inefficiencies, operating voltage and
current, stall voltage and current and
current and work of a motor
Definition of Actuator
An actuator is the actual mechanism
that enables the effector to execute
E.g, electric motors, hydraulic or
pneumatic cylinders, pumps…
Actuators and effectors are not the
Incorrectly thought of the same;
“whatever makes the robot act”
The most common actuator in mobile
robotics is the direct current (DC)
Advantages: simple, cheap, various
sizes and packages.
DC motors convert electrical into
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
The interaction of the fields produces
the movement of the shaft/armature
=> Electromagnetic energy becomes
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.
Level of Efficiency
Good DC motors can be made to be
efficient in the 90th percentile.
Cheap DC motors can be as low as
Other types of effectors, such as
miniature electrostatic motors, may
have much lower efficiencies still.
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
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.
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
The stall current of the motor is the
most current it can draw at its
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.
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
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
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
A motor produces the most power in
the middle of its performance range.
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
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.
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
H-bridges and 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.
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
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.
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