Mechanical Systems of Motor Module
Overview: The motor module which we will be designing will consist of a mix between
mechanical and electrical components. The mechanical components will be explained in
detail below. They consist of four different subsystems which are the motor, braking,
transmission, and steering. All needed calculations are included as well as detailed
explanations for all decisions.
The subsystems will all have to be put together into one unit along with the
addition of all electrical components. A few different architectures have been considered.
The first would be to have all mechanical components working in the same plane and
having steering being applied to only the wheel. Another idea is to have the steering
move the whole module, not just the wheel. The other option to save space or for other
reasons would be to have the system operating in two different planes; the output shaft of
the motor would be 90 degrees from the output of the transmission. Other combinations
of the components will be looked at after each subsystem has been broken down.
Mechanical information is as follows:
1.) Motor
2.) Transmission
3.) Brakes
4.) Steering
5.) Architectures
1) Motor Concept Development
Overview:
The motor will be the means of converting the voltage and current into a mechanical
energy, and ultimately result in torque and angular velocity of an output shaft. The motor
will run on DC as specified in the PRP.
Possible Concepts:
Permanent Magnet Brushed
Permanent Magnet Brushless
Permanent Magnet Stepper
Servo
RC Servo Kit
PM Brushed:
This is the motor that is typically used for this application.
Pros
High torque, low speed applications
Cheaper
Accurate/predictable
Smaller size for same output
PM Brushless:
Used for high speed applications requiring low speed deviations, quick and precise
Pros
Better heat dissipation resulting in ability to run at higher continuous loads
Less maintenance, longer life
Quieter
Cleaner
Quicker Accel/Decel
PM Stepper:
Divides full rotation into certain number of steps
Pros
Less maintenance, longer life
Optimum characteristics for resolution of speed/load
Accurate and has ability to rigidly stay in position
Easy to control
Cons:
Generates a lot of heat at stand still
Open loop- no feedback
Non-continuous- not smooth
Heavy and big
Larger power consumption
Servo
Servos operate on the principle of negative feedback, where the control input is compared
to the actual position of the mechanical system as measured by some sort of transducer at
the output.
Pro:
Don’t have to constantly attach and detach power source
Ability to go in reverse easily
Con:
Motor will slow with increased payload
Needs an encoder
Speed and current draw effected by payload
Complex circuitry
RC Servo
Same as servo, except comes in a kit that includes a gearbox, encoder, and control
circuitry.
Pro:
Complete system including motor, gearbox and feedback device, servo control
circuitry, drive circuitry
Easily controlled
Low Voltage draw
Con:
No flexibility
No Modularity
No Scalability
Analysis:
Comparison with Baseline:
Baseline is PM Brushed but it is not be an efficient solution. While it is very inexpensive,
it is extremely heavy, is very big, and also requires over a 100 amps to operate. We have
to match our required motor output with the specific model we purchase.
Applicable Customer Needs:
Payload range- 10kg to 100kg w/ multiple configurations
Tare weight 40kg
Top speed of 4.5m/s
Controllable speed
Run time of 1 hour (power consumption)
Ability to read angular speed
Feedback
COTS
Durability of 5 + years
Cost
Pugh Diagram:
Motor Concepts
A B C D E
Brushed PM Brushless PM Stepper Servo RC Servo
Selection Criteria Weight Rating Weighted Score Rating Weighted Score RatingWeighted
Score RatingWeighted
Score RatingWeighted
Score
Weight 10% 3 0.3 3 0.3 1 0.1 3 0.3 2 0.2
Operating Speed/Accel 5% 4 0.2 5 0.25 3 0.15 2 0.1 2 0.1
Torque 10% 5 0.5 4 0.4 4 0.4 3 0.3 3 0.3
Controllability 10% 4 0.4 4 0.4 5 0.5 3 0.3 3 0.3
Power Consumption 8% 3 0.24 3 0.24 1 0.08 5 0.4 5 0.4
Feedback 5% 1 0.05 5 0.25 5 0.25 1 0.05 5 0.25
COTS 10% 5 0.5 5 0.5 5 0.5 5 0.5 5 0.5
Durability/Maintainence 5% 2 0.1 4 0.2 4 0.2 2 0.1 3 0.15
Temperature 5% 3 0.15 3 0.15 1 0.05 3 0.15 3 0.15
Cost 15% 4 0.6 2 0.3 1 0.15 4 0.6 2 0.3
Modularity 8% 3 0.24 3 0.24 1 0.08 3 0.24 2 0.16
Size 9% 3 0.27 2 0.18 1 0.09 4 0.36 4 0.36
Total Score
Rank
Continue? Yes Yes No No No
3.40
2
3.17
3
2.55
4
(reference)
3.55
1
3.41
1
Conclusion:
The Pugh clearly shows that Brush and Brushless are the best solutions. They meet every
need to some degree, the big argument here is Maintenance versus cost. We feel like the
small amount of upkeep required for brushed motors isn’t a big deal.
