Sensors
 

Hi! I'm new to robotics this year and I've heard people talk about different types of sensors a lot but I'm not really sure what they mean... What's a gyro, what's an accelerometer, a limit switch, a potentiameter? Are there any other sensors I'm missing? And were do they connect? Is it the VRM? Any help would be greatly appreciated, thanks! :)

Re: Sensors
 

First off... No sensors should EVER be connected to the VRM (unless they need one of the constant voltages it supplies to operate). The Voltage Regulator Module supplies a constant voltage to whatever is connected to it. This probably won't be a good thing for the sensors...

Now, the sensors:
Accelerometer--measures the acceleration of the robot. (AKA: I'm speeding up, how fast am I speeding up?)
Gyro--Similar to an accelerometer, but measures the rate of turn of the robot.
Limit switch--contact sensor. Often used to limit the motion of a moving object.
Potentiometer--used to measure distance of rotation of some part of the robot. Limited motion, usually. (Also known as a "pot"--be careful who you say that in front of, particularly if they aren't familiar with electronics!)

Sensors you're missing (partial list, because there are more that can be rather specialized in application):
Encoder--measures speed of rotation of part of the robot--like the drivetrain.
Ultrasonic sensors--Often used for finding a distance to some other object.
Camera (AKA vision sensor)--usually used for showing drivers what the robot sees, but can be used to track blobs of color.

And where do they all connect? That's a little more interesting. Generally speaking they'll connect to the RoboRIO's DIO or analog pins. They may also connect to a secondary processing board that sends the data back to the RoboRIO. Cameras will usually plug into the RoboRIO's USB ports or into an Ethernet port on the robot's router.

Re: Sensors
 

Some more details: The VRM provides 5V and 12V only, and if your sensor is not 5V or 12V, it should not connect to the VRM. If the sensor is 5V or 12V, then it can be connected to the VRM since the purpose of the VRM is provide regulated 5V and 12V rails despite the voltage peaks and voltage dips seen on the battery rail. One advantage to that is ensuring your sensors don't drop out if the battery voltage dips during high current events.

Always read the datasheet of a candidate sensor to make sure you are meeting the voltage and current requirements of said sensor.

For learning the differences between the general sensor types I think the best place to start is the FRC documentation...(https://wpilib.screenstepslive.com/s...commit=search).
...particularly for sensors that connect to the roboRIO directly. Search each sensor type individually.

Additionally sensors can also be attached directly to motor controllers in the case of the Talon SRX and Jaguar. In the case of the Talon SRX the user's guide goes over wiring, and the software reference guide goes over programming.
http://www.ctr-electronics.com/talon...ical_resources

Re: Sensors
 

Limit switches can be attached to Talon SRX or Jaguars in CAN mode. Also, either a limit switch or encoder can connect directly to a Talon SRX, though you'll probably want one of the cables and breakout boards available through AndyMark, Vex (search Talon SRX Breakout on either), and probably other vendors..

There are also numerous non-contact alternatives to limit switches, which may be based on capacitance, reflection of light, or other effects. We used a Sharp optical interrupter switch coupled with a spring-loaded mechanical mount which pushed a 5/32" shaft through one of the holes in the KoP chassis to detect "contact" with the scoring platforms in 2015.

Potentiometers are also available in linear packages, not just rotational. You can also convert a rotational pot to report linear values through a pulley, either with the "string potentiometer" available COTS at AndyMark, or through something a bit more involved like our "electric tape measure" in 2015.

Encoders essentially count the number of "teeth" of a physical or virtual gear/sprocket/rack as it moves. Most encoders used by FRC teams are "quadrature" encoders, which means that they have two sensors, located about half a "tooth" apart from each other. By using this setup, it is possible to tell what direction a shaft is rotating (or a rack is sliding), based on which sensor transitions a given direction before the other. Encoders are most commonly attached to a "back shaft" of a motor or gearbox so that they are away from the mechanical work, but linked to it. Encoders may be based on optical (counting black lines on a white background), Hall effect (checking magnetic interference of a gear tooth as it passes), or other physical phenomenon.

Devices that measure the battery voltage and current to each circuit are now built into the PDP, but these definitely count as sensors; they give you good insight as to how the battery is performing and how much work is being done by each actuator.

My favorite place to go when I sort of know what I want to detect/measure but don't know what's available is Pololu's Sensor page.

Edit: I was ninja'd a bit by Omar, but not enough to change the post.

Re: Sensors
 

While I love seeing Chief Delphi so helpful, I have to mention that you will save yourself a lot of time by mastering the art of google-fu.

Every one of the sensors you listed gives a full page of extremely useful links when searched in conjunction with "robotics" or "FRC". In fact, the first link for all of them is an FRC specific screenstepslive page on programming them.

Re: Sensors
 

Hey guys! One more question: I know on talons there is a brake mode and a coast mode. What's the difference and what do they do? Thanks!

Re: Sensors
 

This one is a bit more obscure and interesting.

A motor is basically a backwards generator. (and vice versa) You can observe this fact by plugging two nxt motors into eachother and spinning one by hand. Trippy as it is, the second motor will turn. (Because the first motor is generating electricity and sending it to the second one)

But wait, when a motor controller gives voltage to the motor, the motor spins. This means it starts working like a generator and sending voltage at the motor. This is called back voltage.

Current flows only when there is a voltage difference, so if nothing is stopping the motor (theres no load), it speeds up until it basically sends back almost the same voltage, and electrons almost stop flowing. Motors are designed around the idea that they are moving when they are being sent power. When they arent moving (stalled), theres no backvoltage, and the difference is massive.

Now why does this matter for the coast vs break settings? In coast mode it leaves the circuit of the motor controller as is. In coast mode the controller isnt sending any voltage, but the motor (by virtue of inertia) is moving, and thus generating electricity. Since you dont want electricity making the voltage higher than normal the circuit is just kind of cut (someone fact check me on that one because I'm not ENTIRELY sure this is how motor controllers deal with backvoltage) electrons are shoved into the wire, but they cant go anywhere, so once again, theres little difference between what is "being sent to the motor" (by the wire that is charged by the motor) and what the motor is sending.

In brake mode it shorts the motor, connecting the positive to negative. Now the electrons CAN move, directly, through a low resistance circuit. The voltage difference is now large causing more resistance on the motor, stopping it quicker.

I will edit this as people tell me I'm wrong, which is likely to happen.

Re: Sensors
 

I was thinking the same thing. Chalk one up to NOT wrong.

Re: Sensors
 

Hah, in the middle of composing an answer I realized we already put it in the Talon SRX User's Manual.
So here's a snippet...

Re: Sensors
 

If you want to understand the result (how well it works), here's a fairly simple way to do it:

Get a CIM, an 8mm hub and key, and a moderately heavy wheel (e.g. an 8" kit wheel), and connect them in the obvious way, but without a gearbox. Actually we won't be doing anything fast, so don't worry about keeping the hub from falling off the shaft other than orienting the CIM vertically with the wheel above it.

With the CIM leads open, give the wheel a spin by hand and notice how long it runs.

Then, twist the two leads together and repeat.

Re: Sensors
 

Hi! What do the different lights on the solenoid mean? Thanks!

Re: Sensors
 

That would depend on what solenoids you are using. The solenoid valves we currently use don't have indicator lights; the spec sheet notes that they're optional on the model, but doesn't say anything more about them, so I would assume that it just lights when voltage is applied to the solenoid, or when the solenoid actuates. Your quickest answer would be to google your solenoid's model number and look for a user manual or spec sheet.