Archive for category: Spiderbot

By Kristine Abatay – Project Manager
and Matthew Clegg – Computer & Control Systems

The final statement of our mission objective created a new level 1 requirement regarding wireless control of Spiderbot through the Arxterra control panel and Arxterra Android app. The following is the final list of the level 1 requirements for Spiderbot, along with their corresponding verification tests:

1. Completion of the project will be achieved by May 12, 2014.

This is the date of the final assigned specifically for EE400D.

Test: If project Spiderbot is completed by this date, then this requirement will have been achieved.

2. The legs and complete chassis components of Spiderbot will be designed to extend into three spatial dimensions.

Test: This requirement will be verified within the SolidWorks program, which enables our manufacturer to define the three axes of a design. If, at any rotated view of a component design, three separate pieces of the design can be chosen to define a respective x-, y-, and z- plane, then this requirement will have been fulfilled.

3. Match the speed of the track rover project on a flat surface. This value was determined to be 0.2003 m/s.

Test: In order to verify this requirement, a flat surface, straight-lined course will be measured out and Spiderbot will complete the course while being timed. The resulting quotient of the length of the course, with the amount of time it will take Spiderbot to complete the course, will be calculated. If this value is equal to or less than the calculated rover speed, then requirement will have been achieved.

4. Operate in accordance with the CSULB College of Engineering Health and Safety Policy            

(found here: http://www.csulb.edu/colleges/coe/views/safety_and_environment/safety_policy.shtml)

Test: The CSULB College of Engineering Health and Safety Policy states that

“Faculty…shall: Implement the university’s Health and Safety Policy and all other university safety programs in work areas under their supervision/control.”

If Professor Hill, a faculty member of the CSULB College of Engineering, approves the operation of Spiderbot in the classroom, then this requirement will have been met.

5. Have a height clearance of 4 in. and width clearance of 2.5 in.

These values were obtained through measurements of the largest obstacles found in the assigned course. The following image is an aerial view of the course that Spiderbot will have to maneuver. The total length of this route was measured to be 41.80m. It is located in the eastern wing of the CSULB campus.

Test: Verification of this requirement will be done by measuring the height clearance, as well as the length of one leg sweep of the fully constructed Spiderbot.

6. Spiderbot will be capable of being wirelessly controlled using the Arxterra Control panel, in conjunction with the Arxterra Android phone application.

Test: If a command sent to Spiderbot through the Arxterra Control panel matches the command that Spiderbot executes during operation from the Arxterra Android phone application, then this requirement will be satisfied.

Experiment Conducted by: Matthew Clegg (Controls – Spiderbot), Elaine Doan (Systems Engineering Division), and Kristine Abatay(Project Manager – Spiderbot)

Test Setup written by: Matthew Clegg and Kristine Abatay

 Discussion and Results written by: Elaine Doan

A test will be performed in attempt to find the current the Power HD High Torque Servo 1501 MG consumes during operation of the Spiderbot or Biped when 6 volts is applied.  From the test, the idle current of the Power HD High Torque Servo 1501 MG will also be identified.

Test Setup

The following setup was used for this test:

 

The test was run using breakout boards since they will be used to power the many servo motors in the final construction of Spiderbot. A plastic indicator was attached to the wheel of the servo, which was placed in front of a protractor since the test code used was based off of PWM adjustment as opposed to angle. The following image shows the test code that was used. Matthew Clegg modified the code that was provided by Adafruit (the brand of the breakout board used) to cater to this test:

 

The final construction of Spiderbot will involve the use of metal brackets to securely connect the servos with the designed components, so a metal bracket was also used in this test. In order to acquire accurate current readings and properly mimic the motion that Spiderbot’s legs will use, small weights were attached to the end of the bracket.

The estimated weight that a single leg servo on Spiderbot will have to support came out to be 437.55 g. This value was acquired using estimates provided by SolidWorks for the designed components and actual weight measurements from a scale for components that we had. The total is comprised of a single femur and tibia component, as well as three servos to account for a single leg, and 1/6 of the chassis design with all of the components it will hold (i.e. Android phone, pan and tilt platform). The total mass that was tested went up to as high as 500 g in order to account for the possibility of undershooting the final mass, keeping in mind the total mass that the servo can handle with the extended arm attached. The following image provides an idea of the approach taken in finding the current for varying lengths of the servo arm:

 

The first panel of the image shows the metal bracket and the small weights that were attached to it. In order to maintain a constant test mass with a varying arm length, the weights were first placed on the inner and outer portions of the bracket without exceeding the initial length of the arm, as depicted in the second panel of the image. Finally, the masses were stacked on the bracket to extend the length of the arm, as shown in third panel, and more current measurements were obtained.

