how do I wire and configure a servo and then control it from an opmode

Wiring the Servo

Connect the standard 3-wire servo cable to one of the 6 built-in servo ports on the Hub, such as the port labeled "0" .
Ensure that the black ground wire of the servo cable aligns with the ground pin, which is located on the left side of the servo port .
If using extension wires, heavy-duty wires are recommended . Secure the connections between the extension wires and the servo wires with electrical tape to prevent them from loosening over time .

Configuring the Servo

Before programming, you must create a configuration file on the Robot Controller .
On the Driver Station app, open the pop-up menu by touching the three vertical dots, select Configure Robot, and tap New .
Select your Expansion Hub Portal and then select the Expansion Hub to display its available ports .
Touch Servos to open the Servo Configuration screen .
Use the dropdown control to select "Servo" as the servo type for port #0 . To utilize the full 500-2500 microsecond range of your servo instead of the default 600-2400 microsecond range, select "Full Range Servo" instead .
Specify a name for the servo (for example, "Release Servo") and press Done to complete and save the configuration .

Controlling the Servo from an Op Mode

Using Blocks

In the Blocks Programming Tool, you can control the servo by specifying a target position ranging from 0 to 1 . A target position of 0 corresponds to 0 degrees of rotation, and a target position of 1 corresponds to 180 degrees of rotation for a typical servo . You can map gamepad buttons to send specific position values (such as 0, 0.5, or 1) to the servo .

Using Java (OnBot Java or Android Studio)

To control the servo in Java, instantiate the Servo class and use setPosition() to command the servo to a target position between 0 and 1 .

Servo relicServo = hardwareMap.get(Servo.class, "Release Servo");

releaseServo.setPosition(0.75);
telemetry.addData("Release Servo Target", releaseServo.getPosition());

The getPosition() method returns the current target position of the servo, not its physical position . For advanced control, such as setting a custom PWM range or disabling the PWM signal to let the servo de-energize, cast the Servo object to ServoImplEx .

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Connecting a Servo to the Hub > Connecting a Servo to the Hub Instructions

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Connect the servo cable to the servo port labeled "0" on the Hub.

Note that the ground pin is on the left side of the servo port.

Verify that the black ground wire of the servo cable matches the

ground pin of the servo port (which is aligned on the left side of the port).

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Connecting a Servo to the Hub

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The Hub has 6 built-in servo ports. The servo ports accept the standard 3-wire header style connectors commonly found on servos. Note that ground pin is on the left side of the servo port.

Note that it will take an estimated 2.5 minutes to complete this task.

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Configuring a Servo

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You will also want to add a servo to the configuration file. In this example, you are using a standard 180-degree servo.

Touch on the word Servos on the screen to display the **Servo

Configuration** screen.

Use the dropdown control to select "Servo" as the servo type for

port #0.

Use the touch pad to specify the name of the servo ("servoTest"

for this example) for port #0.

Press the Done button to complete the servo configuration. The

app should return to the previous screen.

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Controlling a Servo Blocks

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In the section titled Creating an Op Mode with Blocks you learned how to use the Blocks Programming Tool to write an op mode that controls a 12V DC motor. In this section, you will learn how to write an op mode that controls a servo motor.

A servo motor is a special type of motor that is designed for precise motion. A typical servo motor has a limited range of motion.

In the figure below, a "standard scale" 180-degree servo is shown. This type of servo is popular with hobbyists and with FIRST Tech Challenge teams. This servo motor can rotate its shaft through a range of 180 degrees. Using an electronic module known as a servo controller you can write an op mode that will move a servo motor to a specific position. Once the motor reaches this target position, it will hold the position, even if external forces are applied to the shaft of the servo.

Servo motors are useful when you want to do precise movements (for example, sweep an area with a sensor to look for a target or move the control surfaces on a remotely controlled airplane).

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Controlling a Servo Blocks > Modifying Your Op Mode to Control a Servo

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Let's modify your op mode to add the logic required to control a servo motor. For this example, you will use the buttons on the Logitech F310 gamepad to control the position of the servo motor.

With a typical servo, you can specify a target position for the servo. The servo will turn its motor shaft to move to the target position, and then maintain that position, even if moderate forces are applied to try and disturb its position.

For the blocks Program & Manage server, you can specify a target position that ranges from 0 to 1 for a servo. A target position of 0 corresponds to zero degrees of rotation and a target position of 1 corresponds to 180 degrees of rotation for a typical servo motor.

In this example, you will use the colored buttons on the right side of the F310 controller to control the position of the servo. Initially, the op mode will move the servo to the midway position (90 degrees of its 180-degree range). Pushing the yellow "Y" button will move the servo to the zero-degree position. Pushing the blue "X" button or the red "B" button will move the servo to the 90-degree position. Pushing the green "A" button will move the servo to the 180-degree position.

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Controlling a Servo OBJ

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In this section, you will modify your op mode to control a servo motor with the buttons of the gamepad.

