux. In order to interfae the ommuniation to a host omputer a differential S232 line driver was needed. DL hose the Texas Instruments SN75C3221 S232 line driver for this appliation. The part offers flexible operating voltage, 1Mbit bandwidth, small external apaitors and has an easy to follow referene shemati in the datasheet. During the development yle the group depended heavily on this part for testing ommuniation and in the end design the S232 line driver was used for the host ZigBee interfae for host to node ommuniation. 2.4 - Current Sense Differential OpAmp Eah Node requires the ability to understand the effort it is exerting to work with the other robots in the system. It uses this ability as a way to make better deisions about how it should orient itself to redue power onsumption in the system. Current draw from the servos is used to measure the effort of the system and it sampled with Texas Instrument's INA195 urrent shunt monitor. The INA195 offers ample bandwidth with low error exeeding our design requirements. In addition it has a very small footprint with no need for additional passive parts aside from a single deoupling apaitor and the shunt resistor. Overall, the INA195 not only saved valuable board spae but also dereased our development time and inreased our onfidene in the design of the system. Figure 7: Final Current Sense Design 2.5 2.4Ghz PCB Antenna ZigBee/IEEE 802.15.4 is used for inter node and host to node ommuniation in the DL system. DL s ZigBee design uses a 2.4 GHz arrier frequeny and it is ritial for the board to have adequate range while using a ompat low ost 2.4 GHz antenna. After some areful spatial, ost, and performane analysis between system-on-hip, PCB, and dual band antennas DL onluded a PCB antenna would be well suited for the system. Texas Instrument s 2.4 GHz Inverted-F PCB Antenna design was implemented from Audun Andersen s 2.4 GHz Inverted F Antenna (design note DN0007). The antenna is muh lower ost ompared to system-on-hip antennas, and it is more ompat than a dual band or PCB loop antenna. Though Texas Instruments provided detailed designed information about a disrete balun design DL hose to use an on-hip multi layer balun to redue the foot print on the board.
Figure 8: Photo of DL 2.4GHz antenna (left) Shemati of the antenna (right) Without Texas Instrument s parts and design guidane DL would not have been a funtional system. From the ritial Tool design to the important supporting parts in the system we were able to fully realize all of our design goals. Eah design worked as expeted the first time and beause of the exellent performane and doumentation there are no regrets about any of the parts. Texas Instruments has always had high reputation among analog omponents and DL would like to thank them for their high level of quality that made our system suh a suess. For more information please visit: http://oregonstate.edu/~filihkp/