Using the Potentiometer V2 with the V5 Workcell

Potentiometers are used with the V5 Workcell to ensure that it operates safely and in a repeatable manner.

This article will cover the following:

  • What is a potentiometer?
  • VEX’s potentiometers
  • What is Robot Mastering?
  • Why does the V5 Workcell need potentiometers to operate?

What is a potentiometer?

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Potentiometers are analog variable resistors, which provide a variable voltage value based on the position of the wiper arm (the piece that moves across the resistive track material), inside of the potentiometer.

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These are used on the Workcell to identify the position of the joints on the Workcell at all times, based on the wiper arm’s position of the potentiometer. The 3-Wire Port on the V5 Brain converts the voltage values into a digital value between 0 and 4095.

The bottom part of the potentiometer where the resistive track doesn't cover is called the ‘deadband’ on the potentiometer. If the wiper arm is not connected to the resistive area (in the deadband), then the circuit is open. An open circuit returns 0 volts.


VEX’s potentiometers

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Most potentiometers used in everyday objects, such as volume knobs in a car or on a stereo, or to control a light dimmer in your house, look similar to this.

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Most of these examples have a fixed shaft, similar to this image.

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VEX potentiometers use a through hole instead of a fixed shaft, so you can pass a square shaft through the center to act as the post, controlling the position of the potentiometer.


What is Robot Mastering?

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Industrial robots in general, and specifically the V5 Workcell, need to operate in a way that is safe and repeatable. The Workcell needs a fixed, known location (also called a home location) to base all subsequent movements from.

On the Workcell, the individual motors do not have physical limits in the same way that the metal on the arm does. The metal on the arm has physical limits that prevent it from moving in certain orientations.

Because of this, the motors could potentially continue to spin and force the arm to move beyond its physical limitations, causing the Workcell to potentially break.


Why does the V5 Workcell need potentiometers to operate?

The arm of the V5 Workcell uses the potentiometers in order to track its position while moving around the surface area of the Workcell.

During installation and mastering of the arm, the range of acceptable values from the potentiometers has been set to ensure the V5 Workcell will operate in a safe manner. If the potentiometers begin within this range, the potentiometer’s wiper arm will not enter the deadband of the potentiometer’s resistance track, and ensures the arm knows its position at all times.

The potentiometer ranges for each joint during the mastering process are the following:

  • Joint 1: 1600 - 2000
  • Joint 2: 1900 - 2400
  • Joint 3: 1700 - 2100
  • Joint 4: 200 - 650

The potentiometers are also used to know where the arm of the Workcell is in three-dimensional space. This is an important piece of information that is used when coding the V5 Workcell using VEXcode V5 to ensure that it operates in an accurate and repeatable manner.

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The mastering process ensures that the potentiometers associated with each of the four joints on the Workcell, are within the predetermined ranges mentioned above. When mastering, an example project in VEXcode V5 is used to report if the potentiometers are within the acceptable ranges. If so, these values are then recorded by the user and defined when coding the Workcell in VEXcode V5.

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If any of the four joints are reporting that they are failing, moving the arm could spin the wiper arm in the potentiometer into the deadband zone—this would cause the arm to not know its current physical location and may cause damage to the Workcell or the arm itself.

Using potentiometers on the V5 Workcell ensures that it was built and assembled properly, safely operates within its physical limitations, and can move in a repeatable manner because it has a defined, fixed ‘home location.’