Receiving Info From the Servo Motors

      Since we successfully sent commands to the controller using the sliders, our next endeavour was to get feedback on the actual position of the servos as they move, using the commands Q & QP. The format of the Q command is "#(servo) Q", and it will send back an ASCII '+' or '.'. The plus means that the servo is currently in motion, while the period means the servo is stopped. The QP command takes the form of "QP(servo)", and will return the pulse width that the servo has been delivered. Using these 2 commands, we can track the servo as it moves.


      We added a text box to the right of slider 4 in our control window and modified our code to have the program show the current pulse width as the slider and the servo both move.
     Our first attempt at coding this was to add a subroutine called “getPulseWidth” that sent the SCC-32 the QP command for servo 4, then read the response and put it in the text box. This operated on roughly the same lines as how we sent sent the move commands to the arm, and then it used the 'read' command to get the signal that was sent back. Because the SSC-32 can sometimes take a few milliseconds to respond, we included a timer that pauses the program for 6 milliseconds to account for that. The program then converts the response to to an integer, and places it in the text box.

      This worked, but we wanted something that would constantly display the pulse width as it changed, not just once when the slider was moved. To do this, we used a loop to call “getPulseWidth” multiple times, constantly updating the text box. It kept getting the value but introduced a few other problems. Since that the loop was constantly running, it became somewhat of a power-hog, and was using processing power when we didn't need it to. This caused large amounts of lag, slowing down our program.

          To fix this,we used a special timer that would trigger every 5th of a second and check the pulse width. This worked perfectly but the timer was still running. We fixed this with the “Q” command, so that the timer when the servo stopped moving. It works by sending the Q command to the arm, reading the response, and if it's a period, meaning that the arm has stopped, it kills the timer.

      We also did one last check of the pulse when the arm had stopped to make sure we got a final position value. This ensured that the position in the text box was correct. We also had to add a couple of lines to kill the timer before we started it just to make sure that the timer was completely reset.




Here's a video showing the final program with the pulse value being updated as the servo moves:

Robot Arm: Writing The Controller

     Since my understanding of programming isn't overly advanced yet, the best approach to building a program for the arm would be to find a program that has a similar function, and modifying it for my needs. After browsing the apple forums, we found a program that someone had developed that could output signals from the serial port. 

       Using a programming environment called XCode, we opened up the program and examined it to figure out how it worked. We then stripped out all of the code we didn't need, leaving us with a solid foundation that we could build upon.

     


       From there, we built a simple interface consisting of six sliders that controlled each of the six servos. The program had to be written in a language called Objective C, which has some similarities to the Java programming language we used in my Grade 11 Programming course, so it wasn't that hard for me to understand.

       Here is a look at a portion of the code that handles the interface. The very first line basically waits for any of the sliders to be changed, and when one is, it activates the code within the curly brackets. The next line creates the format for the signal that will be sent to the robot(the red text). Normally, the signal that is sent to the robot is "#(servo) P(desired servo position) S(speed)". For instance, if you wanted to send the shoulder to position 1700 at a low speed, you'd type "#1 P1700 S100" and then hit enter. So in our program, you'll notice the same format, except the servo number and servo position have been replaced by a "%d", which is used in Xcode as a substitution, so you can get the program to replace that "%d" with any set of symbols, letters, or numbers. 

        The next few lines get somewhat complicated, but all they really do is take the decimal data received from the slider and convert it to ASCII, so the arm understands it. Finally, the last line outputs the ASCII command through the serial port.

Robot Arm: Testing the Computer

     To run the robot arm with a computer, we first needed to make sure the computer could send and receive signals. Since the Macintosh does not have a physical serial port, we used a USB to serial cable that we connected to the physical serial port on an older PC. Then, using terminal emulation programs, which allow you to send and receive signals out the serial port. We set both of the programs to accept ASCII characters, as that is what the robot arm uses, then successfully transmitted a signal to the PC using the Mac, with the same speed and conditions required by the robot arm(115.2kb/s, 8 bits no parity).

Here, you can see that the message that was written on the mac was successfully delivered to the PC using the terminal programs.

     We later used the same method to test if our simple program to control the arm was working. It was better to use the PC to test the program, rather than the arm itself, because we can actually see the signals our program is sending. We used RealTerm on the PC ( download at http://realterm.sourceforge.net ) and CoolTerm on the Mac ( download at http://freeware.the-meiers.org/#CoolTerm ). Both are free and work really well.

