Sensors

[Joe_Widen]

Well, the team I am on is currently looking for sensors to increase the performance of our robots. Can you give me some specifics on good sensors that have worked well for your teams. I'm looking into sensors of every type. Please send the name, approximate price, and what it does.

[RbtGal1351]

Okay, the basic sensors that FIRSTers use:

Limit switches: Mechanical switch.
USE: Robot has an extending ladder. Mount limit switch and the bottom, and when the switch is pressed, don't let the motor retract the arm (in software). < $4 per.
(Also, reed switches (magnetic) and optical sensors can be used similarly.)

Potentiometers: Variable resistor. Think of a volume control knob. Plugs into the Analog input; reads 0-1023 in code. You'll probably want LINEAR taper, not Audio taper, so one "tick" is the same # of degrees rotation. 1 turn, 5 turn and 10 turn are the most common.
USE: Robot has an articulated arm. Mount a pot to the joints to see what angle the arm is at. (Good for autonomous control, or making a mini-arm for the OI to control it by position.) < $4 per.

Gyro: Senses the rate of rotation in one plane. You can integrate the data to get the degrees of rotation. (Position from rate of change.) Kevin has code for this. (I think, anyway, you can find what direction you're facing...)
USE: Rotate accurately autonomously. Or interesting driver control of a robot with omni wheels. (Push the joystick forward and the robot goes away from you no matter what, you'd need a gyro for that.) ~$50 per. Kevin's site has suggestions on where to buy and which.

Encoders: Count gear teeth that go by. You can tell how far you've gone or the position of an arm.
USE: Driving accurate distances autonomously, for one. In that use, coupled with a gyro, you can tell where you are on the field at all times. (You'll need sin and cos functions, if you'd like a table look up sin approximation function in software, PM me. I still have to comment it, though.)
I don't know which types of encoders are best (optical? hall effect? etc.), where to buy them, or how much they cost. Could someone reply with that information please?

Dials/dipswitches: Many switches --> multiple combinations.
USE: Set to a position, do a different autonomous mode based on the switch position. If you have 4 switches, you could have 16 different modes.
You can also get encoded dials (easier to solder and attach PWM cables). (I can't find the link to the one we bought on jameco.com).
One set of dipswitches (8): < $1
Encoded dial w/ 16 positions: ~$3
(Our encoded dial was really small and you had to set it with a flathead screwdriver. Has anyone found a bigger one that can be set manually? Thanks.)

Other sensors possibly used: Camera, Infrared, Light sensors (for line following in 2004).


Hope this helped, not really sure what you were looking for.

~Stephanie
Team 1351 Controls Leader and Leader Programmer 2005-2006

[Mike Bortfeldt]

We have used the following components and have found them to be reliable.

Gyro:
Analog Devices ADXRS300EB - $50
Used to calculate robot heading - mainly used for autonomous navigation.

Potentiometer:
Bourns 6639 Series - $11
if you are going to connect one to an analog input on the RC, use one with less than 2.5k ohm resistance (1k works well). Used to know the position of our multi-jointed arm in 2005, and our turret angle in 2006.

Optical Encoders:
Bourns ENT1J Series - $70
We've used these for 2 years in our drive transmissions. Used to measure distance traveled, and control drive speed (PID).
Grayhill 63K or 63R Series - $60
Used to control our 2006 shooting wheel speed.

Be careful when mounting the encoders to minimize side loads on the shaft as this can cause premature failure of the device.

Mike

[Adam Shapiro]

Originally Posted by RbtGal1351

Potentiometers: Variable resistor. Think of a volume control knob. Plugs into the Analog input; reads 0-1023 in code. You'll probably want LINEAR taper, not Audio taper, so one "tick" is the same # of degrees rotation. 1 turn, 5 turn and 10 turn are the most common.
USE: Robot has an articulated arm. Mount a pot to the joints to see what angle the arm is at. (Good for autonomous control, or making a mini-arm for the OI to control it by position.) < $4 per.

Unless something changed drastically this year that I am unaware of, analog inputs read from 0 to 255. This corresponds to 1 byte (8 bits) of data input.

Originally Posted by Mike Bortfeldt

Potentiometer:
Bourns 6639 Series - $11
if you are going to connect one to an analog input on the RC, use one with less than 2.5k ohm resistance (1k works well). Used to know the position of our multi-jointed arm in 2005, and our turret angle in 2006.

