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How to Select a motor? (Part 3) - How To Select A Motor
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How to Select a motor? (Part 3) - How To Select A Motor?
In order to select motor for a robot or a machine, several key questions must be asked::
1. What is the power requirement of the motor?
2. How Fast the motor has to spin?
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How to Select a motor? (Part 3) - How To Select A Motor
3.
4.
5.
6.
7.
Do i need to move something in a linear or rotary motion?
What is the operating voltage required for the motor?
Will the motor work all the time or once in...?
Is a precise movement required?
How much money to invest on the motor?
What is the Power Requirement Of The Motor?
In order to accelerate a body with a mass, forces must be exerted. Meaning, the heavier the
robot or the machine, the stronger the motor has to be.
There are two common motion application in robotics and automated machines:
1. Robot or machine moving independently on a surface.
2. robot or machines moves certain items using mechanical mechanisms.
Motor Selection when the Robots or Machines are Independently Mobile
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How to Select a motor? (Part 3) - How To Select A Motor
A robot that moves on a surface has to overcome the friction of the wheels with the ground and
the internal friction inside the motor. If the robot starts to climb on inclined surface, then it must
overcome the gravity force as well.
A machine spinning a large drum has to overcome internal friction in the motor and the moment
of inertia of the drum in order to accelerate it.
To maintain a constant speed, the motor have to exert a torque that higher than the torque
opposes to the movement (friction, gravitation, etc.). if the motor will be to weak, that will exert
smaller torque then the opposing torque, the motor will stop (motor that is stalled for a large
period might burn due to high current in the motors coils).
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How to Select a motor? (Part 3) - How To Select A Motor
A
. robot that moves on a horizontal plane as friction forces exerted on it – Normal Force.
– Coefficient of friction between the robot and the surface
In case the robot moves over inclined plane, both friction force and gravity forces exerted on it.
Both forces depends on the angle of inclined plane.
.
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How to Select a motor? (Part 3) - How To Select A Motor
The sum of these forces is the total force that opposes to the robot or machine motion. The
motor has to generate enough torque to overcome this total force.
Now when the opposing forces are known, how to calculate the required torque required from
the motor?
You must first define what the diameter of the wheels of the robot is. The torque is calculated by
.the following formula: - Force
- Wheel Radius
the
torque required to overcome the frictional force is calculated as follows: . Hence, .
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How to Select a motor? (Part 3) - How To Select A Motor
The complete forces equation looks like this:
Frictional Force exerted against the direction of motion. When the robots moves forward, at the
point of contact of the wheel with the ground, the wheel speed direction is backwards and the
friction force direction is forward (as mentioned before, friction force is exerted against the
motion direction).
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How to Select a motor? (Part 3) - How To Select A Motor
Reorder
the equation to isolate the torque will obtain:
This formula calculates the required motor torque in order to drive a robot on a inclined surface
at
angle a mass (m), with a wheel radius and acceleration .with
You can use two motors instead of one, thus each motor "participates" in the load and provides
half the torque needed.
If we'll place four motors (a motor on each wheel), the motor torque required to drive the robot
will be equal to quorter of the total torque: When is the number of motos driving the robot.
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How to Select a motor? (Part 3) - How To Select A Motor
Another consideration to be taken into account when calculating the required torque from the
motor is the motor efficiency. As Mentioned earlier there is no motor with 100% efficiency.
There are always internal friction that decrease engine efficiency and reduce its power. When
used in transmission, must also take into account the efficiency of the transmission. Therefore,
the formula takes the following form:
– Overall efficiency of the motor, transmission and other components participating in moving
the robot or machine.
Now that the torque of the motor is known, you can calculate the power of motor by the
following formula:
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How to Select a motor? (Part 3) - How To Select A Motor
– Motor torque
– Angular Speed of the motor
In order to know what is the maximum power required from the motor, you should take into
account the maximum speed the motor will spin.
Last consideration should be taken in selecting the motor power is the safety factor. As
mentioned, it is NOT recommended to drive the motor at 100% of its capacity. Therefore, a
safety factor should be accounted for. That is, if the maximum power required from the motor
has been calculated, it has to be multiplied by additional safety factor. Usually this safety factor
is equals to 1.5 or 2.
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How to Select a motor? (Part 3) - How To Select A Motor
For example, if we find that the maximum power required from the motor is 1W. Multiply with the
safety factor and select a motor with 1.5W or 2W power. This way we can be sure that the
motor will be strong enough to meet all the requirements of the robot and won’t work in its 100%
all the time thus increasing the reliability of the motor and the robot and reducing system failure
rate and degradation.
