Final Report MAL-9000

Transcription

1 Final Report MAL-9000 Abstract: This paper describes the second and final stage of development of a Musical Automated Lifeform (MAL) at the University of Illinois at Urbana- Champaign. It explains design choices and gives an overview of the hardware and software implemented to play a six-string guitar. John Pritz & Michelangelo Scafidi Spring 2012

2 Table of Contents 1. Project Overview Description Outside the Scope of this Document Design Choices The Original Plan Issues that Prompted Change Hardware Micro-controller Servos Printed Circuit Board (PCB) The Guitar Cradle Software Micro-controller Code song.h tab_parser.py How to use all of these parts together Tab Format Description Commands Examples Control Word Description Command Pattern Examples Future Work Credits Contact Information... 9 Appendix

3 1. Project Overview 1.1 Description MAL-9000 is a Musical Automated Lifeform with one purpose: to play guitar. The project was broken up into two stages. The first stage consisted of implementing all of the hardware and software necessary to achieve reasonably competent play on the low E string on a six-string guitar. The success of this stage was recorded in the stage one document. The second stage consists of expanding the first stage into a system that can play all six strings of a guitar. The details of this second stage are covered in this document. A Variax Line 6 guitar was used as the subject to be played. The guitar is secured in a custom made cradle. The cradle contains servos and arms that perform the fretting and strumming operations that produce music. The servos are attached to six printed circuit boards fed with a 5 Volt power supply. The PCB's interpret the output of a micro-controller and feed the servos the proper square waves to control their positions. The micro-controller runs C code that uses commands from a header file (song.h). The header file contains a tempo, the length of the song, and an array of commands that indicate a fret/strum/delay action. A python script, tab_parser.py, can interpret a more humanreadable tab of a song and produce a corresponding header file. 1.2 Outside the Scope of this Document This document does NOT: Teach the tools associated with interacting with a STM32VLDISCOVERY micro-controller evaluation board Teach any of the special C functions associated with GPIO (general purpose input/output) pin interaction on this board. Detail the specific implementation of the software components. 2. Design Choices This section details trade-offs associated with various design choices. 2.1 The Original Plan Fretting was to be implemented with solenoids and strumming was to be implemented with either additional solenoids or stepper motors. I considered creating the solenoids myself to save money. Because of its large number of outputs, I intended to use an Altera DE2 FPGA board to control the fretting and strumming. One FPGA output would control one fret or the strumming of one string. The fretting logic was going to support up to 32 frets for simple expansion up to that point should the system be modified to play some other arbitrary device. 2.2 Issues that Prompted Change 2

4 Solenoids have two measures of force: pushing force and holding force. To test the viability of using a solenoid I pressed an active solenoid against a guitar string, and it seemed like it would hold the string down just fine. The holding force met my requirements. The pushing force did not. When pushing the solenoid could not achieve enough force to activate a fret. The solenoid I used for these rough experiments was larger than would be reasonable for my design. Therefore, I could conclude that solenoids of acceptable size could not produce the force I needed. I then tried a servo. The servo I used for my experiments met my force requirements and was much cheaper than a solenoid. I decided that instead of using solenoids and stepper motors I would use servos. FPGA boards are expensive. I decided to use a micro-controller. Since this decreased the number of outputs I could no longer have one output control one action. Instead, I decided to collapse the output signals into select signals and a few commonly used other signals that would choose an action to perform in a given time step, and to have some logic outside of the micro-controller that would be an interface between it and the servos. To cut down on breadboard area and micro-controller outputs I limited the fretting to 12 frets. This gives each string an octave, but also keeps the physical area of the fret large (as the fret number increases the area of the fret on the fretboard decreases). When hooking elements up on the breadboards, I realized that power was not being distributed properly because the wired connections are not ideal. This was fixed by shifting some wires around, but stage 1 was still a power hog. In the second stage of the project I designed a printed circuit board. When designing the circuit board I picked a shift register rather than flip-flop chips and relays rather than MUX's to lower the chip count. Per string, the chip count reduced from 27 to 9. The PCB's are more stable and easier to manage than the breadboards. I was able to lower the amount of power consumed drastically through a trick in sending position information to a servo at 50 Hz rather than Hz. This method is described in the section about the servos. For the square wave signals a pair of inverters are used as buffers because the micro-controller cannot fan-out as much as it needs to on its own. The HiTEC HS-311 servos burned out after some use. I replaced them with HiTEC HS-322HD servos, which have stronger gears inside of them. 3. Hardware 3.1 Micro-controller The micro-controller used in this project is a STM32 Value Line Discovery Evaluation board (STM32VLDISCOVERY). For the first part of the project the micro-controller was powered by through its USB interface plugged into a PC. The board has 45 GPIO pins (general input/output), 12 of which seem to be dedicated to a specific purpose. That leaves 33 useful GPIO pins. The following outputs are needed to control the breadboard logic: 3

