Mechanical Systems of Motor Module - EDGE



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:

[pic]

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,

• Globe,

• Mabuchi,

• Bodine,

• Leeson,

• Maxon Motors,

•

•

Calculations for PM Brushed Motors:

[pic]

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:

[pic]

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,

• Boston Gear,

• Goodyear,

• Emerson Power Transmission,

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 ................
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