Understanding the Arduino to LabVIEW Interface

Transcription

1 E-122 Design II Understanding the Arduino to LabVIEW Interface Overview The Arduino microcontroller introduced in Design I will be used as a LabVIEW data acquisition (DAQ) device/controller for Experiments 1 and 2 in Design II. The Arduinos provided for the experiments will be preloaded with the necessary software functionality. No programming of the Arduino microcontroller is needed. However, for LabVIEW to communicate with the Arduino you will need to download and use within LabVIEW a predefined subvi (a subroutine). This document describes what you need to do and know so that LabVIEW can communicate with the Arduino during Experiments 1 and 2. It further overviews some things you should consider if you later plan to use the Arduino DAQ in your major design project. In order to use the Arduino as a data acquisition device/controller connected to LabVIEW, the two must be able to communicate with each other. To do this a common physical hardware interface and a common software communications protocol (the language each will use to communicate) must be available. The physical hardware interface is readily available via a standard USB connection. The software communication protocol that enables LabVIEW to communicate with the Arduino was custom designed to support our Design Lab activities. This protocol is implemented in software that resides on both sides of the USB communications link, i.e. in both LabVIEW and the Arduino. It has been designed to operate according to a master/slave relationship. This means LabVIEW (the master) will always be the one to initiate a conversation on the link. The Arduino (the slave) will always be listening, waiting to get a command from LabVIEW (i.e., send me data from analog channel 1.) The protocol does not support the Arduino initiating a communication with LabVIEW. This orderly approach greatly simplifies the interface design and must be kept in mind when designing your projects. Details of the protocol are described in this document. The Arduino Side of the Protocol There is nothing you need to program or load into the Arduino. It will be preloaded with the necessary "firmware" that implements the protocol to listen for LabVIEW commands on the USB link and respond accordingly. Once powered, a blinking yellow LED on the Arduino board is an indication that the protocol software is loaded and is running. The LabVIEW Side of the Protocol The second half of the interface is coded in a LabVIEW subvi, or if you were using a text based program language, a subroutine that will be called in the LabVIEW program when communication with the Arduino is required. The subvi has inputs and outputs which can be wired into your LabVIEW program. The subvi is named Arduino_Communication_SubVI.vi and it must be downloaded onto your computer. You can download the SubVI under this module in Canvas: Weekly Classwork -> Weeks 3&4 -> 1a. Arduino SubVI Download. Remember where you store the file on your laptop, as you will need the path later to bring it into LabVIEW.

2 To load and use the Arduino subvi: 1. Open LabVIEW and select "File, New VI". A "Block Diagram" and a "Front Panel" window will appear. 2. On the block diagram, get the Functions palette by right clicking. Then point and click at the "Select a VI" option at the bottom of the Functions palette and browse to the "Arduino_Communication_SubVI.vi" file you just saved. Click OK and drop it to the block diagram. The following icon will appear: When you place your mouse over the icon, five wiring/attachment points will appear. Note the colors of these attachment points. Colors within LabVIEW indicate data type (ASCII string, Boolean, integer number, floating point number, etc.) and connecting things with different colors typically will cause an error. See the LabVIEW quick reference guide for more information on data types and colors. 3. Right click on the icon and uncheck "View as Icon. You will get a more descriptive box that better describes the connection points. This box should be "stretched" down to expand it, and also right click on this box and select "Size to Text" to properly size the box. You are now looking at the subvi that is used to communicate with the Arduino. It should look like: To use it, as with any other LabVIEW program, you add controls and indicators on your Front Panel and then "write" your program in the "Block Diagram" by wiring various icons together according to your program design, including communicating with the Arduino. The inputs to the subvi, noted by in arrows on the left, are: 1. COM Port to use on the host (laptop or desktop) computer. This COM Port input will be connected to a drop down selection icon on your front panel that will identify all active COM ports on your computer and allow you to select which one is connected to the Arduino. 2. Message (command) to send to the Arduino. All characters in the Message data type must be ASCII characters. If your front panel inputs a number that will become part of the message, you must convert it to a string character (ASCII) before passing it to the subvi.