Welding cast iron brackets

     Today, 2 brackets that hold up a side shelf on our wood stove broke off, so i took them into welding class to see if I could fix them. I was worried they wouldn't weld well, as they are made of cast iron, and neither me, nor my welding teacher knew if it would work. To my surprise, I easily joined them with a MIG welder, and grinded the weld so it was flush with the rest of the piece, then polished it off. Here's a picture I took after I did the first one. It was much easier then I thought it would be, and I am quite pleased with the final product.

Offroad Truck Bumper Part 1

     I've decided to build my own bumper for my truck that will increase it's capabilities, allowing it to be more agile over rough terrain. I've looked into buying a bumper, but there aren't too many available for the 97-03 generation of F-150s, and all the one's I've found are highly expensive. The style of bumper that I've chosen is prerunner/baja. These bumpers are widely used in desert racing and offroading in areas such as Mexico and Southern California.
         




The main point of these bumpers is to improve the clearance of the front end. Normally, if you were trying to drive your truck from flat ground onto a steep incline, the bumper would hit, and you'd get stuck. These bumpers expose much more of the wheel, thereby allowing the truck to handle drastic changes in incline.  




      
     
     To build my bumper, I would first need to find a way to mount it to my frame. The old bumper was bolted onto 2 large brackets that were welded to the end of the frame. To mount my bumper, I would need to chop the ends of the frame off to allow the skid plate to have shallower angle, which will keep air resistance to a minimum.




    

    First, I drew up some designs for the mounting brackets of my bumper. I then cut out the pieces I would need with a plasma cutter. I used 1/4 inch iron, as it would give my bumper a good strong base to sit on.
         





    


   
    I used a mig welder to join the pieces together, and once I had both brackets built, I arc welded a piece of tube in between them to get the width right.

     

Robot Arm: Controlling the servos

Each servo connects to the SSC-32 board using 3 wires. The VC(Red) and Ground(black) wires provide the 5 V DC voltage to power the servo.

The third “Pulse” wire supplies a 5 volt control pulse signal that is repeated 50 times a second (every 20ms). The width of this pulse determines what point the servo rotates to, and can be from 0.9ms to 2.1ms long. For example,an ASCII command from the computer to the SSC-32 control port such as “#2 P1500” would tell the controller to send Servo #2 a signal consisting of 1.5ms pulses which would move the servo to move to it centered position.

The Oscilloscope lets us actually see the pulse by freezing the screen, allowing us to measure it. This is how the Oscilloscope shows the 50ms 5 Volt pulses to the servo.

The black permanent grid squares on the Oscilloscope screen are called “Graticules”, and the numbers at the bottom show the units. In this setting, each graticule is 10 ms wide and 5 Volts high. The two dotted lines are part of the Oscilloscope's measurement tools. The Oscilloscope allows these to be moved to allow vertical or horizontal measurements, and the number at the top op the screen represents the distance between them(20.0ms). The time scale of the scope can be adjusted to look at one pulse and make more accurate measurements.

Below is what the .75ms, 1 ms and 1.5 ms pulses look like for the commands “#1 P750”, “#1 P1000” and “#1 P1500” are sent. 

Using the basic commands the Servos moved very quickly but there is a longer command that allows you to control the speed that they rotate into position. By adding a “S” parameter, the movement speed of the servo can be adjusted. The SSC-32 manual does not explain how this works, but using the Oscilliscope we were able to see how the SSC-32 slowly adjusted the pulse width when the command “#1 P700 S100” was sent (note that the the previous servo position was “#1 P2300”). The “S” value sets the movement speed in us/sec.

Here is a video displaying the changing pulse width.

Robot Arm: The SSC-32 Servo Controller

       The servos on the robot arm are controlled by a controller board called a SSC-32. 

       Here's a diagram from the manual detailing each of its components. 

       As you can see, those pins along the bottom are the connection points for a servo motor. One pin is ground, one pin delivers power, and the remaining pin delivers the pulse that moves the servo. The area of the card that says "Baud" is where the speed that data is passed is selected using 2 jumpers. As you can see in the photo, both jumpers are in, which means that the card will operate at the maximum speed of 115.2 Kilo Bytes per second.