The RC reference guide recommends using a 10k-ohm linear taper potentiometer on analog inputs. They say to use a range from 250-100k, but I have found that their 10k recommendation is usually very good and provides more than enough resolution.

[Mark McLeod]

Originally Posted by Adam Shapiro

Unless something changed drastically this year that I am unaware of, analog inputs read from 0 to 255. This corresponds to 1 byte (8 bits) of data input.

10 bits resolution (0-1023) on the RC analog inputs.
You're thinking of the 8-bits of resolution (0-255) for OI analog inputs

[Dave Flowerday]

Originally Posted by Adam Shapiro

Unless something changed drastically this year that I am unaware of, analog inputs read from 0 to 255. This corresponds to 1 byte (8 bits) of data input.

The ADCs on the 8520 and 8722 are 10-bit (0-1023). It has been this way since the move to the C-based RCs a few years ago.

[Adam Shapiro]

Originally Posted by Dave Flowerday

The ADCs on the 8520 and 8722 are 10-bit (0-1023). It has been this way since the move to the C-based RCs a few years ago.

Wow, I guess I just never took notice of that. I haven't done the programming myself since the Basic Stamp RCs! That's nice to have so much of a higher resolution.

[Donut]

We only really used one sensor other than the camera this year, so I'll list that as well as a few past ones/ones we're looking at:

Gyro: The Analog Devices ADXRS300EB that other people have already mentioned is good, ~$50. The kit ones are just a slower model with the 80 degrees per second, but anything less than 150 degrees per second is (I think) unacceptable, so the 300 degrees per second this sensor can handle is nice. The gyro was used for autonomous turning, but more importantly a very simple piece of code can be written to use it to help make your robot drive straight (we programmed it to work based on the joysticks, so our robot would drive straight during driver operation as well).

Limit Switches: Very cheap (less than $15), can be found at just about any electronics store (Radio Shack sells them in spare parts drawers). These provide nice safety limits for rotating or sliding mechanisms (to make sure you don't break an arm) and if you use them cleverly can be used to tell linear position (we used them in an arm that elevated vertically by having a metal piece slide along a track with multiple limit switches in it, and wrote autonomous code to automatically go to certain switch positions).

Potentiometer: Still pretty cheap (less than $10), available at most electronics stores. Potentiometers provide nice rotational feedback if you want a position rather than just limits. There's an interesting whitepaper on using them in place of encoders as well.

Accelerometer: Can't name a good one as I haven't used them, but most sell for between $30 and $50 (I know analog devices sells a number of them, and one was included in the kit). Accelerometers can be used for inertial navigation, but this would require a nice amount of math for the processor and small errors grow rapidly due to the math (without a high price model, I haven't found a sensor with enough accurate resolution to use for this). They can be used as a sensor for the earth's gravity though, and with a tiny bit of math you can determine the current angle your accelerometer is at with the earth based on the acceleration it reads, so you can make a nice anti-tip function programmed in your robot.

Sonar: Most cost $25-$50, the newer Maxbotix Sonar sensor outperforms most of the others out and only runs around $25 (this is from word of mouth from other sources, I've never used it). The only useful thing sonar can give you is distance of an object in its field of view, so you can use it to avoid walls/find a wall or goal in autonomous. I'm looking into developing a robot-dodging (for offense) and robot-seeking (for defense) autonomous mode with them, but something like this may not be very feasible.

[Ken Leedle]

Slide Potentiometer: These come in various slide lengths and are useful for some applications where linear motion can be measured. We used one in a our control box for controlling the launch angle of the shooter.

IR Photointeruptors: These can either be used as limit switches, rotational encoders or anywhere else you want to see if something is there. We constructed one using a IR led pulsed at 38khz and a IR reciever for sensing if a ball was in our scoop. The effective range of this sensor is between 1 foot and 30 feet depending on the current through the LED.

[intellec7]

Perhaps you could use a cheap Hitachi 44780 based parallel LCD and around 4 I/O lines (clock, data, enable, R/S) and a shift register ?

[Joe_Widen]

I think one of the mods should make this a sticky for all the rookies and teams that want sensor feedback.

[Alekat]

I think it's great that your looking into sensors. We really just started getting our hands dirty with them last season but I have a lot of faith the team will step up to the next level next season.

[987HighRoller]

Can somebody post some more info on accelerometers, I have no experience with these. Also, so more info on gear-tooth sensors would be helpful as well.