3 The outputs of the subvi, noted by out arrows on the right, are: 1. Arduino Reading, the requested data (if any) from the Arduino. This will return to LabVIEW the value on an analog-to-digital converter (ADC) channel (integer data approximately in the range of corresponding to 0-5 volts), or the status of a digital I/O pin (a one or a zero) as requested by the command string sent to the Arduino. The data output from the subvi is a string". That is, it is ASCII data, not a number. It could be converted to a number if desired by your program. 2. Data Input Error - a Boolean output that indicates the data string you tried to send did not meet the built in range checks for the specified bit or channel. For example, you may have requested analog data from analog channel 7, but the valid range is 0 through 5. If a data input error occurs, that string is "thrown away" and not sent to the Arduino. 3. Hardware Error - a Boolean output that indicates a message to/from the Arduino was not successful within the allotted time (I.e. timeout of 3 seconds). This most probably indicates a USB cable fault (disconnected) or the wrong COM port has been selected.. You can (and should for practice) exercise this subvi directly. This subvi has its own Front Panel associated with it; however the subvi s front panel must not be used when you write your own programs. It should only be used to exercise the subvi as an aid to understanding how it works. Typically if you were designing your own subvi you would use the subvi s front panel to verify its design and operation. For the experiments and project in this course, you will write your own programs, which means you will create your own front panels. To see the subvi s front panel, double click on the icon. To see and analyze the program behind the subvi, its Block Diagram, press Control E when the SubVI front panel is open. Be careful not to change the program in the block diagram, otherwise it may not work. Note the "case" structures, the gray boxes. Click on the arrows on top of the boxes to see alternate execution paths based on the input to the case. Right click on just about anything to get "help" on the item. LabVIEW help is excellent and also steers you to examples for a particular feature. The Protocol (Communication Language) Protocols across a USB serial communications line are designed using a series of ASCII characters that are interpreted on each side of the link. Our protocol currently supports five types of commands: 1. Read an analog input channel on the Arduino 2. Set a digital output pin (either high or low) 3. Read a digital input pin 4. Set the speed, direction or on/off status of a motor output 5. Set a digital output pin to function as a Pulse Width Modulated (PWM) control signal The following defines the specific rules of the protocol: The string to write must adhere to the following rules, or else you will get a "DATA INPUT ERROR" from the subvi: 1. To read an analog input (similar to the readadc ( ) function we used in Design I) the first character must be an 'A (for Analog) and the second character must be a number in the range of 0-5, for analog pins 0-5 on the Arduino. So to read analog channel 0, send an A0 character string. For alpha characters, upper or lower case is accepted. The total string length must be two characters. The Arduino Reading window will show the data sent back.

