4.3 Sensors: Switches, Knobs, Encoders, and Gyros#

Why it matters#

Motors let the robot act. Sensors let it know what's happening. The labs use four kinds of sensor, each wired a different way and answering a different question:

This lesson covers the hardware side: what each sensor is, how it's wired, and what can go wrong. The labs cover the code. Lesson 8 covers sensors and units in general.

Read this#

• Digital Inputs – Hardware (WPILib)
• Analog Inputs – Hardware (WPILib)
• Encoders – Hardware (WPILib)
• Gyroscopes – Hardware (WPILib)

Walkthrough#

The roboRIO's sensor ports#

Every DIO and Analog In port has three pins: ground, 5 V power, and signal. Sensors that need power use all three. A plain switch uses just signal and ground.

Limit switches#

The test bed's limit switches are industrial plunger switches. Pushing the plunger flips a set of contacts inside.

The switch connects between a DIO signal pin and ground:

A limit switch wired to a DIO port: signal and ground only. From the WPILib docs, Digital Inputs - Hardware (CC BY 4.0).

So with NO contacts:

That's why Lab 4 uses !m_limitSwitch.get() to mean "pressed."

Potentiometers#

A potentiometer ("pot") is a variable resistor with three pins: two ends of a resistive strip, and a wiper that slides along it as the shaft turns.

5 V ──┬─────────────────┐
      │  resistive strip │
GND ──┴────────▲────────┘
               │
            wiper ──→ Analog In signal

The wiper's voltage goes from about 0 V at one end to about 5 V at the other. The test bed's is a 10-turn pot: ten full turns from end to end, which gives very fine resolution.

A potentiometer on an Analog In port (left) is a voltage divider (right). From the WPILib docs, Analog Potentiometers - Hardware (CC BY 4.0).

Potentiometers are cheap and simple, so teams use them to measure arm angles and as driver knobs, like in Lab 5.

The REV Through Bore Encoder#

The Through Bore Encoder mounts around a 1/2" hex shaft (the shaft passes through its center), and measures the shaft's rotation with a magnet and sensor.

It has two outputs:

Quadrature, explained. Channels A and B each switch on and off as the shaft turns, a quarter-cycle apart. One full cycle steps through four states, and the order of the states tells the roboRIO the direction:

Turning forward, A switches first. Turning backward, B switches first. Every one of those switches is one count, so one cycle is 4 counts.

The direction comes from which channel (A or B) changes first. From the WPILib docs, Encoders - Software (CC BY 4.0).

The roboRIO counts these in hardware (its FPGA chip), so it never misses a count, even though your code only runs every 20 ms.

Wiring the Through Bore Encoder to the roboRIO#

The encoder has a small white 6-pin JST socket. Its cable splits into four 3-pin Dupont plugs, one per signal, each with its own red (5 V) and black (ground) wire. The color of the third wire tells you which signal it is:

The encoder end of the cable is the white JST plug (right). The four 3-pin Dupont plugs go to the roboRIO. Each has a red wire (5 V), a black wire (ground), and one colored signal wire.

Which way around? Each roboRIO DIO port is three pins in a row, and the roboRIO has a label printed next to each row of pins: the ground symbol (⏚), V (5 V), and S (signal). Turn each plug so the black wire is on ground (⏚), the red wire is on V, and the blue or yellow wire is on S. Every plug on this cable has its own red and black, so each one powers itself; that's normal.

The two DIO port numbers become EncoderChannelA and EncoderChannelB in Constants. Which robot uses which ports:

The NavX2-MXP gyro#

The NavX2 is a small board with its own processor and three kinds of sensors: gyros (rotation), accelerometers (acceleration), and a magnetometer (compass). It combines them to report the robot's orientation.

Yaw is turning left and right; pitch and roll are tipping. From the WPILib docs, Gyroscopes - Hardware (CC BY 4.0).

Which sensor for which job?#

Try it#

• Find each sensor on the test bed and WCD, and follow its wire to the roboRIO. Write down the port number for each.
• On a limit switch, find the NO and NC terminals. Which one is wired on the test bed?
• Find the Through Bore Encoder's two quadrature wires and which DIO ports they go to
• Find the NavX2 on the WCD's roboRIO. Is it mounted flat?
• Fill in a wiring table for your team: every sensor, its port, and which lab uses it. Post it near the test bed.

Check yourself#