A servo motor is a special type of motor. A servo motor is designed for precise motion. A typical servo motor has a limited range of motion.

In the figure below, "standard scale" 180-degree servo is shown. This type of servo is popular with hobbyists and with FIRST Tech Challenge teams. This servo motor can rotate its shaft through a range of 180 degrees. Using an electronic module known as a servo controller you can write an op mode that will move a servo motor to a specific position. Once the motor reaches this target position, it will hold the position, even if external forces are applied to the shaft of the servo.

Servo motors are useful when you want to do precise movements (for example, sweep an area with a sensor to look for a target or move the control surfaces on a remotely controlled airplane).

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Controlling a Servo AS

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In this section, you will modify your op mode to control a servo motor with the buttons of the gamepad.

A servo motor is a special type of motor. A servo motor is designed for precise motion. A typical servo motor has a limited range of motion.

Servo motors are useful when you want to do precise movements (for example, sweep an area with a sensor to look for a target or move the control surfaces on a remotely controlled airplane).

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Reading and Writing to Hardware > Examples of Using Common Hardware Components > Servo

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Servo relicServo = hardwareMap.get(Servo.class, "Release Servo");

After instantiating a Servo, there are two main functions that can be called: setPosition() and getPosition().:


releaseServo.setPosition(0.75);
telemetry.addData("Release Servo Target", releaseServo.getPosition());

setPosition() sets the position of the servo. The SDK will use a built-in control loop with the servo's potentiometer to drive the servo to that position and hold that position. setPosition() takes in a double between 0 and 1, where 0 is the servo's lower limit of rotation and 1 is the servo's upper limit of rotation. Everything between is directly proportional, so 0.5 is the middle, 0.75 is 3/4 the way up, etc.

getPosition() does not return the servo's current position, rather its current target position. If a variable for the servo's current target position is stored properly, this function should never be needed.

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Creating Op Modes Blocks > Controlling a DC Motor

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In this section, you will add some blocks to your op mode that will allow you to control a DC motor with a gamepad.

Note that you will need an estimated 15 minutes to complete this task.

Important: The programming blocks for user configured devices (motors, servos and sensors) will only be visible in the Blocks tool if there is an active configuration file with the configured devices included in the file. If a type of device is not included in the active configuration file, then its programming blocks will be missing from the palette of blocks.

If you did not create and activate a configuration file yet please follow this link to do so. After you created and activated your configuration file, you can close and then reopen your op mode so that the programming blocks for the newly configured devices will be visible.

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Getting Started

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Before you can communicate with the motor, servo and sensors that are connected to the Control Hub or Expansion Hub, you first must create a configuration file on your Robot Controller, so that the Robot Controller will know what hardware is available on the Control Hub's or Expansion Hub's external ports.

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Wiring Guide > Specific Recommendations > Servo Wires

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Using heavy-duty extension wires are recommended.

Tape the connections between extension wires and servo wires with electrical tape. This is as the connections can become loose over time and are easy to pull out.

The VEX Motor Controller 29 has the wrong gender connector on the 3-pin end. You are required to either use an adapter cable, or add the right connector to the wires (recommended). Be sure to protect the MC29, as it is fragile and prone to failure if it takes impact from another object.

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Controlling a Servo Blocks > Modifying Your Op Mode to Control a Servo > Modifying the Op Mode to Control a Servo Motor Instructions

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This set of blocks will send the current servo position value to the DRIVER STATION while the op mode is running.

Save your op mode and verify that it was saved successfully to

the Robot Controller.

Follow the procedure outlined in the section titled :doc:`Running Your

OpMode <../running_op_modes/Running-Your-Op-Mode>` to run your updated op mode. Also, make sure that your gamepad is designated as User #1 before running your op mode.

You should now be able to control the servo position with the colored buttons. The servo position should be displayed on the DRIVER STATION.

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Choosing a Servo > Servo Range

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The angle over which a non-continuous servo can rotate while retaining position feedback is called the range. When choosing a servo, it is important to make sure you have enough range for the movement you need.

By default, the FTC control system outputs 600-2400 microsecond signals. However, this range can be expanded to 500-2500 microseconds. When choosing a servo, it is important to make sure that its range will be usable for your application inside of 500-2500 microseconds.

Note: The default 600-2400 range of the FTC control system when selecting the "Servo" configuration limits the range of many servos to less than the advertised range. Make sure you select the "Full Range Servo" configuration instead to get the full 500-2500 range.

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Getting Started > Creating a Configuration > Creating a Configuration File Using the DRIVER STATION > Creating a Configuration File on the Robot Controller using the DRIVER STATION Instructions

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Touch the three vertical dots in the upper right hand corner of

the Driver Station app. This will launch a pop-up menu.

Select Configure Robot from the pop up menu to display the

Configuration screen.

If your Robot Controller does not have any existing configuration

files, the screen will display a message indicating that you need to create a file before proceeding.

Hit the New button to create a new configuration file for your Robot Controller.