Additionally, we have found that Brushed DC motors meet our calculated needs very
well. Dr. Hensel also made a big deal about finalized production cost of the motor
module. The final product will be a Brushed PM Motor unless we can find no
manufacturer to make something to match our needs.
Manufacturers/Vendors:
Source Engineering, http://www.sei-automation.com/products.html
Globe, http://www.globe-motors.com/home.html
Mabuchi, http://www.mabuchi-motor.co.jp/en_US/index.html
Bodine, http://www.bodine-electric.com/
Leeson, http://www.leeson.com/products/stock_dcmotors.htm
Maxon Motors, http://www.maxonmotorusa.com
www.robotmarketplace.com
www.robotics.com
Calculations for PM Brushed Motors:
Weight 140 kg Driven Wheels Max Motor Torque (N-m) Max Motor Torque (Oz-in) Max Motor Torque (in-lbs)MaxForce Friction (N) 27.32 N 1 2.41 342 21.34Max Force(N) 170.88 N 2 1.21 171 10.67Max Wheel Torque (N-m) 8.68 N-m 3 0.80 114 7.11
4 0.60 86 5.335 0.48 68 4.27
max incline= 6 deg 6 0.40 57 3.560.10 rad
veh. Velocity= 3 m/scoef. of rolling res. 0.02g= 9.81 m/s^2 T=Ts-Ts/Wn*WWheel diameter= 0.1016 m T=torque 0.35 N-mGear Ratio= 4 Ts=stall torque 1.97 N-mGear Efficiency= 0.9 Wn=Free speed 2740 rpm
W=speed 2255.74 rpmWheel Shaft Speed= 563.9 rpmMotor Speed= 2255.7 rpm
Power= 43.9 W0.059 HP
Specific Model Possibilities:
Bodine Electric Model N4802
Dewalt 24v Hammerdrill Motor
Maxon F 2260, Winding 881
SEI Automation, Model ZY125-249-12
2) Transmission
Overview: The transmission for our motor module has a few characteristics that are
important to its use. The first is that it must have some sort of mechanical advantage,
meaning that it has to help us choose the most efficient motor. Another key characteristic
is that our transmission must be small, light, and inexpensive. Also there are factors like
noise, maintenance, and assembly ease. All these characteristics will be discussed in
further detail and conclusions below.
Concepts: The concepts chosen to discuss for further review are as follows:
1.) V-belts
2.) Synchronous Belts
3.) Gears
4.) Chains
5.) Direct drive
V-Belts: This is the most popular type of belt used for transmissions. The v-shape
causes the belt to wedge tightly into the pulley which increases friction and allows for
higher operating torque. The belts contain tensile members which are the main load
carrying elements. The rest of the belt is made from an elastomer which transmits the
load from the tensile fibers to the flanges of the pulley. There is jacket/skin around the
entire belt that protects the belt for the environment.
There are three types of designs that v-belts consist of:
Narrow design: Narrower and lighter than the classic design for low
power and high RPMs.
COG design: This has grooves in the inner surface in order to increase
belt flexibility, allowing the belt to turn a smaller radii, thus it
can be used with smaller pulleys. This also increases the
durability of the belt.
