Commanding Servos

Servo Control Basics

Servo motors are specialized motors that can be controlled to move to a specific angle instead of continuously rotating like a DC motor. For more general information on servos, see this page; for more detailed information, see https://en.wikipedia.org/wiki/Servo_control.

Setting up Servo Hub

Servo control in REVLib is accessed through the ServoHub's object. This object manages the individual ServoChannel objects and monitors the overall device. The ServoChannel object controls each servo motor and monitors its operation. It also contains all the methods to configure and control your servo. It can be accessed as shown below:

Java

// Initialize the servo hub
ServoHub m_servoHub = new ServoHub(deviceID);

// Obtain a servo channel controller
ServoChannel m_channel0 = m_servoHub.getServoChannel(ChannelId.kChannelId0);
ServoChannel m_channel1 = m_servoHub.getServoChannel(ChannelId.kChannelId1);
...
ServoChannel m_channel5 = m_servoHub.getServoChannel(ChannelId.kChannelId5);

API Docs: ServoHub, ServoChannel

C++

using namespace rev::servohub;
using namespace rev::servohub::ServoChannel;

// Initialize the servo hub
ServoHub m_servoHub{ deviceID };

//Obtain a reference to a servo channel controller
ServoChannel& m_channel0 = m_servoHub.GetServoChannel(ChannelId.kChannelId0);
ServoChannel& m_channel1 = m_servoHub.GetServoChannel(ChannelId.kChannelId1);
...
ServoChannel& m_channel5 = m_servoHub.GetServoChannel(ChannelId.kChannelId5);

API Docs: ServoHub, ServoChannel

Setting the Servo Pulse Period

With a servo motor, the width of the pulse will determine how far the motor turns. The Pulse Period, on the other hand, will determine how often the pulse is sent to the servo. The ServoHub supports a Pulse Period of 4.5 - 20ms (specified in microseconds). The ServoHub supports a separate Pulse Period for each bank, composing servos 0-2 and 3-5, respectively. These settings can be accessed as shown below:

Java

// Set the pulse period for channels 0-2 to 5ms (5000 microseconds)
m_servoHub.setBankPulsePeriod(ServoHub.Bank.kBank0_2, 5000);

// Set the pulse period for channels 3-5 to 20ms (20000 microseconds)
m_servoHub.setBankPulsePeriod(ServoHub.Bank.kBank3_5, 20000);

API Docs: setBankPulsePeriod

C++

using namespace rev::servohub;

// Set the pulse period for channels 0-2 to 5ms (5000 microseconds)
m_servoHub.SetBankPulsePeriod(ServoHub::Bank::kBank0_2, 5000);

// Set the pulse period for channels 3-5 to 20ms (20000 microseconds)
m_servoHub.SetBankPulsePeriod(ServoHub::Bank::kBank3_5, 20000);

API Docs: SetBankPulsePeriod

Controlling an Individual Servo

Individual servos are controlled via the ServoChannel objects. As shown above, you obtain a reference to a ServoChannel object by calling the getServoChannel() method on the ServoHub. You may set the following on a ServoChannel:

• PulseWidth - determines the servo's position (500 - 2500 microseconds).

• Enabled - enables/disables the servo - when disabled, the servo will maintain power according to the DisableBehavior configured for the specific channel.

• Powered - turns on/off the power to the servo

Java

// Power on channels 0, 1, and 4
m_channel0.setPowered(true);
m_channel1.setPowered(true);
m_channel4.setPowered(true);

// Enabled them as well
m_channel0.setEnabled(true);
m_channel1.setEnabled(true);
m_channel4.setEnabled(true)

// Set the servo on channel 0 to the center (1500 microseconds)
m_channel0.setPulseWidth(1500);

// Set the servo on channel 1 to the far left (500 microseconds)
m_channel1.setPulseWidth(500);

// Set the servo on channel 4 to the far right(2500 microseconds)
m_channel4.setPulseWidth(2500);

API Docs: setPulseWidth, setPowered, setEnabled

C++

using namespace rev::servohub;

// Power on channels 0, 1, and 4
m_channel0.SetPowered(true);
m_channel1.SetPowered(true);
m_channel4.SetPowered(true);

// Enabled them as well
m_channel0.SetEnabled(true);
m_channel1.SetEnabled(true);
m_channel4.SetEnabled(true)

// Set the servo on channel 0 to the center (1500 microseconds)
m_channel0.SetPulseWidth(1500);

// Set the servo on channel 1 to the far left (500 microseconds)
m_channel1.SetPulseWidth(500);

// Set the servo on channel 4 to the far right(2500 microseconds)
m_channel4.SetPulseWidth(2500);

API Docs: SetPulseWidth, SetEnabled, SetPowered