Drivetrain Encoders - Blocks

We're updating this documentation!

During 2024 Summer, "Hello Robot" will be getting a refreshed look to reflect changes to the latest versions of the REV Hardware Client and Robot Controller App.

While this guide is still generally relevant, you may see differences in the naming of devices or commands along with visual updates to the guide.

In the previous section you learned about how to use Elapsed Time to allow your robot to navigate the world around it autonomously. When starting out many of the robot actions can be accomplished by turning on a motor for a specific amount of time. Eventually, these time-based actions may not be accurate or repeatable enough. Environmental factors, such as the state of battery charge during operation and mechanisms wearing in through use, can all affect time-based actions. Fortunately, there is a way to give feedback to the robot about how it is operating by using sensors; devices that are used to collect information about the robot and the environment around it.

With Elapsed Time, in order to get the robot to move to a specific distance, you had to estimate the amount of time and the percentage of duty cycle needed to get from point a to point b. However, the REV motors come with built in encoders, which provide feedback in the form of ticks ( or counts) per revolution of the motor. The information provided by the encoders can be used to move the motor to a target position, or a target distance.

Moving the motors to a specific position, using the encoders, removes any potential inaccuracies or inconsistencies from using Elapsed Time. The focus of this section is to move the robot to a target position using encoders.

There are two articles in that go through the basics of Encoders. Using Encoders goes through the basics of the different types of motor modes, as well as a few application examples of using these modes in code. In this section we will focus on using RUN_TO_POSITION.

The other article, Encoders, focuses on the general functionality of an encoder.

It is recommended that you review both articles before moving on with this guide.

Basics of Programming with Encoders

Start by creating a basic op mode called HelloRobot_EncoderAuton.

When creating an op mode a decision needs to be made on whether or not to set it to autonomous mode. For applications under 30 seconds, typically required for competitive game play changing the op mode type to autonomous is recommended. For applications over 30 seconds, setting the code to the autonomous op mode type will limit your autonomous code to 30 seconds of run time. If you plan on exceeding the 30 seconds built into the SDK, keeping the code as a teleoperated op mode type is recommended.

For information on how op modes work please visit the Introduction to Programming section.

For more information on how to change the op mode type check out the Test Bed - Blocks section.

Add the block to the op mode under the .This will change the direction of the rotation of the right motor to be the same direction as the left motor.

For more information on the directionality of motor check out the Basics of Programming Drivetrains section.

Recall from Using Encoders that using RUN_TO_POSITION mode requires a three step process. The first step is setting target position. To set target position, grab the block and add it to the op mode under the comment. To get a target position that equates to a target distance requires so calculations, which will be covered later. For now, set target position to 1000 ticks.

When there are multiple of the same type of variable (such as multiple Dc Motor variables) the variable specific blocks will choose a default variable based on alphabetical order. For this example Op Mode Dc Motor blocks will default to the arm variable. Click the arrow next to the motor name to change the arm motor variable to the rightmotor variable. Use the variable drop down menu on the block to change from arm to rightmotor.

The next step is to set both motors to the RUN_TO_POSITION mode. Place the block beneath the block.

The main focus of the three step process is to set a target, tell the robot to move to that target, and at what speed (or velocity) the robot should get to that target. Normally, the recommended next step is to calculate velocity and set a target velocity based on ticks. However, this requires quite a bit of math to find the appropriate velocity. For testing purposes, its more important to make sure that the main part of the code is working before getting too deep into the creation of the code. Since the function was covered in previous sections and will communicate to the system what relative speed (or in this case duty cycle) is needed to get to the target, this can be used in the place of velocity for now.

Add the block to the op mode beneath the block. Change the duty cycle (or power) of both motors to 0.8, instead of 1.

Now that all three RUN_TO_POSITION steps have been added to the code the code can be tested. However, if you want to wait for the motor to reach its target position before continuing in your program, you can use a while loop that checks if the motor is busy (not yet at its target). For this program lets edit the block.

Recall that, within a linear op mode, a while loop must always have the Boolean as a condition. This condition ensures that the while loop will terminate when the stop button is pressed.

Grab an block from the logic menu and add it to the while loop. On the left side of the block add the block. On the right side add the block.

Embed the in another block. Place the on the right side of the block. On the left side add the block.

Right now the while loop is waiting for the right and left motors to reach their respective targets. There may be occasions when you want to wait for both motors to reach their target position, in this case the can be used.

Save and run the op mode two times in a row. Does the robot move as expected the second time?

Try turning the Control Hub off and then back on. How does the robot move?

In the Basic Encoder Concepts section, it is clarified that ell encoder ports start at 0 ticks when the Control Hub is turned on. Since you did not turn off the Control Hub in between runs, the second time you ran the op mode the motors were already at, or around, the target position. When you run a code, you want to ensure that certain variables start in a known state. For the encoder ticks, this can be achieved by setting the mode to . Add this block to the op mode in the initialization section. Each time the op mode is initialized, the encoder ticks will be reset to zero.

For more information on the motor mode STOP_AND_RESET_ENCODERScheck out the STOP_AND_RESET_ENCODERS section of the Using Encoders guide.