Multiple design: Several v-belts connected side by side. This increases
the amount of power transferred.
These designs will be researched further if v-belts are chosen as the component of
our transmission. Here is the breakdown of advantages and disadvantages to v-belts:
Pros
Smooth
Quiet
Inexpensive
Ease of assembly
No lubrication
Cons
Creep
Not good for high temp
Low torque only
Slip
70-96% efficiency
± 3% speed
Rely on friction to transmit power
It is easy to see here that this belt is no good because of the efficiency range that we
get and that it is prone to creep. It also does not transmit speed perfectly and there will be
slip. But on the other hand there is no lubrication, it is quiet, and very inexpensive.
Synchronous Belts: These belts are used where input and output shafts must be
synchronized. They combine the advantages of flat
belts with the positive grip features of gears and
chains. These belts do not rely on friction to
transmit power so they have a high efficiency. The
belts are made of an elastomer but are reinforced
with glass or aramid fibers. This allows for
maximum performance and gets rid of slipping and creep. Synchronous belts require low
belt tension so there is much less load on the bearing that support the sheaves and shafts.
There are two general designs of synchronous belts which are the standard design
(trapezoidal teeth) and the HTD design (curvilinear teeth.) The HTD design is usually
used for high torque applications. The synchronous belt has many pros and some cons
about it, they are:
Pros
98% efficiency
Input/output shafts synchronized
Do not rely on friction to transmit power
Less slip and creep
Low belt tension
Cons
Not good for high temp
It seems here that the synchronous belt is really the choice to go with over the two
belts, but we need to compare it to some other systems. It is very efficient and reasonable
inexpensive. But it is not good for high temperatures because it is made of an elastomer.
This belt does not require any lubrication and its operation is quiet.
Gears: Gears are used to transmit power between rotating shafts at different speeds and
high torques. They offer perfect synchronization. There are many different types of
gears:
Spur gears: These gears have straight teeth parallel to the akis of rotation.
They are easy to manufacture and cheap.
Helical gears: Have teeth inclined at an angle with respect to the axis of
rotation. This angle is termed the helix angle and is usually
at 45 degrees. This angle provides a more gradual
engagement of the teeth during meshing and produces less
impact and noise. This smooth operation makes them good
candidates for high speed applications, but because of the
helix angle, thrust forces are produced.
Herringbone gears: Has two opposite-hand helical gears butted against
each other with the purpose of counterbalancing the trust
forces.
Bevel gear: Have teeth formed on a conical surface and are used to
transmit motion between non parallel shafts. Used for
reducing speed.
Worm gear: Used to transmit motion between nonparallel shafts. Very
efficient when higher ratios are necessary.
Each of these specific designs have valid reasons to their benefits but we
are going to break down the good and bad of these gears as a whole:
Pros
Small packaging size
Handles high torques
98-99% operating efficiency
Perfect synchronization
Can orient input and output in different planes
Cons
Needs lubrication
Center distance is not flexible
High cost
Gears give us the highest operating efficiencies as well as the smallest packaging
size capabilities. They offer perfect synchronization and they can orient the input and
output in different planes. The key player here is that we can orient the input and output
shafts to the transmission in different planes. The problems with gears are that the center
distance is fixed, they need lubrication, and they are expensive.
Chains: Chains transmit power through interlocking links wrapping on a sprocket.
These drives are less expensive than gears and they can transmit high loads. They have a
long service life and are not effected by temperature. Here are some types of chains:
Roller chain: The most common chain. It has pins that pivot inside a
roller bushing.
Inverted tooth chain: Use in applications for high speed, smooth, and
quiet operation is required. Expensive.
Looks like our project would use the inexpensive common (roller) chain. It has
some advantages and disadvantages:
Pros
Less expensive than gears
Transmit high torque
No slippage
98% efficiency
Long service life
No temperature limits
Do not require initial tension
Cons
Fatigue
Noise
Lubrication
Chains look like a good choice because they are cheaper than gears, transmit high
torque, have long life, and there is no initial tension required making for easy installation.