When you know what the maximum power of the motor and the operating voltage is known, the
maximum current that will flow in the motor coils can be calculated by the following formula:
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How to Select a motor? (Part 3) - How To Select A Motor
–
Motor Power
–
voltage
– motor
currentOperating
in the
motor
Isolating
the
current
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give:
Or
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for
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each
each
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state.
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state
all
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Safety
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// -- Maximum
Motor Selection when the Robots or Machines are Mobile using
mechanical Mechanisms
A common Mechanical mechanism in robotics is robotic arm. Robotic arm consists of links that
connected in series (like a chain). Each link is driven by a separate motor. At first sight it’s easy
to see that the first motor should be the strong one. The last motor in the chain has to carry the
weight of the last link the load the robot arm is lifting thus it can be the weakest motor in the
chain.
Analysis of the maximum forces required by the motor will always consider the worst case. This
case assumes that all links of the robot arm is horizontal, that is, the weight is perpendicular to
the robot arm (if the link is vertical, the required torque from the motor will be zero in this case).
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How to Select a motor? (Part 3) - How To Select A Motor
The motor required torque depends on the weight of the link and the distance of the links center
of mass from the motor shaft. (If the link is symmetrical, it can be assumed the center of mass
of the link located in the center of the link)
– Mass of the first link
– Gravitational acceleration
– Length of the first link
– Load weight in the end of the first link
– Required static torque by motor 1.
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How to Select a motor? (Part 3) - How To Select A Motor
The
torque
Is the
static torque. That is, the torque required to hold the link in place and prevent it from
falling. If you intend to accelerate the link so dynamic torque should be added. Dynamic torque
is
thethe
product
of torque
inertia of the link angular
acceleration .with
Reminder:
When Calculating
Moment
of inertia
ofrotation.
link , Note the geometric
structurethe
of link
and the
point of
Moment of Inertia in link that
rotates around its center is different than link that rotates around its edge (as in our case). You
can use the table that summarizes moment of inertia of different bodies.
General torque required from the motor is the sum of the static and dynamic torque.
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How to Select a motor? (Part 3) - How To Select A Motor
What happens if we have two links in the arm? In this case the load “contribution” of each links
is added to the first motor.
Static torque calculation will look like this:
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How to Select a motor? (Part 3) - How To Select A Motor
So what was added? The length of the load that the arm lifts increased from L1 to L1+L2. Motor
2 is added also so its weight has to come into account and added to the equation as Wm2 and
finally the weight of the 2 nd link is added also.
In this case the dynamic torque has to be considered also and calculated as follows:
Therefore, the final torque required by the motor is:
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How to Select a motor? (Part 3) - How To Select A Motor
In case of several links in the arm, the same method of calucation is performed.
An example of the final formulas of a robotic arm that has three links:
After we got the theoretical torque, we have to consider the practical torque. As calculated in the
case of a mobile robot, motor and transmission efficiency has to come into account in the
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How to Select a motor? (Part 3) - How To Select A Motor
calculations. And don’t forget the safety factor. The formula looks like this:
– Final required torque of the motor
– General theortical torque
– Motor efficiency
– Transmission efficiency
- Transmission Ratio
– Safety factor (Usually equals to 1.5 or 2)
How Fast the Motor has to Spin?
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How to Select a motor? (Part 3) - How To Select A Motor
In the case of a mobile robot or a machine moving an item from place to place, a linear speed of
the robot has to be defined and the wheel radius of the robot or the mechanism.
Example 1:
Robot need to travel at 10 kph. Radius of each wheel is 50 cm. Wheel speed is calculated using
the following formula:
– Wheel Speed
– Robot Speed
– Wheel Radius
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How to Select a motor? (Part 3) - How To Select A Motor
Hence the robot's wheel speed (RPM units) is:
If a gear transmission that attached to the motor shaft is used in order to increase the motor
output torque, the motor speed will be reduced accordingly (this is how transmission works).
That is, if we calculated that the required speed of the wheel is omega and we’re using
transmission (say a 1:10 gear ratio) it reduces the motor output speed and we’ll have to select a
motor with a speed 10 times more than the calculated wheels speed.
In this calculation it is also recomended to take the safety factor into account.
Example 2:
Machine has to punch holes in a surface that moves on a track. The punch is performed every 1
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How to Select a motor? (Part 3) - How To Select A Motor
second using a pointed rod. The distance of the rod movement is 10 cm per punch and the gear
radius that moves the rod mechanism is 10 cm.
From here:
Speed of movement of the rod is 10cm per second (or 0.1 m/s). because the rod has to punch
and then return to its upper position, the speed of motion has to be 2 times faster, that is, 0.2
m/s.
