Range Finder?
 

I'm not sure if this is the right category, but I hope I can find some help. For this years competition we're looking at possibly using a range finder, however the one we have tried in the past (Not sure the exact model, it used radio waves is all that I know) didn't work out very well and wasn't consistent. Has anybody had luck with a good (Possibly infrared?) range finder that isn't very expensive? Looking for around a $100-$150 range. Any help would be greatly appreciated.

PS: Also, what makes a range finder legal/illegal for FIRST FRC?

Re: Range Finder?
 

A number of teams have used the VEX one with success in the past.

Re: Range Finder?
 

I believe that was the one we tried before, but as it got farther out it became very inaccurate unless you had perfect conditions.

Re: Range Finder?
 

It's range is 1.5" to ~9.5'. Were you operating outside this window.

We used it in 2011 to detect our distance from the wall in autonomous. We were looking for relatively close distances, somewhere around a foot away. I think we missed a total of two tubes in auto at our two competitions that year. So depending on application it can work quite well.

I've also had good luck (outside of FRC) with the Parallax PING sensors.

I haven't used them myself, but there is quite the range of options available from maxbotix. They have sensors that supposedly can measure to the cm resolution at a range of up to 35ft, which is pretty amazing. You have a $30 voucher for this company in your kit of parts.

There aren't any rules specific to the use of range finders in FRC. Read the robot section of the manual for electrical rules and custom circuits.

As far as what you should use. Nobody is going to be able to tell you that unless you describe your application. When are you envisioning using this sensor. What range of distances will you be from your target. How large is your target. What material is your target made of?

In general ultrasonic sensors will be used for lower resolution higher range applications. Infrared is typically limited in range, but can provide good resolution over that range.

Re: Range Finder?
 

We have good luck with these...

http://www.maxbotix.com/Ultrasonic_S...ngefinders.htm

Re: Range Finder?
 

Well, this is second hand information from a mentor (my first year on the team). He said that after they got to a certain distance (I assume not to far out of range) the cone of the radio waves became big enough that it had a half second delay, along with it not being accurate enough to pin point the target, and was picking up things farther away and occasionally registering them.

Re: Range Finder?
 

As long as you are in range of the sensor you shouldn't have much of a delay. The ultrasonic sensors use sound waves, which travel ~1116 ft/s

For the maxbotix sensors the max range is 255 inches or 21 feet, so at the max range, the sound travels a total of 42 feet for a travel time of ~37 ms.

If you look at the maxbotix datasheet they show approximate detection fields based on various targets. For long distance they use a 12" wide target and can detect it within a 2' variation side to side. For closer range they use small dowels to show the side to side bounds of where the sensor can detect. Different sensors from them have different side object detection ability.

I would highly recommend either of these. With the maxbotix sensors you get a much wider range of sensors to choose from, and they also have a higher resolution product line which may be useful. Their standard sensors have a resolution of 1 inch, and their high resolution sensors claim to have a resolution of 1mm. Definitely use the KOP voucher at least.

Re: Range Finder?
 

Ultrasonic range sensors may give unpredictable results if aimed at the player station. The diamond plate gives unusual reflections. There are laser sensors as well and the rule that comes into play for those is the one covering Class I lasers only. I believe Digikey has some ultrasonic that are inexpensive and easy to use.

Re: Range Finder?
 

In the 2014 Aerial Assist game my team and I tried to use Ultrasonic sensors, but with no luck. The Ultrasonic sensor could not get a constant reading, and would often jump from 15ft to only a few inches which would triggers safeties and our launch system. So unless we were just unlucky with our three sensors ( all failed) I would say to avoid them. Maxbotix was the brand, I'm interested to see if the VEX Range Finder works better, and will pitch it to my team.

Re: Range Finder?
 

Are the unpredictable results consistent with:

Our software team had similar spikes, and think that it is due to noise on the analogue signal.

Could those of you who used the Maxbotics sensors successfully provide some more info on how you did so? Which signal did you use (analogue/PWM/UART)? If you did anything non-trivial in reading the input (such as filtering), could you also provide some sample code?

We are also at a loss on how to read in the PWM signal (to get over the analogue noise above) directly into the cRIO (and probably roboRIO), if anyone has experience with reading in PWM signals it would be great to hear.

Re: Range Finder?
 

We used a Maxbotix ultrasonic sensor this year. We wired it directly to our cRIO's analog breakout and it worked well for detecting how far away the goals were. What language are you programming in and how many other devices are plugged into your analog breakout?

Re: Range Finder?
 

We currently use Java.

Last year we had two sensors connected, the Maxbotics ultrasonics sensor (the one from the KoP a season or two ago) and a sharp infrared sensor (to channels 4 and 5 respectivly).

Re: Range Finder?
 

I think it could be analog noise from the IR sensor, because iirc running two rangefinders at once is known to cause interference issues.

Unfortunately I don't have access to my team's equipment right now so I can't test my theory on how to read from PWM.

Re: Range Finder?
 

We also saw occational spikes when trying to use this sensor, I don't recall exactly how we had it set up when we were testing but it would spike to full distance. We went back to the datasheet for the maxbotix and noticed our sensor has a 20 Hz reading rate. We changed are loop rate to 20 Hz and never saw spikes again.

Our competition bot uses the PWM signal from the Maxbotix sensor and is used to detect the ball.

Re: Range Finder?
 

The diamond plate will produce unusual reflections due to surface texture for IR and ultrasonic. In addition, objects striking the surface of the diamond plate may also produce ultrasonic signals that can confuse or overwhelm the transducers. It is best to take a series of readings and either average the results, do some noise filtering or reject readings that are outside the realm of your environment. i.e. if the readings have been in the 3-4 foot range and you suddenly get a few inches reported, wait for more data. If you have several reporting inches then you might be inches. If you only get one and then it goes back to feet, the later is likely true. Remember in both cases, the rangefinders are looking for reflections.