The Potentiometer is a sensor which uses a variable resistor to measure the angular rotation (up to 265o) of a shaft. It is one of the 3-Wire series sensors.
The Potentiometer is designed with a “D-hole” in its center. This allows a square shaft to be inserted through the hole and change the sensor’s hub position as the shaft rotates.
The sensor’s housing has two mounting arc slots which allows the Potentiometer position to be fine tuned up to 90o after it has been attached to the robot’s structure. This can be accomplished by loosening the attachment screws, adjusting the sensor, and then retightening the screws.
The 3-Wire Sensors are compatible with the V5 Robot Brain or the Cortex. The sensor’s cable can be extended using a 3-Wire Extension Cable.
In order for the Potentiometer to be functional with the V5 Brain, the sensor cable needs to be fully inserted into a V5 Brain 3-Wire Port.
|Potentiometer||Sensor Cable fully Inserted|
How the Potentiometer Works
The Potentiometer functions by having the variable resistor change its value as a shaft rotates the sensor’s central hub. This change in resistance modifies the V5 Brain’s output signal. The returned input signal will have a different voltage. The V5 Brain in concert with a user program can convert this change in voltage into the percent of shaft angle rotation or degrees of shaft rotation. This measurement can detect either an increase or decrease in value.
An advantage of the Potentiometer is it will return the same reading even if the V5 Brain is turned off and then turned back on. If the shaft is rotated while the Brain is off, the value the Potentiometer returns will be the same as if Brain was powered on the entire time. In other words, the Potentiometer will always return a value which is dependant on its central hub’s position. This is unlike the Optical shaft encoder which loses its reading every time the Brain is powered down.
The Potentiometer needs to be paired with a programming language such as the VEXcode V5 or VEXcode Pro V5 to create a user program for the Brain to utilize the change in signal voltage to control the robot’s behavior.
The central hub of the Potentiometer can be rotated by a shaft 265o. This makes the sensor ideal for measuring a shaft which has a limited rotation such as an arm shaft or a claw gripper shaft.
Do not force the Potentiometer to rotate beyond 265o. If the central hub is forced, the sensor’s internal stops can be broken allowing the hub to free rotate. If this happens the sensor should be recycled/discarded because its values will not be true.
The measurement range of the Potentiometer can be extended by placing it on the driven shaft of a “torque” gear ratio and have the shaft being measured on the driving side of the ratio. However, if this method is used, the resolution of the sensor’s values will not be as great.
If a shaft is free-rotating, an Optical Shaft Encoder should be used to measure the shaft’s rotation and not a Potentiometer.
Common Uses for the Potentiometer:
The two applications Potentiometers are commonly used for are controlling the position of an assembly or providing varying feedback to the V5 Brain to select a function or change a variable value. Some classroom examples of these would be:
Controlling position: The shaft for the 84T gear (step 32 V5 Clawbot build) found on the V5 Clawbot’s arm could be replaced with a longer shaft so a Potentiometer can be inserted on the shaft and mounted to the tower of the Clawbot (steps 35,36). Once the sensor is in place, the students can be challenged to use the feedback from the sensor to move, stop, and hold the V5 Clawbot’s arm at 3 different heights when buttons are pressed on the V5 controller.
Adjusting variables/functions: A short shaft can be inserted through a Potentiometer and fixed to the sensor’s hub with Rubber Shaft Collars, then a small component (such as a Driver Shaft Lock Bar) can be added to the shaft to serve as a knob.
Once this assembly is mounted to a robot drivetrain, the students can be challenged to use the feedback from the sensor to change the degrees the robot turns. In this case, each time the program is run, if the knob on the Potentiometer is turned to a different location, the robot will turn a different amount.
Another fun activity using this assembly would be to divide the values returned from the Potentiometer into seven ranges. Challenge the students to create a user program so as the knob is turned from one range of values to the next on the Potentiometer, a different day of the week (or other cute message) is displayed on the Color Touch Screen of the V5 Brain.
Uses of a Potentiometer on a Competition Robot:
Shaft control verification: The V5 Smart Motor’s encoders are excellent when used for control of the rotation of a shaft during a competition. However, there may be cases where a Potentiometer may be useful to verify a secondary shaft is rotating to the correct angle. For example, a Potentiometer could be added to the wrist shaft (step 43, Flip build) of the “Hero” Robot - Flip.
In this example, the sensor will provide feedback for an autonomous program to assure the chain drive for the wrist has not skipped a tooth on the wrist’s sprocket and is in sync with the V5 Smart Motor’s encoder.
Program Function Selection: The V5 Brain with its ability to hold 8 different programs can provide many programmed routines. These programs can be selected by the Brain’s Touch Screen prior to the start of a match. However, a Potentiometer with a knob could be used to select functions within a program or adjust variables in the last moment prior to setting the robot on the field without having to remove the Touch Screen’s protective shield.
For example, the knob on the Potentiometer could be rotated from one side (low range) to the other side (high range) to switch the robot’s autonomous routine from a Blue Alliance routine and a Red Alliance routine.