By Trent Jarvi, MathWorks
Many MATLAB users who analyze data, develop and verify algorithms, or create software applications leave the MATLAB environment to import data from external hardware. This extra step creates inefficiencies by forcing the user to manually transfer data from one environment to another and to learn and maintain multiple software tools.
Using a Garmin handheld Global Positioning System (GPS) receiver with an RS-232 serial interface as an example, this article demonstrates how you can use MATLAB and Instrument Control Toolbox to work with diverse external devices without leaving the MATLAB environment. It explains how to communicate with a serial device, interact with the device in MATLAB, and automatically generate a MATLAB script that can be shared, reused, or incorporated into a MATLAB application.
The MATLAB scripts used in this article are available for download.
RS-232 serial devices, such as GPS receivers, and instruments, such as oscilloscopes and signal generators, can be configured and controlled in MATLAB with Instrument Control Toolbox (Figure 1). Our example uses a consumer-grade Garmin ETrex 12-channel GPS device, typically used while hiking and boating. The example uses electrical and data communications specified by National Marine Electronics Association (NMEA) 183 communication, which should work with any GPS device capable of serial communication. You could use other protocols, including Garmin’s proprietary protocol, if you have access to the specifications.
To find out what serial devices and instruments are available for communication, we can use MATLAB command-line functions, such as
serial. Alternatively, we can use TMTool, a graphical user interface provided by Instrument Control Toolbox that lets you locate, configure, and control serial devices and instruments without writing MATLAB script (Figure 2). First, we will explore available hardware assets using a hierarchical tree.
Communicating with a device to acquire data often requires establishing a connection to its communication interface. With TMTool we can configure, control, and acquire data from devices through well-established interfaces, including serial, GPIB, TCP/IP and UDP (Figure 3). We can also use TMTool to communicate with instruments using VISA, LXI, or IVI and VXIplug&play drivers.
In Figure 4, the serial node has been expanded to show the serial ports available for communication. The GPS is configured to broadcast NMEA format data, a common option in consumer-grade GPS receivers.
After connecting the GPS receiver to COM1, we select the Configure tab for COM1 and set it to match the serial port parameters. NMEA defines these parameters as baud rate 4800 bits per second, 8 data bits, 1 stop bit and no parity.
After configuring the serial port parameters, we open the connection via the serial port. The Connection status changes to Connected, indicating that communication with the GPS through the Communicate tab is now possible. The NMEA standard specifies that data must be transmitted as ASCII characters in new line-terminated strings. Using the dropdown menu, we set the “Receiving data” parameters to comply with this specification.
We can now send data to the hardware and read the responses. For our example, all we need to do is read the data broadcast from the GPS. We can confirm that we are communicating properly and that the data of interest is available by clicking the Read button several times to observe that properly formatted strings are being acquired in the sequential log. After a few attempts, the GPS coordinates of interest appear in the results, together with several other NMEA data types. Finally, we close the connection to the GPS device to capture the disconnect as MATLAB script in the session log for later reference.
Once we have configured the port, read the strings, and acquired the data in which we are interested, we can repeat this series of tasks in the future by simply exporting the generated script to a MATLAB file called
get_gps_location.m. We can then execute the MATLAB script that was automatically generated when we interacted with TMTool (Figure 6).
So far, we have configured the hardware and acquired the data, but some of the data that we acquired, such as waypoints and time information, is not specific to the GPS location. With TMTool, we can automatically filter out this unwanted data, just as we automated the hardware configuration and data acquisition by adding MATLAB script to a MATLAB instrument driver. TMTool automatically updates the MATLAB script generated in the Session Log tab. Once we have exported the updated script
MATLAB_GPS_example.m, we can insert MATLAB routines to filter the data stream.
% Create a serial port object.
obj1 = instrfind('Type', 'serial', 'Port', 'COM4', 'Tag', '');
% Create the serial port object if it does not exist
% otherwise use the object that was found.
obj1 = serial('COM4');
obj1 = obj1(1);
% Set BaudRate at 4800.
obj.BaudRate = 4800;
% Connect to instrument object, obj1.