4 2. To set a digital output (similar to the outputlow ( ) and outputhigh ( ) functions in Design I) the first character must be a 'D' (for Digital), the second character must be a 'O' (letter O, not zero for Output), the third (or third and fourth) character(s) is a number either in the range of 4-9, 10 or 12, corresponding to the Arduino digital pin, and the last character is either a 0 (zero) or a 1 to indicate whether to set the bit low or high. So to turn on the red on-board LED at pin 10, send a DO101 character string to the Message input of the subvi. The total string length must be four or five characters, depending on the number of characters of the pin index. 3. To read a digital input (similar to readinput ( ) used for bumpers in Design I), you need to specify D for Digital, I for Input, and a number 2-9 indicating the pin index. So to read the data on pin 2 (which is normally pulled up to 5 volts) send a DI2 character string. The total string length must be three characters. The state of the specified input pin, a one or a zero, will be returned in the Arduino Reading window. 4. To set the status or speed of one or both of the motors controlled by the serial motor controller output (similar to the motors ( ) function in Design I), the first character must be an M for Motor. The second character (Motor #) must be a 1 or a 2 corresponding to the two available motor circuits, or a B indicating the command will be sent to Both motor circuits. The third character (Motor Status) must be either an A or B (where A and B correspond to the motor s spin direction, based on the your wiring), the letter O (not a zero), which means to turn the specified motor(s) Off, or an X meaning to do nothing with the status of a motor(s), i.e. we may just want to change speed. The final one, two or three characters (Motor Speed) must be a number in the range of 0-255, corresponding to the speed you want to set on the specified motor(s). Where 0 indicates no modification to the current operating speed, i.e., we may just want to change Motor Status, and 255 indicates the maximum/full speed. The three character Motor Speed data can be represented as X, XX, or XXX (i.e., 9, 09, 009) as long as it is within the range of The total string length for the motor command must be four, five or six characters, depending on the number of characters in the Motor Speed parameter. As examples, to set the speed on motor #1 to approximately 50% without changing its current status (i.e., on/off or direction), send a M1X127. To turn both motors off while leaving their individual speed settings unchanged, send a "MBO0". Also note that the Design II protocol allows for greater resolution on motor speed control than Design I did during the robot project. There are now 255 speed steps to full speed versus only 100 steps in Design I. This will enable more precise speed control, especially during Experiment To set a digital output pin to function as a Pulse Width Modulated (PWM) control signal the first character must be a P for PWM. The second character must be 5, 6 or 9 corresponding to the allowable digital pins on the Arduino that can support PWM. The final one to three characters must be a number in the range of corresponding to the desired PWM duty cycle (speed), 255 representing full on (maximum input VDC) and 0 representing full off (0 VDC). Like the motor command, speed can be represented as X, XX or XXX. The total string length for the PWM command must be either three, four or five characters, depending on the number of characters in the speed parameter. As an example, to set pin 6 as an output at 50% duty cycle send P6127. THE PWM FEATURE CAN BE USED TO CONTROL THE SPEED OF AN ADDITIONAL THIRD MOTOR, HOWEVER IT IS IMPORTANT TO NOTE THAT THE PWM DIGITAL OUTPUT FROM THE ARDUINO CAN ONLY BE USED AS A CONTROL SIGNAL. IT CANNOT SOURCE CURRENT DIRECTLY TO DRIVE A LOAD, SUCH AS A MOTOR. CONNECTING A MOTOR OR OTHER LOAD DIRECTLY TO A DIGITAL OUTPUT PIN WILL INSTANTANEOUSLY BURN OUT THE ARDUINO MICROCONTROLLER ($$$).

5 THE PWM CONTROL SIGNAL CAN ONLY BE USED BY CONNECTING IT TO THE BASE OF A TRANSISTOR THROUGH A 5K-10K OHM RESISTOR. THE TRANSISTOR WILL FUNCTION AS A HIGH CURRENT SWITCH TO DRIVE THE MOTOR. (Similar to the Floor Sensor Module circuit used in Design I.) NEVER USE A PWM DIGITAL OUTPUT PIN WITHOUT FIRST HAVING YOUR CIRCUIT APPROVED BY YOUR INSTRUCTOR. Table 1. summarizes the valid command sequences that can be sent to the subvi. Remember, all command sequences must be sent as ASCII characters and all data returned by the subvi is in ASCII. If other than a valid command sequence is presented to the subvi, it will ignore the command and return a "Data Input Error". Command First Parameter (*) Second Parameter (*) Third Parameter (*) Fourth Parameter Read an Analog Input A Set a Digital Output low or high D O 4-9, 10, 12 0 or 1 Read a Digital Input D I Control one or both of the two motors connected to the Serial Motor Controller Board (i.e., the robot wheel motors) Set a Digital Output to function as Pulse Width Modulation "control signal" (to possibly regulate the speed of a third motor) M 1, 2, B A, B, O, X P 5, 6, Table 1. Valid Input Strings (ASCII) for the "Arduino_Communication_SubVI" (*) Note: Any letter can be either lower or upper case.