In Unit 5: Speed, Power, Torque, and DC Motors, you build a VEX test stand winch that enables you to learn key engineering concepts and principles so you can directly apply mathematics and science to robotics.

After completing Unit 5: Speed, Power, Torque, and DC Motors, you will be able to:

Demonstrate the physical concepts of speed, force, torque, power, acceleration, and characteristics of DC Motors.

Test a VEX motor and build a simple winch.

Determine and calculate the free speed and stall torque of a VEX motor.

Key terms and definition chart page 5-2

Acceleration A change in speed over time.

Chamfer A placed feature that bevels a part edge and is defined by its placement, size, and angle.

DC motor An electric rotating machine energized by direct current and used to convert electric energy to mechanical energy.

Direct current (DC)

An electric current flowing in one direction only.

Force Accelerations are caused by forces. For example, when a robot is accelerating it does so because of the force its wheels exert on the floor

Keyway A slot for a key in the hub or shaft of a wheel. This permits the shaft and wheel to turn together.

Power-- Energy that is produced by mechanical, electrical, or other means and used to operate a device. The time-rate of doing work, measured in watts or less frequently horsepower.

Relief --An undercut on a shaft. Typically used between the shoulder of a shaft and a threaded section. This makes it easier to cut a thread on a shaft.

Shaft-- Often referred to as a drive shaft. A mechanical device for transferring power from the engine or motor to where it is wanted.

Speed Measure of how fast an object is moving, that is, how much distance it will travel over a given time.

Torque Torque is the application of force where there is rotational motion.

Work --The measure of a force exerted over a distance.

Wrench flat-- Flat surfaces cut on opposite sides of a shaft. These flats are sized for standard wrench openings and allow for assembly of the shaft.

Speed, Power, Torque, and DC Motors To understand the performance of a robot one must first understand the basic concepts of physics. Speed The first concept is the concept of speed. Speed is a measure of how fast an object is moving, that is, how much distance it will travel over a given time. This measure is given in units distance per time (some common ones include miles per hour or feet per second). Rotational Speed Speed can be expressed rotationally as well. This refers to how fast something is moving in a circle. It is measured in units of angular-distance per time or rotational cycles per time. Common examples include degrees per second or revolutions per minute (RPM).

Acceleration A change in speed over time is known as acceleration; the higher the acceleration the faster the change in speed. If something is moving at a constant speed, it is not accelerating. Force Accelerations are caused by forces. When you press on something you are exerting a force on it. When a robot is accelerating it does so because of the force its wheels exert on the floor. Force is measured in units such as pounds and newtons. Torque Force directed in a circle is called torque. Torque is a spinning force; however in the instance of a wheel, this spinning force creates a linear force at its edge. This is how we define torque, as a linear force at the edge of a circle. Torque is described by the magnitude of the force multiplied by the distance it is from the center of rotation (Force x Distance = Torque). Torque is measured commonly in units of inch-pounds and newton-meters.

So if we know how much torque is applied to an axle with a wheel on it, we can find out how much force the wheel is applying on the floor.

Force = Torque / Wheel Radius In this case, the wheel radius would be the distance from the center of rotation.

Work Work is the measure of a force exerted over a distance. If I lift something 5 feet, it requires less work than if I lifted it 10 feet. It can also be thought of as a change in energy.

Power Power is another concept important in robotics. Most people are more familiar with power as an electrical term, but it is part of mechanical physics as well.

Power is defined as the rate that work is performed. How fast can you do your work?

In robotics, it is handy to think of power as a limit. If you need to lift a 10 lb weight (exerting a 10 lb force) the amount of power you have available limits how fast (the rate) at which you can lift it. If you have lots of power available, you will be able to lift it quickly; if you don’t have a lot of power, you will lift it slowly.

Power can be defined as force multiplied by velocity. (How fast can you push with a known force?) Power is frequently measured in terms of watts.

Power = Force [Newtons] x Velocity [Meters / Second]

1 Watt = 1 (Newton x Meter)/ Second

The descriptions above only scratch the surface of these physical properties. The concepts discussed above are significantly more advanced that touched upon here. However, the basic understanding is enough to apply them.

DC Motors DC (Direct Current) motors convert electrical energy into mechanical energy. Motors can have very different characteristics depending on their manufacture.

When a voltage is applied to the motor, it outputs a torque inversely proportional to its speed. (That means the faster the motor is going the LESS torque it outputs, and the slower the motor is going the MORE torque it outputs.) The motor will also draw current proportional to this torque (more torque means more current draw).

You can think of the motor as working to overcome a load. With NO load on the motor, it will spin very fast and draw almost no current. As the load increases on the motor it must output more torque to overcome the load, and as it increases the torque it draws more current. Eventually if enough load is placed on the motor, it will stop moving or “stall."

There are four main characteristics which define DC motor performance and define the relationships described above:

*Stall Torque (N*m): The amount of load placed on a motor which will cause it to stop moving.*Free Speed (RPM): The maximum rotational speed a motor will run at when it is under no load.*Stall Current (Amp): The amount of current a motor will draw when it is stalled.*Free Current (Amp): The amount of current a motor will draw when it is under no load.

In this phase, students assemble a test stand that can be used to determine the specifications of a VEX motor. The completed test stand is shown in the following image.

Phase Objectives After completing this phase, you will be able to: Test the specifications of a VEX motor.

Build a simple winch. Prerequisites and Resources Before starting this phase, you must have:

Completed Unit 5: Speed, Power, Torque, and DC Motors > Think Phase.

Disassembled Protobot used in Unit 3: Building a Protobot.

Complete activity on page 5-31

Then complete the tests on page 5-38/ 5-39