% We only want the strings with $GPGLL…
data = fscanf(obj1);
% Parse the string to obtain coordinates
[lat,data] = strtok(data,',');
[lat,data] = strtok(data,',');
[nsCardinal,data] = strtok(data,',');
[long,data] = strtok(data,',');
[ewCardinal,data] = strtok(data,',');
lat = str2double(lat);
long = str2double(long);
% Display the results
% Free the serial port
To acquire and process the GPS data, simply execute the
MATLAB_GPS_example.m file on the command line.
While the MATLAB workflow discussed so far will be sufficient for some applications, others might benefit from incorporating the MATLAB script into a reusable driver. We can incorporate the MATLAB script into a reusable driver by using MIDEdit, a driver development tool in Instrument Control Toolbox (Figure 7). MIDEdit lets you incorporate lower-level commands into higher-level commands that are easier to access. After launching MIDEdit from the command line, we create a new generic instrument driver through the File->New context menu.
To demonstrate the use of a self-contained, reusable driver in MATLAB, we will add just two pieces of functionality to the driver: configure the GPS device and acquire the GPS location. With MIDEdit we could also create more advanced drivers comprising hundreds of lines of code.
To configure the GPS device, we browse to the automatically generated MATLAB script in the TMTool session log. This script sets the baud rate and then opens the port. We can now copy this information into a driver. Within MIDEdit, we select the Connect tab from the Initialization and Cleanup node and set the Function Style to M-Code. We then paste the script from TMTool into the function created in the driver editor. Noting that the serial interface may be obtained from the variable obj passed into this function, the MATLAB script is adjusted to use the serial port interface.
To acquire the GPS location, we select the Function node in MIDEdit and add a new function called
getLocation(). This function will obtain the latitude, longitude, and cardinals. In the MATLAB code editor pane, we paste in the MATLAB script that reads the NMEA lines. As in our first example, we modify the function to return an array representing the GPS location. We add logic that instructs
getLocation() to find only the GPS coordinates in the NMEA stream and to return the location. More functions could be added for each type of NMEA information found in the stream. Finally, we save the driver to our workspace as
The driver is now ready to be used with the GPS device. We use TMTool to expand the Instrument Drivers node and create a new interface object by right-clicking the Interface Objects node under the Instrument Objects node. We select a serial port interface object and set the port to the serial port to which the GPS is connected. The interface object could be configured, opened, and read using the process described in the previous section, but this time we will use the instrument driver to perform our acquisition and analysis tasks.
We select the Instrument Objects node and enter the information for the instrument driver
nmeareceiver.mdd by right-clicking and selecting new Device Object. We then connect the GPS to the serial port represented by the interface object. It is possible to change the interface if the device and driver support more than one interface. When completed, the
nmeareceiver instance will appear under the Device Objects node.
We can now connect to the GPS using the Connect button. Under the Functions tab, we can execute the
getLocation() function, resulting in the current location being acquired. We can export the results to the workspace for further analysis.
MATLAB script is automatically generated when you interact with the driver in TMTool. As when working with the GPS device without using a driver, this generated script lets you reuse your work later to communicate with the device.
% Create a SERIAL object.
interfaceObj = serial('COM4');
% Create a device object.
deviceObj = icdevice('nmeareceiver.mdd', interfaceObj);
% Connect device object to hardware.
% Call the getLocation function.
% Disconnect device object from hardware.
We can enhance the reusability of the driver by adding functions, such as the datum being used, date and time, or waypoints. The driver can be shared with users who do not need to understand in detail how to communicate with their device.
We used a GPS device to demonstrate how to configure and acquire data from external hardware without leaving the MATLAB environment. We used MATLAB and Instrument Control Toolbox to communicate with this hardware without writing a MATLAB script, demonstrated how to reuse our work using a MATLAB script automatically generated to communicate with the hardware, and how to incorporate functionality into MATLAB function calls through drivers. Using MATLAB improves work efficiency by eliminating the need to manually transfer data from one environment or learn and maintain multiple software tools.