But the problem is that there will be fatique in the chanin along with noise and lubrication
issues.
Direct Drive (no transmission): The idea of a direct drive was looked into because
maybe there was no need for a transmission. This is not the most efficient idea since
there will be no mechanical advantage but maybe we don’t need that advantage. The
pros to this system are that there is not transmission which makes it very inexpensive and
efficient. The only con to the idea is that there is no mechanical advantage. This factor
alone rules out the idea of direct drive.
Pugh Diagram:
Transmission Concepts
A B C D E
Gears V-belt Sync. Belt Chain Direct Drive
Selection Criteria Weight Rating Weighted Score Rating Weighted Score RatingWeighted
Score RatingWeighted
Score RatingWeighted
Score
Out of Plane Flex. 5% 5 0.25 1 0.05 1 0.05 1 0.05 1 0.05
In Plane Flex. 15% 2 0.3 5 0.75 5 0.75 5 0.75 1 0.15
Weight 5% 2 0.1 4 0.2 4 0.2 3 0.15 5 0.25
Efficiency 10% 4 0.4 1 0.1 4 0.4 4 0.4 5 0.5
Scalability 5% 4 0.2 4 0.2 4 0.2 4 0.2 5 0.25
COTS 15% 5 0.75 5 0.75 5 0.75 5 0.75 5 0.75
Durability/Maintainence 10% 3 0.3 3 0.3 4 0.4 3 0.3 5 0.5
Cost 5% 3 0.15 5 0.25 5 0.25 4 0.2 5 0.25
Size 10% 3 0.3 4 0.4 4 0.4 4 0.4 5 0.5
Noise 2% 3 0.06 5 0.1 5 0.1 2 0.04 5 0.1
Torque Transfer 10% 5 0.5 2 0.2 5 0.5 5 0.5 5 0.5
Mech. Adv. 6% 5 0.3 5 0.3 5 0.3 5 0.3 1 0.06
Temperature 2% 5 0.1 2 0.04 3 0.06 5 0.1 5 0.1
Total Score
Rank
Continue? Yes No Yes No Yes
4.14 3.96
4 5 1 2 3
(reference)
3.71 3.64 4.36
This diagram shows us how our concepts performed when placed against the others
according to customer needs. The top three are the synchronous belt, the chain, and the
direct drive. We don’t want to look at chains due to its low ranking on noise and
maintenance and we don’t want v-belts due to there very low efficiency. The gears are
chosen over the chain because they offer the unique characteristic of out of plane flex,
when none of the others do.
Analysis:
Comparison with Baseline:
It seems that the timing belt option would be the lightest, easiest to assemble, cheap, and
efficient option that we have. They seem to have the highest amount of pros and the
lowest amount of cons. We just need to see how it fits into our customer needs and if it
will transfer over to the 10 kg and 1000 kg models. It exceeds most of the qualities of
our baseline of drive gears, but the baseline offers easy combinations of one or two
motors and I am not sure if that changeover is so easy with timing belts.
Customer Needs:
Payload range- 10kg to 100kg w/ multiple configurations
Tare weight 40kg
Top speed of 4.5m/s
COTS
Durability of 5 + years
Cost
Conclusion:
The baseline gears seem to be a good option because of the variety of gearing ratios
available when using gears. But the problem of lubrication and noise is not the best thing
to have. The gear drive is the only option that allows for the input and output shafts to be
in different planes. The chain drive seems like a pretty good option because of its
flexibility and that it does not have any initial tension on it. I still don’t like the
lubrication and noise issues that it seems to have. The V-belts are great because they are
very common but they are not really efficient at all. That leaves us with the timing belt
which seems to have the greatest qualities. It has all of the qualities of the chain and gear
drives plus it is quiet, requires no lubrication, and it is lightweight compared to those.
Also this choice is supported by the pugh diagram which is based on our customer needs.
The timing belt will be the option if we can stay in one plane because it meets our
customer requirements the best of all the choices.
