Archive for category: Spiderbot Generation #6

BY: Andrew Saprid ( Manufacturing Engineer)

Introduction:

By assembling the gears into the bottom plate, making the connection was going to be an issue of stability and movement for the gears. The project manager and manufacturing engineer went to Lowes to buy screws for the connection. But turns out that the holes on the bottom plate did not print the exact measurement on Solid-works, making the holes too small or big.

Some tests were being made for the small motors and big motors by the electronics engineer and manufacturing engineer.

Requirements

Level 2 system requirement follows:

• The 3DoT David shall use two micro motors for the movement system of the robot.

Testing #1

The project manager suggested that the screw would fit exactly on the gear, but make the hole a little bit bigger on the bottom plate, so that the gear would rotate on the bottom plate. The manufacturing engineer drilled the hole on the bottom plate, but broke some of the planks that were super glued into the bottom of the bottom plate. He cut off the screw to cut the weight of the screw, which may not make a difference for stability. He noticed the friction of the bottom plate may affect the movement of the gears.

  
Gorilla tape was used to tape the planks to the bottom of the bottom plate. With using the small motor that the project manager bought in Amazon, they tested the motor to see if it will work, and it turns out that the friction, the screws, and the gorilla tape were causing problems with the motor turning the gears.

Testing #2: White gears

Since red gears may be heavy for the small motors to run, white gears are placed on the wooden prototype for testing. They tested them with the small motor the project manager bought. They were running smoothly. The white gears were light. Driving a nail through the center of the hole of the gear will insure stability if the nail is driven to the wood straight downward. The small motor will run well with these gears, but the project manager wanted to use the red gears because they were stable and strong to withstand the weight of the legs. With legs and the joint attached to the white gears, the leg was putting weight into the gears, which caused instability of the gear train.

White Gear Testing With Motor VIdeo Link

Testing #3:Testboard, Oil, and Washers

In order to make the gears move freely on the bottom plate, washers and oil will be added. These were bought at Home depot.

TestBoard

A test board was made in Solid-works to test the gear stability, movement, and friction. The original test board was not documented. The test board below has been tested, and the results of the testing have damaged the cylindrical extrusions by hammering a nail to it to make the gears stay in place as the motors drive the gear. By putting oil into the plate, and washer underneath the gear, the gear was able to move freely. The cylindrical extrusions did its job stabilizing the gear train. Finding a nail, screw, or etc to hold the gears in place is in progress.

Conclusion

The electronics and manufacturing engineers did 3 tests for the white gears, red gears, and the extrusion of the bottom plate. The white gears are not going to be used because they are too light. The red gears will be used instead. They will be able to withstand the weight of the legs. Kent used the big motors. Big motors were able to move the red gears. Motors are still being investigated and researched by the electronics engineer for a small motor with more torque to the turn the gears better.Testing #3 could be the solution to the gear stability, friction, and movement. In order for the gears to move properly, perfection and accuracy is key.

Red Gear Testing With Motor Video Link

 

BY: Kent Hayes (Electronics and Control)

Introduction

A servo is an assembly of 4 different things: a DC motor, a gear-reduction unit, a position sensing unit, and negative feedback to control the motor’s speed/position. Unlike the DC motor, servos use a 3 wire connection for the power, ground, and control. The servo receives a control signal (PWM) that represents a desired position and powers the motor until its shaft reaches this desired position. They are typically able to rotate 200 degrees back and forth.

A DC motor uses only 2 wires, through which all of the power is supplied. The power level is controlled by PWM which is a ratio of the on time and the off time or the duty cycle. So if the power is on for half the time, then the motor operates with half of the power of its full-on operation. However there are various types of motors, so in the resources section you will see a link which has nice summaries of the different motors and which ones will be of the best use to the reader.

Types of Servos:

• RC Servos. The most common and economical type of servo motor that normally come with 180 degree range for rotation. You can drive these through servo controllers or through GPIO pins from a microcontroller. A major disadvantage of these is that there is no feed back to the program of which you use to control them, so one cannot be entirely sure that it is operating properly.
• Analog Feedback Servos. Come with an extra feedback wire that you can connect to an analog input pin in order to get feedback to the user’s program.
• Continuous Rotation Servo. They are “hacked” RC servos, meaning that the controller feedback is hard-wired to believe that it is always at a middle position, resulting in a reversible, speed controlled gear-motor. A disadvantage is that it can be difficult to find the neutral point in the control signal where the motor stops using all together.

Types of Motors:

• Brushed Motors. The most common type of motor because they are light, inexpensive, efficient, and have reasonable torque at low speeds. Can be used in toys, RC servos, and even gear motors.  