When the new configuration screen appears, the Robot Controller

app will do a scan of the serial bus to see what devices are connected to the Robot Controller.

It will display the devices that it found in a list underneath the words "USB Devices in configuration." You should see an entry that says something like "Expansion Hub Portal 1" in the list.

Your Expansion Hub is listed as a Portal because it is directly connected to the Robot Controller phone through the USB cable or in the case of the Control Hub through the internal serial bus.

If you do not see your Expansion Hub Portal listed and you are using a smartphone as a Robot Controller, check the wired connections to make sure they are secure and then press the Scan button one or two times more to see if the smartphone detects the device on a re-scan of the USB bus.

Touch the Portal listing ("Expansion Hub Portal 1" in this

example) to display what Expansion Hubs are connected through this Portal.

Since we only have a single Expansion Hub connected, we should only see a single Expansion Hub configured ("Expansion Hub 2" in this example).

Touch the Expansion Hub listing ("Expansion Hub 2" in this

example) to display the Input/Output ports for that device.

The screen should change and list all the motor, servo and sensor ports that are available on the selected Expansion Hub.

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SDK Servos > Hidden Methods

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All REV hub servos are instances of ServoImplEx, which exposes some more methods to the user, such as setting the servo PWM range.

Note: There is no downside to using ServoImplEx, in order to convert a Servo to a ServoImplEx the user simply just needs to cast the Servo returned by the hardwareMap to a ServoImplEx.

Tips:

You can use setPwmRange() to increase the servo range to a maximum of 500 to 2500 microseconds. This changes the SDK mapping, with 0 becoming the minimum microseconds and 1 becoming the maximum microseconds. Increasing the range to 500-2500 can give more range when controlling servos that use 500-2500 PWM Range, such as goBILDA servos and REV Smart Robot Servo
setPwmDisable() and setPwmEnable() can be used to turn on and off the PWM signal to a servo. On some servos, this will cause the servo to de-energize and "turn off" with no holding power. Other servos may stay on and hold their position. Behavior will vary from servo manufacturer to servo manufacturer. It will also disable 5V power if the servo is the only one in its group of ports.

Tip: Both REV Smart Robot Servos and goBILDA servos will stop holding their position when setPwmDisable is called, regardless of if the 5v power is still on or not.

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Using Servos on Your Robot > Servo Power Injectors

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Servo Power Injectors have been used in FIRST Tech Challenge for several years now, but do you really understand what they are and how they work? What is a Servo Power Injector and how might servos behave differently when used with one?

A servo connection is a 3-wire combination that combines power, ground, and a signal. The actual command signal for the servo travels on the signal wire, and the power used to power the servo travels on the other two wires. A servo power injector is a device that removes the power provided by the servo controller (REV Control Hub or REV Expansion Hub for FTC) and provides a new, usually higher wattage power source. Both the REV Servo Power Module and Studica Servo Power Block replace the 5V/10W power provided by the REV Control/Expansion Hubs with a 6V power source with a higher maximum wattage.

So what does higher voltage do for a servo? Servos operate on a power range; the more power they get, the faster and stronger they can become, up to a certain limit. Servos operating at 5V get a noticeable boost in speed and output power when used at 6V. The same servos may seem superhuman at higher voltages!

The downside of servo power injectors is that teams are now responsible for managing their own power usage. On the REV Control Hub, for instance, each servo port pair is limited in how much power it can draw (at least there's a limit on how long it can draw high loads). When using a servo power injector, the pool of power for a servo is much larger and less restricted since it pulls its power directly from the robot battery - using power injectors means you could consume all of the power on the robot just from the servos alone! This will result in the robot power system browning out (resulting in loss of communications or loss of power to the control system) or even blowing the 20A battery fuse.

Using a servo power injector can also expose different behaviors in servos that were not present when using the REV Control/Expansion Hub directly. The biggest behavior is the "Lost Signal" behavior. When an OpMode ends, the REV Control/Expansion hubs stop the signal and also cut power to the servo ports - this leads to the servos "going limp" as they lose power. With a servo power injector, the servos never lose power, and so "lost signal" behaviors will often then take over which may cause the servo to move to a "default" position (which is virtually never advantageous for robots but definitely advantageous for R/C planes for example). The Axon MAX+ servo and several higher-power HiTec servos have this behavior, the Axon MAX+ behavior is at least configurable with a servo programmer.

Finally, when using a servo power injector it's of VITAL importance that you cover unused ports with tape or other debris-limiting measures to protect the ports. It's very easy to get metal swarf in open servo ports, and that metal can short out the power output pins - especially lower-cost power injectors cannot tell when they're being used, or don't have protections against short circuits, but they still have all output pins powered. This can quickly turn your servo power injector into an expensive paperweight when the power regulator overloads and burns out.

Tip: Wondering how to compare one servo against another? See Calculating Motor and Servo Power.

Got any questions about servos? Come start or join the conversation on the FTC Community Forums (https://ftc-community.firstinspires.org/)!

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