Lecture Outline Lecture Outline DC motors

inefficiencies, operating voltage and current, stall voltage and current and torque

current and work of a motor

Definition of Actuator Definition of Actuator

An actuator is the actual mechanism that enables the effector to execute an action.

E.g, electric motors, hydraulic or pneumatic cylinders, pumps…

Actuators and effectors are not the same thing.

Incorrectly thought of the same; “whatever makes the robot act”

DC Motors DC Motors

The most common actuator in mobile robotics is the direct current (DC) motor

Advantages: simple, cheap, various sizes and packages.

DC motors convert electrical into mechanical energy

How?

How DC Motors Work How DC Motors Work

DC motors consist of permanent magnets with loops of wire inside

When current is applied, the wire loops generate a magnetic field, which reacts against the outside field of the static magnets

The interaction of the fields produces the movement of the shaft/armature

=> Electromagnetic energy becomes motion

Motor Inefficiency Motor Inefficiency

As any physical system, DC motors are not perfectly efficient.

The energy is not converted perfectly. Some is wasted as heat generated by friction of mechanical parts.

Inefficiencies are minimized in well-designed (more expensive) motors, and their efficiency can be high.

How high?

Level of Efficiency Level of Efficiency Good DC motors can be made to be

efficient in the 90th percentile. Cheap DC motors can be as low as

50%. Other types of effectors, such as

miniature electrostatic motors, may have much lower efficiencies still.

Operating Voltage Operating Voltage A motor requires a power source

within its operating voltage, i.e., the recommended voltage range for best efficiency of the motor.

Lower voltages will (usually) turn the motor, but will provide less power.

Higher voltages are more tricky; they increase power output at the expense of the operating life of the motor ( the

more you rev your car engine, the sooner it will die)

Current and Work Current and Work When constant voltage is applied, a

DC motor draws current in the amount proportional to the work it is doing.

E.g., if a robot is pushing against a wall, it is drawing more current (and draining more of its batteries) than when it is moving freely in open space.

The reason is the resistance to the motor motion introduced by the wall.

Stall Current Stall Current

If the resistance is very high (i.e., the wall won't move no matter how hard the robot pushes against it), the motor draws a maximum amount of power, and stalls.

The stall current of the motor is the most current it can draw at its specified voltage.

Torque at the Motor Shaft Torque at the Motor Shaft

Within a motor's operating current range, the more current is used, the more torque or rotational force is produced at the shaft.

The strengths of the magnetic field generated in the wire loops is directly proportional to the applied current and thus the produced torque at the shaft.

Stall Torque Stall Torque

Besides stall current, a motor also has its stall torque.

Stall torque is the amount of rotational force produced when the motor is stalled at its operating voltage.

Power of a Motor Power of a Motor The amount of power a motor

generates is the product of the shaft's rotational velocity and its torque.

If there is no load on the shaft, i.e., the motor is spinning freely, then the rotational velocity is the highest

but the torque is 0, since nothing is being driven by the motor.

The output power, then, is also 0.

Free Spinning and Stalling Free Spinning and Stalling

In contrast, when the motor is stalled, it is producing maximum torque, but the rotational velocity is 0, so the output power is 0 again.

Between free spinning and stalling, the motor does useful work, and the produced power has a characteristic parabolic relationship

A motor produces the most power in the middle of its performance range.

Speed and Torque Speed and Torque Most DC motors have unloaded

speeds in the range of 3,000 to 9,000 RPM (revolutions per minute), or 50 to 150 RPS (revolutions per second).

This puts DC motors in the high-speed but low-torque category (compared to some other actuators).

How often do you need to drive something very light that rotates very fast (besides a fan)?

Motors and Robots Motors and Robots DC motors are best at high speed

and low torque. In contrast, robots need to pull loads

(i.e., move their bodies and manipulators, all of which have significant mass), thus requiring more torque and less speed.

As a result, the performance of a DC motor typically needs to be adjusted.

How?

Control of Motors Control of Motors Motors require more battery power

(i.e., more current) than electronics E.g., 5 milliamps for the 68HC11

processor v. 100 milliamps - 1 amp for a small DC motor).

Typically, specialized circuitry is required

H-bridges and pulse-width modulation are used

Pulse-Width Modulation Pulse-Width Modulation

The exact width/length of the pulse is critical, and cannot be sloppy.

Otherwise the motor can jitter or go beyond its mechanical limit and break.

In contrast, the duration between the pulses is not critical at all.

It should be consistent, but there can be noise on the order of milliseconds without any problems for the motor.

Why?

Noise in Modulation Noise in Modulation When no pulse arrives, the motor

does not move, it simply stops. As long as the pulse gives the motor

sufficient time to turn to the proper position, additional time does not hurt it.

On the other hand, if the duration of the pulse is incorrect, the motor turns by an incorrect amount, so it reaches the wrong position.