• A gear-motor is a brushed motor with reduced speed but increased torque through the use of a gear-train.  The reduction of speed is actually an advantage because most DC motors spin too fast. A disadvantage is with the extra resistance, the gear-trains will become unresponsive at lower voltages.
• Brushless Motors. Replaces the brushes with electronic communication to switch the current flow to drive the motor, while maintaining its efficiency. Typically used in laptops as fans and quadcopters. The main disadvantage is that it normally requires a separate controller for operation.
• Stepper Motors. DC motors that move is discrete steps so they have much more precise speed control. Another advantage includes having a reasonable low-speed torque. The main disadvantages include reduced efficiency and being subject to missing steps if overloaded.

Servo Trade-off Study Guiding Factors

The purpose of the servo was to control the rotation of the head, while the motors would control the movement of the legs. In choosing a servo, we would need it to rotate 360 degrees continuously within our voltage range of 3.7V to 5V. In addition, it should be lightweight, small in size, and inexpensive so that it not bring us over our budget. The following table are the results from what Kent was able to find online:

Motor Trade-off Study Guiding Factors

After reading about the different types of motors, I  began to look at motors that would work for the 3DoT David team. The main factors that contributed to his search were the following: size, cost, and voltage rating. The size is important to our project since the mechanical design has been shrunk in size compared to previous generations of spider bots. Our budget for the project is not supposed to exceed $80.00 so it is best to be aware of how much each type of motor will cost. Finally, it is important to have a motor that can operate within 3.7V~ 5V since the 3DoT board will allow us to operate within this range. After searching online, the only motors that were able to fit our specifications were of the brushed motor type. All other stepper motors and brushless motors were either too large or had too high of a voltage rating. In addition, they both require separate controllers and are much more complicated to control.

Update:

After our team team received new gears, we began testing to see if the motors would still be able to work. Unfortunately, they did not have enough torque to turn our gear train. Kent then began to test with the DC hobby motor, which did have enough torque but was too heavy and bulky to work with our design. He then turned to the E&C division manager for possible solutions and he recommended that we take a second look into geared motors.

After looking online, Kent found a list of micro geared motors from the Servo City website. They had various motors from 45RPM to 2500RPM and as the speed increased the torque decreased. Therefore when searching, we had to make sure we had enough torque at a speed that would make our legs rotate at a reasonable rate. We agreed that 2 cycles/sec would be an appropriate speed to enable the spider to walk. In order to acheive this, hr did the following calculations:

2 cycles/sec * 60 sec/min = 120 cycles/min (RPM)

The gear ratio we are using is 3:1 so,

120 RPM * 3 = 360 RPM

So 360 RPM is the minimum speed we need motor to rotate in order for the micro geared motor to rotate our legs at 2 cycles/sec. Therefore we purchased 2 micro geared motor that have 450 RPM at 6V. We will not reach 6V due to our board only being able to supply 5V, so as the PWM signal gets closer to 5V we should be able to reach 360 RPM if not greater. Once they arrive, we can begin to test to see how reliable they are and if we will need to choose something else.

Trade-off Studies Tables

Servo-Trade Off Study

Requirements Met	Servo Name	Price	Torque (kg)	Speed	Voltage (V)	Weight (g)	Dimensions
-Supply voltage is realizable since we can do anything between 3.7 and 5V.	Single Cell 3.2g / .16kg / .10sec Digital Ultra-Micro Servo	$7.95	.16 ~.19	.1s~.08 sec/60 degrees	3 ~ 4.2	3.2	28mm x 9 mm x 22mm
-Supply voltage is realizable since we can do anything between 3.7 and 5V. -Weighs less than 2 g	Ultra Micro Servo 1.7g for 3D Flight	$4.34	0.08	0.12 sec/60 degrees	3.7 ~ 4.2	1.7	12 mm x 21 mm x 0mm
-Supply voltage is realizable since we can do anything between 3.7 and 5V. – Affordable since it is less than $4.00 -Weighs less than 2 g	HK-5330 Ultra-Micro Digital Servo 0.17kg / 0.04sec / 1.9g	$3.64	.12 ~.17	.06~.04 sec/60 degrees	2.8 ~ 4.2	1.9	20 mm x 6 mm x 23 mm
-Supply voltage is realizable since we can do anything between 3.7 and 5V. -Affordable since it is $3.99   -Weighs about 2 g	HK-282A Single-Screw, Ultra-Micro Servo 2g / 0.2kg / 0.08sec	$3.99	0.2	0.08 sec/60 degrees	3~4.8	2	23mm x 8mm x 20mm
-Can rotate 360 degrees	Continuous Rotation Micro Servo FS90R	$7.50	1.3~1.5	110~130 rpm	4.8~6	10	32mm x 12mm x 30
-Can rotate 360 degrees	Continuous Rotation Servo – FeeTech FS5103R	$11.95	3~3.2	.18sec/60 degrees	4.8~6	40	37mm x 20mm x 54mm