Companies/Manufacturers:
Dodge-PT, http://www.dodge-pt.com/index.html
Boston Gear, http://www.bostongear.com/
Goodyear, http://www.goodyearindustrialproducts.com/powertransmission/
Emerson Power Transmission, http://www.emerson-ept.com/
3) Braking Subsystem:
Overview:
The braking subsystem will be responsible for stopping the rotary shaft motion of the
module. Through an input voltage and current the system will utilize one or several
means of stopping the wheel from spinning.
Possible Concepts:
Dynamic Braking
Power-off Mechanical Spring
Motor Shorting
Via Speed Controller
Combination
Automotive Style Disc Brakes
Automotive Style Drum Brakes
Dynamic Braking:
Dynamic Braking requires a switching device, resistor and circuit. It detects differences
in motor speed and desired speed to convert mechanical energy to electrical energy that
can be used as utility power.
Pros
Simple design
Reduced power consumption due to regenerative property
Cheap $5-$35
Smooth operation
Good for regular deceleration
Cons
Low output torques-rule of thumb-stopping distance=accel distance (dependant on
motor)
Will not work without power (emergencies)
Dependant on load and motor used
High motor temperatures
Custom H-bridge required
Power-off spring:
A spring actuated disc brake is released when power is on, when the power is cut the
brake is applied.
Pros
Safety consideration for power cut-off
High output torques
Doesn’t effect motor
Very scalable
Cons
More expensive
Heavier
Not necessarily smooth
Always consuming power
Mounting hardware required
More space
Motor Shorting
The motor is short circuited, causing it to generate an opposing magnetic field. This
greatly increases the motor’s resistance.
Figure 1: Possible Motor Shorting Circuit
Pros
No more major parts added
o Light weight
o Cheap <$10
Simple design
Cons
Low output torques
Will not work without power
Difficult to predict analytically
Using Speed Controller:
Some Speed controllers come with a Brake/Coast feature that would ultimately act as a
motor shorting technique. See Motor Shorting for Pros/Cons
Combination:
By combining the use of the power-off brakes with either dynamic braking or short
circuiting there may be a considerable gain in advantages.
Pros
Reduce the size required for Power-off brakes, thus price and weight
Greater braking torque
Fail Safe
Con
More complex
Harder to characterize
Automotive Style Disc Brakes:
A power-on mechanical brake used in most modern cars.
Pros:
High output torque
Easy to find manufacturers
Cons:
Won’t work if power is cut
Too heavy
Too complex
Automotive Style Drum Brakes:
A cheaper power-on mechanical brake found in most older cars.
Pros:
Relatively cheap
Cons:
Low output torque
Too heavy
Won’t work if power is cut
Analysis:
Comparison with baseline:
No mechanical braking system provided
Short Circuit and Characterize Motor
Utilize Victor Speed Controller to characterize braking capabilities
Applicable Customer Needs:
Fail-safe braking
Durability for 5 years
COTS Item
Run time of 1 hour (power)
Controllable speed
Tare weight of 40kg
Brake within 1m
Cost
Scalable
Pugh Diagram:
Braking Concepts
A B C D E
Dynamic Power-Off Short Circuit Combo
Selection Criteria Weight RatingWeighted
Score RatingWeighted
Score RatingWeighted
Score RatingWeighted
Score RatingWeighted
Score
Fail-Safe 20% 1 0.2 5 1 1 0.2 5 1 0
Weight 5% 4 0.2 2 0.1 5 0.25 2 0.1 0
Braking Torque 18% 3 0.54 5 0.9 3 0.54 5 0.9 0
Modularity 7% 5 0.35 3 0.21 5 0.35 4 0.28
Controllability 5% 5 0.25 3 0.15 3 0.15 5 0.25 0
Power Consumption 5% 3 0.15 4 0.2 4 0.2 3 0.15 0
Scalability 8% 4 0.32 5 0.4 4 0.32 5 0.4 0
COTS 12% 4 0.48 4 0.48 4 0.48 4 0.48 0
Durability/Maintainence 3% 5 0.15 4 0.12 5 0.15 4 0.12 0
Cost 13% 4 0.52 2 0.26 5 0.65 2 0.26 0
Size 4% 4 0.16 2 0.08 5 0.2 2 0.08 0
Total Score
Rank
Continue? No Yes No Yes
4.02 0.00
4 2 3 1
(reference)
3.32 3.90 3.49
Conclusions:
Combination is the only way to go due to the fail-safe requirement as well as the large
cost of fail-safe mechanical brakes.
