Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Overview In this lesson you will learn: Current flow Causes of excessive current flow How current is affected by work How to measure DC motor current with a multimeter

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Powering a vehicle is a basic concern of any robot designer. In this lesson you will investigate how the amount of power needed for a vehicle varies according to how much work it performs. Since it is not convenient to measure power directly, you will measure current, which varies directly with power. In order to do this, you will modify the battery powering your robot so that you can put it in series with the multimeter. Then, you will observe and record the amount of current the robot needs as it performs increasing amounts of work. You will increase the amount of work required for the robot by increasing the angle the robot climbs by regular increments. After plotting your data on a graph, you will analyze the graph and demonstrate the relation between work and current, and, by extension, between work and power.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Materials needed: Digital multimeter Vex Squarebot Radio control transmitter Board or plywood (8 ft. by 12 in.) White foam board (8 ft. by 4 ft.) or other board suitable for drawing. Protractor

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction I I I I I What is current? Current is the movement of electrons through a circuit. In the Ohm’s Law formula, I=E/R, the letter I is used to represent current. E represents volts (electromotive force), and R is resistance.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Why is it important to understand the concept of electrical current flow? In any circuit, the amount of current flow will determine how much work is being performed. Proper control of current will allow efficient operation of DC motors and servo devices.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction In robotic applications, the following items will determine how much current is flowing through the DC motor circuits: Wheel size Gear ratios Weight (how heavy is the moving robot?) Mechanics (Friction) Terrain (Are there hills or obstructions in the running surface? Battery voltage and condition

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Keep in mind: In general, a small motor does less work than a larger one Motor size does not necessarily determine a required battery voltage Current and voltage ratings are determining factors in how much work can be performed by a particular DC motor

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Gear Ratios: There is an inverse relationship between gear ratio and current draw, if all other factors are held constant. The higher the gear ratio, the less current will be required for the same driving gear speed. The lower the gear ratio, the more current will be required for the same driving gear speed.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Wheel size: There is a direct relationship between current draw and wheel size, if all other factors are held constant. The smaller the wheel diameter, the less current will be required for the same driving gear speed. The greater the wheel diameter, the more current will be required for the same driving gear speed.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Weight: There is a direct relationship between weight and current draw, if all other factors are held constant. Increasing the weight the motors move will increase the current draw, if the driving axle speed remains the same.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Gears, alignment, and leverage: Improper alignment of axles, shafts, and gears will have a braking effect, which will increase the motor current, compared to proper alignment.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Battery voltage: If a 6 volt battery is used on a 12 volt motor, it will run more slowly (or may not run at all) and will not produce the proper power. If a 12 volt battery is used on a 6 volt motor, the motor will draw excessive current and be destroyed. Weak or dead batteries will not provide adequate voltages for correct motor operation.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Introduction Terrain: Irregular surfaces and steep grades will also affect current draw, if all other factors are held constant In this lesson, we will investigate what happens to current draw when we require a robot to navigate progressively steeper grades.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Set up Build the incline indicator. You will need: A protractor A straight edge A flat 8’ x 12” board Any 8’ x 4” white board that can easily be drawn on. A whiteboard or white foam board will work well. Set angles in 5° increments Starting point

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Set up Starting point Use the protractor and the straight edge to draw lines on the white board:

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Set up Watch the set up video. It covers: Wiring the robot battery in series with the multimeter Modifying Squarebot so you can mount the multimeter on it securely.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Lesson Watch the lesson video. It covers: The experimental procedure. You will find the current that the robot uses when traveling on different inclines. You will then use that data to draw conclusions about current and work.

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Sample Data Angle of InclineVoltage

Vex 1.0 © 2005 Carnegie Mellon Robotics Academy Inc. CURRENT DRAW Extension Activity Compare dragging weight to rolling weight and see if you can find a proportional relationship. How do current readings from dragging weight compare to current readings from rolling weight?