Motor Trade-Off Study

Name	Requirements Met	Rated Volt	Max Volt	Min Volt	No load Current	Stall Current	No Load RPM	Power Rating	Weight	Size	Price
DC Toy/ Hobby Motor	Cost effective	6V	9V	2V	70mA	500mA	7300~10900	420 mW	17.5 g	27.5mm x 20mm x 15mm	$1.95
Hobby Motor	-Cost effective	3V	12V	1V	110mA	N/A	6000~7200	330mW	26g	2.75cm x 2.75 cm	$1.95
3.7V 50,000 RPM Small Colorless Motor 716	-Operational voltage is reachable. -Cost Effective -7mm diameter	3.7V	N/A	N/A	100mA	1.35A	50000	5W	2.5g	16.5 mm x 7mm	$1.95
FA-GM6-3V-25 Micro Motor Mini Motor	-6mm diameter	3V	N/A	N/A	100mA	N/A	1200	300mW	1.2g	20 mm x 6mm	$9.00

 

Conclusion

• Servos are excellent in terms of precision of control which is why we first considered using them for our robot.
• In addition, they have a great amount of torque for dealing with heavier loads.
• However, after looking into greater detail, we realized they might not be as useful to our project as we previously thought.
  • They are much too bulky to fit inside the spider.
  • Their price is quite unreasonable when being compared to the motors.
  • Would only be able to get the CR servos(for the 360 degree rotation of the head)

 

• The 3.7V 50,000 RPM Small Colorless Motor 716 has more advantages for our application when compared to the rest of the available options.6
  • Its rated voltage is realizable since our battery is going to be 3.7V,

• Max(stall) current is 1.5A which is excellent since our motor driver will might output 1A maximum
• It is only 2.5 grams
• Reasonably priced, $1.95
• Only disadvantage of this motor is that we do not know the torque specifications and will have to do my own calculations based on the information given. These calculations will be placed in a separate blog post  if you wish to see them.
• Project Update
  • The mechanical design of our 3DoT david changed in order to make the assembly much simpler than before, and will therefore not require us to use servos. The new design has it set where one motor will control the right side of the legs and the other will control the left side.

Resources:

https://learn.adafruit.com/adafruit-motor-selection-guide/types-of-motors

https://www.adafruit.com/products/711

https://www.sparkfun.com/products/11696

http://www.seeedstudio.com/depot/37V-50000RPM-Small-Coreless-Motor-716-p-1884.html

https://www.firgelliauto.com/products/mini-motor-micro-motor

https://www.servocity.com/html/micro_gearmotorblocks.html

http://www.hobbyking.com/hobbyking/store/index.asp

http://handyboard.com/hb/faq/hardware-faqs/dc-vs-servo/

https://learn.adafruit.com/adafruit-motor-selection-guide/continuous-rotation-servos

https://learn.adafruit.com/adafruit-motor-selection-guide/rc-servo-control

 

BY: Andrew Saprid ( Manufacturing Engineer)

Introduction:

Since the first prototype made in solidworks was difficult to 3D print, the parts had to be redesigned. They have to be as flat as possible to make it easier for the 3D print the parts.

Related requirements:

Level 2 system requirement follows:

• The 3DoT Spider shall incorporate 3D printed parts for the legs, body, or head. This follows from the level 1 requirement dictating the limit on 3D printing times.

3DoT David Model exploded view and simulation

Exploded View

Simulation

A simulation is shown to clearly define the movement system in different views: 

Full view  Corner view

Top view

New Design Parts

Chassis

The new design is made in solidworks.These are the top and bottom plates. The top and bottom plate will be assembled together as a chassis for 3Dot David Spider. The dimensions for these plates are 12 centimeters in length and 7 centimeters in width with an extrusion of 0.2 centimeters for each plate. For the bottom plate, holes are 0.39 centimeters. They are made to fit the gears as they rotate on the surface of the plate. Small holes, dimensioned at 0.2 centimeters in diameter,  on each plate connect them together. For the top plate, 0.2 centimeters holes are made in the middle to connect the PCB box on top of the plate.

Gears

Gears are bought in Amazon.The large gear has 30 teeth, and measures to be 3 cm. The small gear has 10 teeth, and measures to be 1 cm. 6 large gears and 4 small gears will be used to make the gear train. Calculations are done to obtain the gear ratio.