We will combine Dynamic Braking (because of its predictability and controllability over
short circuiting) with the safety of Power-off mechanical braking. Additional we will
consider utilizing regenerative braking with the Dynamic Braking over an H-Bridge. A
possible circuit to accomplish this is shown below.
Figure 2: Possible Regenerative Braking Circuit
Manufacturers/Vendors:
MC Supply Co., http://www.mcsupplyco.com
Acroname, http://www.acroname.com/index.html
Texas Instruments, http://www.ti.com
Ogura Industrial, http://ogura-clutch.com
See Controllers for additional Dynamic Braking products
Calculations:
Power Off Dynamic Brakes CombinedWheel rad 0.0508 m Motor Power 80 W Brake torque=Wheel torque= 5 N-mStatic Torque= 5.65 N-m Motor Efficiency 0.7 Wheel force 98.4252 NBrake torque=Wheel torque= 5.2545 N-m Voltage During Braking 15 Volts # braking wheels= 2Wheel force 103.435 N Motor Speed 2740 RPM Total braking force= 196.8504 N# braking wheels= 2 Dynamic Resistor Current 0.07 Amps veh. Mass= 140 kgTotal braking force= 206.8701 N Resistance 0.38 Ohms deceleration= 1.406074 m/s^2veh. Mass= 140 kg Braking Power 590.55 W velocity 3 m/sdeceleration= 1.477643 m/s^2 Braking Torque 2.06 N-m Stopping distance= 3.2004 mvelocity 3 m/s Brake torque=Wheel torque= 1.914085 N-mStopping distance= 3.04539 m Wheel force 37.67885 N
# braking wheels= 2Total braking force= 75.35769 Nveh. Mass= 140 kgdeceleration= 0.538269 m/s^2velocity 3 m/sStopping distance= 8.360129 m
Current Current Current
Current Rotary Motion Rotary Motion
Position
Steering
Stepped down rotary
motion that can stop
and go in any chosen
direction
RPM, temp, etc. Slip, Speed, etc.
Wheel
Rotary Motion
Stopped or
Engaged
Stepped Down
SpeedMotor Brake Tranny
Current Current Current
Current Rotary Motion Rotary Motion
Position
Rotary Motion
Stopped or
Engaged
Stepped Down
Speed
Stepped down rotary
motion that can stop
and go in any chosen
direction
Motor Tranny Brake Steering
RPM, temp, etc. Slip, Speed, etc.
Wheel
Current Current
Current Rotary Motion Wheel
PositionRPM, temp, etc. Slip, Speed, etc.
Stepped down rotary
motion that can stop
and go in any chosen
direction
Motor Tranny Steering
Variable rotary
motion
controlled by
the motor inputStepped Down
Speed
Current Current
Current Rotary Motion
Position
Current
Slip, Speed, etc.
Steering Motor
RPM, temp, etc.
Multi
directional
motor
module
Rotary Motion
Stopped or
Engaged
Stepped Down
SpeedBrake Tranny Wheel
Motor module that has
stepped down rotary
motion and can stop
and go in any chosen
direction
5.) Architectures
This architecture shows the brake before the transmission and the steering connected
directly to the wheel. This setup is advantageous because the brake will have less torque
to stop in this format
This format puts the brake after the transmission. This may be the best way to organize
the size and layout of the module regardless of whether or not is advantageous to the
brake.
The above architecture is a representation what we would have without a brake, using the
motor to brake. It is a nice setup because it is a smaller package size and less items to
monitor and program.
The final architecture discussed has the entire module being steered. This option came
up when looking at steering options and it seems that instead of steering the wheels we
have the ability to steer the whole module. Everything will be fixed to the module but the
entire module will rotate when needed.