Programming ultrasonic rangefinders

[ekapalka]

Hello CD! I have a pretty good targeting method for launching Frisbees figured out, but there's one thing I need to know wow to do first. How to program ultrasonic rangefinders. Basically, I need to get the range and put it into an equation, either in real time or at the push of a button. How do I get it to return the distance (in inches)? I've looked through CD, and found some examples that are pretty elaborate for doing something similar, but not quite what I was looking for. Is there any easy way to do this? Thanks!

[Alan Anderson]

What specific sensor are you planning to use, and what kind of signal does it provide to the control system? Some have an analog voltage which is proportional to the range. Others have a PWM signal that varies depending on the range. Still others are a serial (e.g. SPI or I2C) connection that encodes a number.

Whatever you get, it's almost certainly going to be a simple matter of multiplying the value by an appropriate constant to turn it into a distance in whatever units you want.

[ekapalka]

We intend to use one Maxbotix MB1010 LV-MaxSonar-EZ1 (from the kit of parts last year (?)) and one Maxbotix MB1361 XL-MaxSonar-AEL0. They both support 3 sensor outputs: Analog Voltage, Serial, ands Pulse Width. I'll keep researching. If it's as simple as multiplication, I may try to find out how to return metres as well.

[DonRotolo]

The Maxbotix sensor analog signal provides 0.01 V per inch. That means that 1.2 V is 120 inches. Just read in the analog voltage and the math is pretty straightforward.

Of course you can use the PWM or digital outputs, but I can't help you there.

[ekapalka]

Thank you very much for the information. As it turns out, somebody <.< soldered our rangefinder for pulse width. Nothing I can't fix in a day or two once I acquire a soldering iron. In the meantime, is there an easy way to get input from the pulse width?

[Alan Anderson]

Getting input from a PWM signal is one thing that I've never found a good way to handle using the available hardware and software of an FRC robot. You can play around with using a filter circuit to turn the signal into an analog voltage and read it that way, but it would take a lot less time to move the output wire to the voltage pin of the sensor.

[ekapalka]

Hypothetically, if we did have the sensor wired properly, is it categorized with the WPI class for rangefinders, or would it just be a digitalinput? I've been looking through the WPI documentation and it says that the Ultrasonic class specifically supports the Devantech SRF04 and Vex ultrasonic sensors. I'm not sure if that means it ONLY supports those two, or if those were just examples.

[DjScribbles]

You may find this helpful (be sure to grab from my second posting):
http://www.chiefdelphi.com/forums/sh...d.php?t=103028

I posted this last year, it did work, however I wasn't very happy with the sensor itself since it's very easily confused.

Another good method for determining range is to roughly compute it from a known target (such as the 3pt goal).

[ekapalka]

Thanks! How would I call the functions from the main robot cpp file? Is it just

void GetDistance()
{
AnalogChannel *range;
range = new AnalogChannel(1,1);
float volt;
volt = range->GetVoltage();
float inches;                      //float meters;
inches = volt * 100;               //meters = volt * ???;
return inches;
}

?

[Alan Anderson]

Originally Posted by ekapalka

Thanks! How would I call the functions from the main robot cpp file? Is it just

//schnip

?

Don't use that code. It's simple, obvious, and wrong. You don't want to keep instantiating a new AnalogChannel every time you need to read the distance.

You can use the fancier .h file from the thread where you got that code, but that might be more than you're ready to handle. Joe ("DjScribbles") is right here and can help you understand how to use it. Or you can define the AnalogChannel object once at the beginning of your program and leave out the pair of lines that create it every time in the code you copied.

[ekapalka]

Sorry about that! I knew exactly what you meant, but copied/pasted the pseudo-code from that page assuming that AnalogChannel was the name of the .h file (Having not noticed it at the bottom of the post and not being familiar with the actual AnalogChannel WPI class. Additionally, I'm at school, and the router prohibits me from downloading it for some reason). What I meant was, if I'm using the .h file, how would I make references to it in my own code. Thanks!

[DjScribbles]

You would instance it much like you would a jaguar, or any other device.

#include "AnalogRangeFinder.h"

#define ANALOG_CHANNEL_FOR_SONAR 1

class Robot2012 : public IterativeRobot
{
   AnalogRangeFinder sonar;
   public:
   Robot2012(void):sonar(ANALOG_CHANNEL_FOR_SONAR)
   {
   }

   void AutonomousPeriodic(void)
   {
       float distance = sonar.GetRangeInches();
   }
}

START_ROBOT_CLASS(Robot2012);

[mrklempae]

am I the only person who can't seem to get this to work? I'm not completely sure how to do this, but one of the other programmers (better than myself) tried to get it working to no avail...

[DjScribbles]

Originally Posted by mrklempae

am I the only person who can't seem to get this to work? I'm not completely sure how to do this, but one of the other programmers (better than myself) tried to get it working to no avail...

The way the sensor is wired is also important, the code I provided is for operating in analog mode (not PWM mode), and the wiring must account for that.

For debugging the sensor itself you can put a multimeter probe on the analog line and see what value it is sending to the cRio (a voltage level that should change the closer the sensor is to something).

Once you see values on the multimeter that correspond to distance, then you debug code. If you need more help, please post actual details on what you've done and what you see.

[ScottW of MaxBo]

Good day! This is Tom Bonar from MaxBotix Inc.

I was looking through the Chief Delphi forum, and I have noticed that you were having difficulty getting the sensor connected to the cRio. You may wish to consider looking at the quick-start guide (http://maxbotix.com/articles/016.htm) that we created last year. This guide has a walk through on connecting MaxSonar sensors to control devices. The fundamentals are the same for all MaxSonar sensors, with the big difference for connection purposes being the scaling factor of the Analog Voltage output. For calculating this output please view our Analog Voltage article (http://maxbotix.com/articles/032.htm). We have examples for all of our sensors, as well as integrating directly with a 10 bit Analog Digital converter.

We have also kept the notes from last year's FIRST competition on our Facebook page (link to the 3 links from first email)

https://www.facebook.com/photo.php?f...type=3&theater
https://www.facebook.com/notes/maxbo...77662868954588
https://www.facebook.com/notes/maxbo...45549128832627

Please let me know if you have any questions by emailing frc@maxbotix.com.

On behalf of MaxBotix Inc., I wish all the teams a successful build season.

Best regards,

Tom Bonar
Technical Support
of MaxBotix Inc.
Phone: (218) 454-0766
Fax: (218) 454-0768
Email: frc@maxbotix.com
Web: www.maxbotix.com
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