5 4 shift register control lines for each string = 4*6 = 24 outputs One strum output for each string = 6 outputs Two square wave outputs that describe an active position (strummer in one position or fret pressed down) or an inactive position (strummer in the other position or fret not pressed down) to a servo. This means that 32 of the micro-controller outputs will be used. When active, the micro-controller runs C code detailed in Section 4.1. Micro-controller outputs Purpose String E (low) String A String D String G String B String EE/e (high) STRUM C6 B5 C4 A2 A8 A3 Square Wave 1* A1 A1 A1 C7 C7 C7 Square Wave 2* C7 C7 C7 A1 A1 A1 R/W C10 B6 C5 A4 A9 C0 SHCP_NOT C11 B7 B0 A5 A10 C1 STCP_NOT C12 B8 B1 A6 A11 C2 SHIFT_IN D2 B9 B2 A7 A12 C3 * The square wave wires could be flipped, and this flip could be accounted for in software by setting the default servo positions to the inverse of their present values. 3.2 Servos See Figure 1. I used HiTEC HS-322HD servos for the fretting and strumming. Here are a few specs from the spec sheet: Control System: +Pulse Width Control 1500usec Neutral Required Pulse: 3-5 Volt Peak to Peak Square Wave Operating Voltage: Volts Operating Temperature Range: -20 to +60 Degree C Operating Speed (4.8V): 0.19sec/60 at no load 4

6 Operating Speed (6.0V): 0.15sec/60 at no load Stall Torque (4.8V): 42 oz/in (3.0 kg/cm) Stall Torque (6.0V): 51 oz/in (3.7 kg/cm) Current Drain (4.8V): 7.4mA/idle and 160mA no load operating Current Drain (6.0V): 7.7mA/idle and 180mA no load operating Dead Band Width: 5usec Operating Angle: 40 Deg. one side pulse traveling 400usec Direction: Clockwise/Pulse Traveling 1500 to 1900usec Motor Type: Cored Metal Brush Potentiometer Drive: 4 Slider/Direct Drive Bearing Type: Top/Resin Bushing Gear Type: Karbonite 360 Modifiable: Yes Connector Wire Length: 11.81" (300mm) Weight: 1.52oz (43g) The inputs to this device are ground (black), power (red), and control (yellow). The control signal should be fed a square wave to determine what position the solenoid should be adjusted to. The solenoid will rotate to the indicated position and hold there. It is limited to a rotation of 180 degrees. To save power, you do not need to send it a square wave with a 50% duty cycle. Sending an active high signal of the proper duration at 50 Hz is what I ended up doing. This lowered the amperage for one string controlled through a PCB from 5 Amps to 1.5 Amperes. Two strings maxed out at two Amperes, and three strings did not exceed 2.7 Amperes. 3.3 Printed Circuit Board (PCB) Six PCB's were made: one for each string. The printed circuit board logic is the interface between the micro-controller and the servos. It contains a shift register that remembers fret positions and relays/multiplexors and inverters to interpret which fret may change orientation and to determine what square wave should go to a given servo. The following chips were used: 74F675 (16 bit shift register) HD74LS04P (Hex Inverter) CD74HC4053E (3 single pole double throw bilateral switch) SN74ALS257AN (quad 2:1 mux with a common select line) The cradle is built such that the square wave for not pressing a fret and the square wave for pressing a fret is flipped between frets 1-6 and frets 7-12 (See images of the cradle in the Appendix to see why this is). The PCB is wired to automatically handle this problem, so feeding the shift register all zeros to start with sets the servos to not press down any frets. The servos for strings D and G need to be 5

7 initialized in a special manner. A possible change to the PCB would be to have the square waves all routed the same way and just adjust for different servo rotations completely in software. See figures The Guitar Cradle The cradle was constructed by the University of Illinois ECE machine shop. See figures Software This section describes the various pieces of software associated with this project and how they all work together. 4.1 Micro-controller Code This code is responsible for interpreting song.h and sending the appropriate control signals to the printed circuit boards. It begins by initializing the output pins. Then it initializes the timers that will drive the square waves and maps them to output pins. After all of the GPIO (general input output) pins are initialized the main function enters an infinite loop that interprets song.h and plays the song by sending out the proper signals to the PCB logic. Once the end of the song is reached the software immediately starts over at the beginning with the first command word in the song array (in song.h). At this time no reset is done when a song repeats. 4.2 song.h This file describes a song. It has three parts: 1) BPM (beats per minute) is a tempo description used by the Micro-controller code to determine the appropriate delays to set between time steps. 2) song_len is the number of command words in the song array 3) song is a character array that contains control words that the Micro-controller code interprets to send out signals to the breadboard logic. The format of the control words is described in a later subsection. 4.3 tab_parser.py This script runs through a tab text file and outputs a song.h file. The tab format is described in a later subsection. BPM is set to a default value which can only be manually changed at this time. 4.4 How to use all of these parts together The simplest way to write a song is in the tab format described in a later subsection. After a song is written use tab_parser.py to convert the tab to a song.h file. This song.h file will then be included into the micro-controller code to build the elf file that will be exported onto the microcontroller. Once the elf file is on the micro-controller, you can press the RST button to reset the state of the micro-controller and start the code from the beginning. 6

