Servo - December 2013

faces north which is 180° out of phase with what you would want if the tablet is mounted on a robot as shown in Figure 1. As another example, it is not inconceivable that someone might prefer to think of north as a specific wall in the room where your robot operates. Since the compass reading should always have a value between 0 and 359, the offset value is so adjusted, if necessary, before placing it into the designated variable.

Figure 3 shows the data displayed when the program of Figure 2 is run. The top of the figure shows the parsed data displayed directly from the Sensors[] array. The bottom of the figure shows the modified data displayed using the variable names passed to the Orientation function.

Before you execute the program shown in Figure 2, you must ensure that the Sensors Interface Utility is running. The utility program and its handshaking text file should reside in the same directory as the application program unless you add path names to all the associated commands.

If you are using RobotBASIC, you have the option of setting the default directory either through the FILE menu or within a program using the DirSet() function. We assumed that some users might prefer to have their application start the Sensor Utility, so we added support for this option by having it terminate if another copy is already running. You can run the utility from RobotBASIC with the command Spawn (“RBsensorUtility.exe”, P_NOWAIT).

Since the Sensor Utility runs entirely in the background, we have it automatically start in the minimized mode. If you click the tray icon though, you will see the splash screen as shown in Figure 4 which has a button for terminating the program. You can also terminate the program through the command interface by placing the word Stop on the clipboard and renaming the text file to tell the utility to process the current command.

The Sensor Utility has numerous commands for obtaining the desired sensor data. Placing the command word Raw on the clipboard, for example, will request the raw data from the gyroscope and the accelerometer (eight total items including the two headings). The command BasicSensors will return all of the readings shown in Figure 5, and the command Sensors returns that data plus eight more readings associated with the full orientation sensor. There are other commands for obtaining images from cameras and GPS data, but they will be discussed in future articles.

We had problems with the orientation sensors that are worth discussion. On two occasions, the compass heading and the tilt values on our tablet become erratic or zero. This appeared to happen when the tablet was used near a TV or other appliance with a significant magnetic field, but we cannot confirm that as the cause. An Internet search indicated that other Windows 8 tablet owners also had this problem.

Slowly rotating the tablet (without the keyboard) on all three axes fixed the problem for us, but only if no programs were accessing the sensors (so close your utility before performing this procedure). We assume the rotation causes the tablet’s firmware to perform an internal calibration procedure, but no one at Samsung tech support could verify this.

As nice as it is to have access to the internal tablet sensors, most robotic projects will also require some form of proximity and/or ranging sensors, as well as some way to control the robot’s motors. This generally means your tablet needs to have a USB connector on the tablet itself and not just on the keyboard. Most Windows tablets do have a USB connector, but even the ones that don’t could use the built-in Bluetooth for I/O operations. Let’s examine four basic ways of interfacing a tablet with external sensors and motors:

• USB I/O: Adding a USB I/O board is a straightforward way of adding I/O pins to the tablet. We have used the U4xx devices (see the U401 in Figure 6)