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.