In the Encoder Navigation section the concept of moving the motor to a specific position based on encoder ticks was introduced. The process highlighted in Encoder Navigation focused on how to convert from encoder ticks to rotations to a linear distance. A similar procedure can be utilized to move the arm to a particular position. However, unlike the drivetrain, the arm does not follow a linear path. Rather than convert to a linear distance it makes more sense to convert the encoder ticks into an angle measured in degrees.
In the image below two potential positions are showcased for the ClassBot arm. One of the positions - highlighted in blue below - is the position where the arm meets the limit of the touch sensor. Due to the limit, this position will be our default or starting position. From the Class Bot build guide, it is known that the Extrusion supporting the battery sits a 45 degree angle. Since the arm is roughly parallel to these extrusion when it is in the starting position, we can estimate that the default angle of the arm is roughly 45 degrees.
The goal of this section is to determine the amount of encoder ticks it will take to move the arm from its starting position to a position around 90 degrees. There are a few different ways this can be accomplished. An estimation can be done by moving the arm to the desired position and recording the telemetry feedback from the Driver Station. Another option is to do to the math calculations to find the amount of encoder ticks occur per degree moved. Follow through this section to walk through both options and determine which is the best for your team.
To estimate the position of the arm using telemetry and testing, lets start with the Gamepad if/else ifcode.
For now you can comment out the limit switch related code.
Within the while loop add the linestelemetry.addData("Arm Test", arm.getCurrentPosition()); and telemetry.update();
Save the op mode and run it. Use the gamepad commands to move the arm to the 90 degree position. Once you have the arm properly positioned read the telemetry off the Driver Station to determine the encoder count relative to the position of the arm.
Recall from the Basic Encoder Concepts section that the encoder position is set to 0 each time the Control Hub is turned on. This means that if your arm is in a position other than the starting position when the Control Hub is turned on, that position becomes zero instead of the starting position.
The number given in the image above is not necessarily an accurate encoder count for the 90 degree position. To get the most accurate encoder reading for your robot make sure that your starting position reads as 0 encoder counts. To further increase accuracy consider doing several testing runs before deciding on the number of counts.
Recall that in order to execute RUN_TO_POSITION the following three lines of cod need to be added to both sections of the Gamepad if/else if block.
When DpadUp is pressed, the arm should move to the the 90 degree position. WhenDpadDown is pressed the arm should move back to the starting position. To do this set the firstarm.setTargetPosition(0); equal to the number of ticks it took your arm to get to 90 degrees, for this example we will use 83 ticks.
Since we want DpadDown to return the arm to the starting position, keeping the arm.setTargetPosition(0); set to 0 will allow us to accomplish this. Set both arm.setPower(0); equal to 0.5.
Note: the code above was given a file name Target Position if/else if this code will be referenced again.
Recall that the target position dictates which direction the motor moves, taking over the directionality control from arm.setPower(); so both blocks can be set to a positive value since they will control the speed.
If you try running this code you may notice that the arm oscillates in the 90 degree position. When this behavior is present you should also notice the telemetry output for the encoder counts fluctuating. RUN_TO_POSITION is a Closed Loop Control, which means that if the arm does not perfectly reach the target position, the motor will continue to fluctuate until it does. When motors continue to oscillate and never quite reach the target position this may be a sign that the factors determining tolerances and other aspects of the closed loop are not tuned to this particular motor or mechanism. There are ways to tune the motor, but for now we want to focus on working with the arm and expanding on how limits and positions work with regards to the mechanism.
In the initial introduction to run to position, you worked through the calculations needed to convert the ticks per rotation of a motor into ticks per mm moved. Now we want to focus on how to convert ticks per rotation of the motor to ticks per degree moved. From the previous section you should have a rough estimate of the amount of ticks you need to get to the 90 degree position. The goal of this section is to work through how to get a more exact position.
To start you will need some of the same variables we used in Encoder Navigation:
Recall, that ticks per revolution of the encoder shaft is different than the ticks per revolution of the shaft that is controlling a mechanism. We saw this in the Encoder Navigation section when the ticks per revolution at the motor was different from the ticks per revolution of the wheel. As motion is transmitted from a motor to a mechanism, the resolution of the encoder ticks changes.
For more information on the effect of motion transmission across a mechanism check out the Compound Gearing section.
The amount of ticks per revolution of the encoder shaft is dependent on the motor and encoder. Manufacturers of motors with built-in encoders will have information on the amount of ticks per revolution.
Visit the manufacturers website for your motor or encoders for more information on encoder counts. For HD Hex Motors or Core Hex Motors visit the Motor documentation.
In the Core Hex Motor specifications there are two different Encoder Counts per Revolution numbers:
At the motor - 4 counts/revolution
At the output - 288 counts/revolution
At the motor is the number of encoder counts on the shaft that encoder is on. This number is equivalent to the 28 counts per revolution we used for the HD Hex Motor. The 288 counts "at the output" accounts for the change in resolution after the motion is transmitted from the motor to the built in 72:1 gearbox. Lets use the 288 as ticks per revolution so that we do not have to account for the gearbox in our total gear reduction variable.
Since we built the the gear reduction from the motor gearbox into the ticks per revolution the main focus of this section is calculating the gear reduction of the arm joint. The motor shaft drives a 45 tooth gear that transmits motion to a 125 tooth gear. The total gear ratio is 125T:45T. To calculate the gear reduction for this gear train, we can simply divide 125 by 45.