8 4.5 - Tab Format Description This tab format exists to make it easier to write a song for the guitar robot to play. It contains shorthands for what I consider to be common strings of commands as well as basic commands that can be used by more advanced users to achieve complicated behavior Commands Note: '#' in this chart indicates a valid fret number. In my implementation this number can be indicates the open string and 1-12 indicates frets. Command Format Example Description of Action # 3 Plays a specific fret. Toggles the fret position, strums the fret, toggles the fret position (press down, strum, pull off) #h# 3h4 Does a Hammer on Toggles the first fret, strums, toggles the second fret, delays, toggles both frets (press down first fret, strum, hammer on second fret, delays, releases both frets) #p# 6p3 Does a Pull off Toggles the first fret, toggles the second fret, strums, toggle the first fret, delay, toggle the second fret (press down first fret, press down on second fret, strum, pull off first fret, delays, releases second fret) \# \7 Toggles fret depression '# '6 Plays a specific fret but does NOT release afterward. s s Strum r r Rest After any command you can append a delay marker. In this way you can extend an action. For example, 4#3 plays fret 4, and delays for three times as long as it normally would. r#4 would rest for four times as long as it normally would. All commands delay for one time step by default. For some commands this delay can be removed. They are '#, s, and \#. After writing the command, it needs to be assigned to a string number (1-6). String 1 corresponds to the bottom or High E string and string 6 7

9 corresponds to the top Low E string. In order to do so, it is written as #;command, where # is the string number that the command is to be played on. A ; is used to separate the command from the string number. If we wish to play multiple commands at the same time on multiple strings, it is written as ;{command1, command2, command3, command4, command5, command6}, where each command is designated to its respective string number 1-6. If you do not wish to play a command on a particular string, simply replace command# with x. To delay multiple commands playing at the same time, we can add the delay symbol at the end of the command string. For example, to delay a string three times as it would normally, it is written as ;{command1, command2, command3, command4, command5, command6}# Examples Daisy Bell Ode to Joy 6;12#3 6;9#3 6;5#3 6;0#3 6;2 6;4 6;5 6;2#2 6;5 6;0#4 6;7#3 6;12#3 6;9#3 6;5#3 6;2 6;4 6;5 6;7#2 6;9 6;7#4 6;4 6;4 6;5 6;7 6;7 6;5 6;4 6;2 6;0 6;0 6;2 6;4 6;4#2 6;2 6;2 6;4 6;4 6;5 6;7 6;7 6;5 6;4 6;2 6;0 6;0 6;2 6;4 6;2#2 6;0 6;0 6;2 6;2 6;4 6;0 6;2 6;4h5 6;4 6;0 6;2 6;4h5 6;4 6;2 6;0 6;2 6;7 6;4 6;4 6;5 6;7 6;7 6;5 6;4 6;2 6;0 6;0 6;2 6;4 6;2#2 6;0 6;0#8 4.6 Control Word Description These control words sit in the array song in song.h Command Pattern See figure Examples 0x#00 Rest for one timestep 0x#80 Strum, delay for one timestep 0x#15 Toggle Fret 5, delay for one timestep 0x#9A Toggle and strum fret 10, delay for one timestep 0x#58 Toggle fret 8, do not delay for the timestep *Note: # represents which string number that command should be played on. 8

10 5. Future Work A possible change to the PCB would be to have the square waves all routed the same way and just adjust for different servo rotations completely in software. A dedicated power source is needed. When a song reaches its end and repeats, reset the state of everything. 6. Credits ECE 395 Staff: Thank you Professor Lippold Haken for helping me to plan the project to fit within the time constraints of two semesters and TA Zuofu Cheng for his miscellaneous advice. ECE Machine Shop: Thank you Scott McDonald, the Research Lab Shop Supervisor and David Switzer, the Senior Lab Mechanic who constructed the guitar cradle. ECE Electronics Service Shop: Thank you Shop Manager Dan Mast for finding me cheap servos and helping me find the TTL parts that I needed. Thank you to my father, Michael Pritz, for exposing me to 2001: A Space Odyssey. The first song played on MAL-9000 was Daisy Bell. Thank you to Lacey O'Brien, who came up with the words to make MAL an acronym. 7. Contact Information Future ECE 395 students, or anyone who is interested, can contact me at johnpritz@gmail.com with questions. 9

11 Appendix Figure 1: HiTEC HS-322HD Servo Figure 2: The gist of the PCB Logic 10

12 Figure 3: The PCB Layout Figure 4: A photo of one of the PCB's 11

13 Figure 5: A PCB hooked up to the micro-controller (wires sticking out the top), servos (yellow, red, black wires), and power (green and red wires in the lower left). Figure 6: The cradle with the G, B, and high E string wired to play. 12

14 Figure 7: The cradle during construction. Figure 8: Detail on fretting servos. 13

15 Figure 9: Detail on strum mechanism. Figure 10: Another angle of the strumming mechanism. 14

16 Figure 11: Bit descriptions for the command pattern. 15