Refer to Gear train blog post for details (Spring 2016: 3DoT David Gear Train)

Legs

Femur and the tibia are combined as one leg part. The length of the femur is 4.78 centimeters. The length of the tibia is 5.3 centimeters. The hole on the femur is 0.25 centimeters in diameter. The hole will connect the joint as shown on the right. Two legs will have the same dimensions as it connects to joint. The space between the two legs will have 0.5 centimeter cushion.

     

Joint

The joint measures to be 0.8 centimeters in length and 0.6 centimeters in width with an extrusion of 0.5 centimeters. The hole in the box is 0.20 centimeters. The cylinder connected to the box is 0.29 centimeters in length with a diameter of 0.39 centimeters.

Plank, Solidworks modeled gear, and leg studies

This part is called the plank that will be attached to the bottom of the bottom plate. The reason for the plank is to make the leg, connected to the gear,  lift up in an angle as the gear rotates in a full 360 degree rotation. In the 3 centimeter gear, the radius 0.76 centimeters. The gear is modeled to simulate the gear train in Solidworks.

Refer to leg study blog post for calculations ( Spring 2016: 3DoT David Leg Movement Angle Study)

      

 

In the middle gear of the gear train, the leg is lifted because of the plank. The lift is about 13.30 degrees according to the leg study. This is the position that it will start for the middle gear.

The position that the corner gear of the gear train will start at is 5 degrees as the leg rests on top of the plank, where the cylinder stops the leg from going anywhere else.

 

Gear view

A better view is shown from the top as it shows the gear train and the position of all the legs. 12 legs will be used, and 2 legs will act as one part with the same dimensions.

 

Conclusion:

The previous design was difficult and complex to build similar to the Hexbug Spider. The new design is made to simplify the CAM Movement system of the HexBug Spider, which will be applied to the legs connected to the gears, as the two motors (located on the bottom plate) will rotate both sides of the gears in a full 360 degree rotation. The two motors will attach to the 1 cm small gears.

 

BY: Andrew Saprid ( Manufacturing Engineer)

Introduction:

Calculations were done by adding the quantity to the table and adding the total amount of printing time for each part. The calculations for all the parts are then added and highlighted on each of the three tables for the total print time of all the parts.

Related requirements:

• As part of our level 2 requirements, 3D printing time shall not exceed the 6 hour limit. Each part will not exceed the 2 hour limit of printing. Three tables are shown in the figure below to compare scenarios, if all the parts are to be 3D printed.

All parts printed

If the parts are all printed, it takes 7.78 hours, which exceeds the 6 hour limit of printing.

12 legs to be printed takes 132 minutes to print. Each leg to print takes 11 minutes.

4 connectors takes 8 minutes. Each connector to print takes 2 minutes.

6 joints to be printed takes 6 minutes. Each joint to print is 2 minutes.

6 planks to be printed takes 8 minutes

Each part did not exceed the 2 hour limit print time.

Bottom plate and Top plate excluded

By excluding the top and bottom plates to be laser cutted, it takes about 4.49 hours, which goes below the 6 hour limit of printing.
Excluding the PCB box and cover

By excluding the PCB box and PCB box cover, it takes about 5.5 hours, which also goes below the 6 hour limit of printing. The PCB Box and the cover makes the PCB invisible to the eye, which does not affect the level 1 requirements, and saves printing time.

Conclusion
The best results are excluding the top and bottom plate to be laser cutted, or excluding the PCB box and PCB box cover. The best scenario would be to exclude the PCB box a
nd cover. The rest of the parts will be 3D printed, which takes about 5.5 hours.

Sources:

Thanks to Min, the manufacturing engineer from the Velociraptor Team for printing all the parts, the results are in for the 3D printing time.

BY: Andrew Saprid ( manufacturing engineer)

Introduction:

The mechanical system for the 3Dot David is the gear train. Gears have teeth, which are designed to make the gear train rotate 360 degrees possible. The motor will operate around 5 volts, and it will be connected to the small gear as it drives the large gear. With the gear train setup, the 3Dot David will be able to move across the lyceum floor.

Related requirements

Level 2 system requirement follows:

• The 3DoT David shall use two micro motors for the movement system of the robot.

 

Motor calculations:

The motor previously used at 50,000 RPM was not the right value because it was unrealistic for the spider to run at that pace, which could easily break components inside of it. In order for the legs to move at a slow steady pace, the motor using will give 360 RPM at 5 volts as the legs will run at a pace of 2 cycles per second.

The team will use the motor at 360 RPM to drive the small to large gears.
120 RPM / 60 sec = 2 cycles per second for the large gear The gear train is 120 RPM

Conclusion

The team will use the motor at 360 RPM to drive the small to large gears.

120 RPM / 60 sec = 2 cycles per second for the large gear

Source:

http://www.engr.ncsu.edu/mes/media/pdf/gears