Radius of the gear that moves the mechanism is 10cm (or 0.1m) that is the final motor speed (in
RPM units) is:
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How to Select a motor? (Part 3) - How To Select A Motor
In summary, to calculate the required motor speed, the following data is required:
- Required linear speed
- Wheel radius
) - Transmission Ratio (if used) (
) - Total motor and tramission ratio (
) - Safety factor (
- The final formula is:
Do I need to Move Something in a Linear or Rotary Motion?
The question is a little tricky. Why? Since everything that is moving in a linear or rotary way
depends on the mechanism that drives it. Not the motor.
A motor that spins a wheel it is understandable, linear motor or Solenoid or piston that pushes
and pulls in a linear fashion that is understandable, but there are lots of options to combine
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How to Select a motor? (Part 3) - How To Select A Motor
different types of motion. That is, you can move linearly by using a rotary motor and vice versa you can rotate wheels by using a linear motor or piston. How?
Drive mechanisms!
The following example shows how using a circular motion and linear motion applications:
:
- A motor that rotates back and forth, pushing and the rack in linear motion.
A motor rotates in one direction pushing back and forth in a linear motion the two pistons.
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How to Select a motor? (Part 3) - How To Select A Motor
What is the Operating voltage required for the motor?
A motor in a robot or automation machine considered to be a “heavy” energy consumer. As the
load on the motor will be greater the larger the current the motor will draw from the power
source. There are several ways to reduce motor current:
1.
2.
3.
4.
A motor with lower power
Using a transmission
Reducing the load on the motor (smaller mass, friction reduction, etc.).
Higher operating voltage
According
to the
forumla , A motor with
a power
of 24W that operates with a voltage of 12V will consume a current of
24W / 12V = 2A.
If we'll reduce the motors power (for example 12W) the motor current will be 12W/12V=1A.
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How to Select a motor? (Part 3) - How To Select A Motor
Conversely, if we decide to select a motor with 24V operating voltage instead of 12V the current
consumption of the same motor will be 24W/24V = 1A.
This is plain math but remember that the motor power, output torque and operating voltage has
to meet the technical requirement of the application. Selecting a motor with less power or
increasing the voltage just like that will affect in some way of the robot performance so don’t
forget to take into account all the variables.
Is the motor will work all the time or once in ...?
The technical term that describes the operating time of the motor is called Duty cycle. this
parameter is indicated by parentage and describes the nature of operation of the motor.
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How to Select a motor? (Part 3) - How To Select A Motor
For example, motors that rotate fans have a very high duty cycle and even 100% (meaning,
they work continuously until malfunction).
However, a duty cycle of a motor that turned on each time a certain condition occurs depends
on the frequency of this condition. For example, a motor turns a fan each time the ambient
temperature rise above 20 degrees. Duty cycle of this motor in this application will be 70% when
operating in Africa and 5% when operating in the North Pole.
To select the proper motor one has to define first the expected duty cycle. A higher value of
duty cycle will require more reliable and powerful motor.
For example, for low duty cycle we choose brushed DC motor and for higher duty cycle we’ll
choose brushless DC motor. By the way, this is one of the reasons that cooling fans are based
on brushless DC motors.
International Standard specifies eight levels of duty cycle:
Description
Level
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How to Select a motor? (Part 3) - How To Select A Motor
Continuous duty. The motor
S1 works at a constant load for enough time to reach temperature equilibr
Short-time duty. The motor
S2works at a constant load, but not long enough to reach temperature equilib
Intermittent periodic duty.S3
Sequential, identical run and rest cycles with constant load. Temperature eq
Intermittent periodic duty with
S4 starting. Sequential, identical start, run, and rest cycles with constant lo
Intermittent periodic duty with
S5 electric braking. Sequential, identical cycles of starting, running at cons
Continuous operation withS6
intermittent load. Sequential, identical cycles of running with constant load
Continuous operation withS7
electric braking. Sequential, identical cycles of starting, running at constant
Continuous operation withS8
periodic changes in load and speed. Sequential, identical duty cycles of sta
Is a Precise Movement required?
If the robot has to move from one building to the second, than millimeter resolution is not
needed (and step motor will not be selected).
Does the machine have to to place electrical component on the printed board? In this case we
prefer stepper motors with high power transmission.
How Much Money to invest on the motor?
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How to Select a motor? (Part 3) - How To Select A Motor
The answer is simple - buy the best motor at the cheapest price. Each and every one has his
financial limitations. After you answer all questions above, perform some market research
between different motor suppliers and manufacturers, select the motor that meets the
requirements and choose the cheapest one. Simple, right?
Written by Eran Cenciper (Robot-and-Machines-Design Webmaster)
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