To summarize, for the Class Bot V2 the following information is true:
Now that we have this information lets create two constant variables:
COUNTS_PER_MOTOR_REV
GEAR_REDUCTION
The common naming convention for constant variables is known as CONSTANT_CASE, where the variable name is in all caps and words are separated by and underscore.
Add the COUNTS_PER_MOTOR_REV and GEAR_REDUCTIONvariables to the op mode beneath where the hardware variables are created.
Now that these two variables have been defined, we can use them to calculate two other variables: the amount of encoder counts per rotation of the 125T driven gear and the number of counts per degree moved.
Calculating counts per revolution of the 125T gear (or COUNTS_PER_GEAR_REV)is the same formula used in Converting Encoder Ticks for ourCOUNTS_PER_WHEEL_REVvariable. So to get this variable we can multipleCOUNTS_PER_MOTOR_REV by GEAR_REDUCTION.
To calculate the number of counts per degree or moved or COUNTS_PER_DEGREEdivide the COUNTS_PER_GEAR_REVvariable by 360.
Add both these variables to the op mode.
Finally we need to create a non-constant variable that will act as our position. Create a variable called armPosition above the waitForStart(); command.
Add this variable to the if(gaempad1.dpad_up) section of the Target Position if/else ifstatement, as this section dictates the 90 degree position. To get to the 90 degree position, the arm needs to move roughly 45 degrees. Set arm position equal to COUNTS_PER_DEGREEtimes 45.
Recall that setTargetPosition()requires an integer to be its parameter. When defining armPosition remember to add the line (int) in front of the double variable. However, you need to be cautious of potential rounding errors. Since COUNTS_PER_MMis part of an equation it is recommended that you convert to an integer after the result of the equation is found.
Set target position to armPostion.
We could change what armPosition is equal to in the gamepad1.dpad_down portion of the if/else if statement such as:
In this case we would consistently redefine armPosition to match the needs of whatever positions we want to create. Since our only two positions at the moment are our starting position and our 90 degree position it isn't necessary However, it is a good practice to create a variable in situations like this. If we want to add another position later, we can easily edit the variable to fit our needs.
Ticks per revolution
288 ticks
Total gear reduction
2.777778
In the previous sections you worked on some of the building blocks for restricting an arms range of motion. From those sections you should have the foundation you need to perform basic arm control. However, there are some other creative ways you can use encoder positions and limits to expand the control you have over your arm.
This section will cover two additional types of control. The first type of control we will explore is the idea of soft limits. In the Adding a Limit Switch section we discuss the concept of physical limits of a mechanism however, there may be times you need to limit the range of motion of an arm without installing a physical limit. To do this position based code can be used to create a range for the arm.
Once you have a basic idea of how to create soft limits, we will explore how to use a limit switch (like a touch sensor) to reset the range of motion. This type of control reduces the risk of getting stuck outside of your intended range of motion, which can affect the expected behavior of your robot.
To set the soft limits we will use some of the basic logic we established in previous sections, with some edited changes. Start with a Basic Op Mode and add the constant variables from the Calculating Target Position section to the op mode.
Next we need to create our upper and lower limits. Create two new integer variables one called minPosition and one called maxPosition. Add both of these to the in the initialization section of the op mode above the waitForStart(); command.
For now we want the minPosition set as our starting position and the maxPosition set to our 90 degree position. Set minPositionequal to 0 and set maxPositionequal toCOUNTS_PER_DEGREE times 45 .
Remember you need to make a data type conversion!
An if/else if statement needs to be added to control the arm, for this we can use the same basic logic we use in the Estimating the Position of the Arm section.
To set the limit we need to edit our if/else ifstatement so that the limits are built in. If DpadUpis selected and the position of the arm is less than the maxPosition then the arm will move to the maxPosition. If DpadDown is selected and the position of the is greater that the minPosition then the arm will move towards the minPosition.
The current code configuration will stop the motor at any point that the conditions to power the motor are not met. Depending to factors like the weight of the mechanism and any load that it is bearing, when the motor stops the arm may drop below the maxPosition. Take time to test out the code and confirm that it behaves in the way you expect it to.
One of the benefits of having a soft limit is being able to exceed that limit. Since encoders zero tick position is determined by the position of the arm when the Control Hub powers on; if attention is not payed to what position the arm is on power up the range of motion of the arm is affected. For instance, if we have to reset the Control Hub while the arm is in the 90 degree position, the 90 degree position is equal to 0 encoder ticks. One way around this is to create an override for the range of motion.
There are a few different ways an override of sorts can be created, in our case we are going to use the a button and touch sensor to help reset our range.
Start by editing the if/else if statement to add another else if condition. Use the line gamepad1.a as the condition. Add a the line arm.setPower(-0.5); as the action item.
Now that we have this change in place, when the a button is pressed the arm will move toward the starting position. When the arm reaches and presses the touch sensor we want toSTOP_AND_RESET_ENCODER .
We can create another if statement that focuses on performing this stop and reset when the Touch Sensor is pressed.
So, if the Touch Sensor returns true (or is pressed) the motor run mode STOP_AND_RESET_ENCODERwill be activated causing the motor encoder to reset to 0 ticks.
Now that this code is done, try testing it!