 The current read through the multimeter varied constantly throughout the test, so the values that were recorded were the largest values that appeared consistently on the multimeter for each angle value. A quick video depicting the setup of this servo test can be found in the link below:

http://youtu.be/b5Dv5LfQsCQ

Discussion and Results

When working with servos, there are two currents worth noting: the idle current and the stall current. The idle current is the current consumed by the servo when it is not performing but connected to the power source. It is important to identify the idle current because it will provide users with the upper bound on how long a battery powered application can run. The stall current is the maximum current drawn by the servo when it is performing. The stall current will provide users with the minimum current requirement for the power supply. The actual current a servo consumes during operation will vary between the idle current and stall current. Servo currents usually are not specified by the manufacturer and the user is required to run tests to find the idle current and stall current. It’s easy to measure the idle current since it is low, constant, and only requires a simple connection to a power supply. A standard servo such as the Power HD High Torque Servo 1501 MG will have an idle current around 0.01 -0.03 amps depending on the power supply. Stall currents are more difficult to measure because stalling a servo can result in a damaged servo. A standard servo such as the Power HD High Torque Servo 1501 MG will have a stall current around 1 amp (finding the stall current of the Power HD High Torque Servo 1501 MG will not be attempted in this test.  Finding the current a servo draws during normal operation of the Spiderbot and Biped will be attempted instead). The current will be approximately linear with the supply voltage, so the currents drawn at 7 V will be almost double those at 4 V. Adjusting the rotational degrees of the servo will typically draw more current linearly. Increasing the torque by ten times will typically draw ten times the current.

Part A
IDLE CURRENT

Idle Current at 6.0 V = 0.03 amps

Part B
SHAFT ROTATION VS CURRENT

 

The graph shows how the current draw is affected by the increasing rotation of the servo’s shaft from 10 to 90 degrees. The current drawn increases steadily by approximately 10 mA as the rotation degrees increase by increments of 10 degrees. 

Part C
TORQUE VS CURRENT

 

 

The graph shows how the current draw is affected by the increasing torque from 0.10 to 6.48 kg•cm. The current drawn increases by approximately 40 mA as the torque increase by increments of 1 (kg•cm). 

By Kristine Abatay, Project Manager

Following the group’s preliminary design review presentation, we met with the Robot Company’s President and modified our mission objective. Our new mission objective is:

Complete a hexapod robot project whose creation will achieve a speed that matches the current Robot Company rover project, operate safely, be capable of maneuvering a predetermined route in a forest-like setting, and have a body comprised of three-dimensional components.

Trickling down from this objective allowed us to create a fifth project requirement:

5. The legs and chassis components of Spiderbot will be designed to have three dimensions.

The other project requirements can be found ina previous post where the project was introduced:

 https://www.arxterra.com/spiderbot-life-times-vol-2/

Additionally, the following tests have been created to verify the project requirements that have been written (they are listed in the order from which they have been introduced):

Verification Tests:

Test 1
If project Spiderbot is completed by May 12, 2014, then this requirement will have been achieved.

Test 2
In order to verify the speed requirement, a flat surface, straight-lined course will be measured out and Spiderbot will complete the course while being timed. The resulting quotient of the length of the course, with the amount of time it will take Spiderbot to complete the course, will be calculated. If this value is equal to or less than the calculated rover speed, then requirement will have been achieved.

Test 3
The CSULB College of Engineering Health and Safety Policy states that “Faculty…shall: Implement the university’s Health and Safety Policy and all other university safety programs in work areas under their supervision/control.”

If Professor Hill, a faculty member of the CSULB College of Engineering, approves the operation of Spiderbot in the classroom, then this requirement will have been met.

Test 4
Verification of the height requirement will be done by measuring the height clearance, as well as the length of one leg sweep of the fully constructed Spiderbot.

Test 5
The requirement of a three dimensional design will be verified within the SolidWorks program, which enables our manufacturer to define the three axes of a design. If, at any rotated view of a component design, three separate pieces of the design can be chosen to define a respective x-, y-, and z- plane, then this requirement will have been fulfilled.

By Simon Abatay, 3D Modeling and Manufacturing

Picking up from last semester’s Spiderbot project, the design for this project will have 6 legs. Though the name ‘spider’ implies 8 legs, translating it to a project would become very pricy in the long run since it would require more materials and power to construct.

Despite the difference in leg amount, we still wanted the design to be similar to an actual spider. The image below is of an actual spider leg. From this, we can see that spiders have 7 joints.

(Image from: http://havoc20.wordpress.com/2010/11/13/spider-tibia-structure/)

This would translate to seven servo motors per leg, and at 6 legs that would mean 42 servo motors! To save money on the final design, we’ve reduced our spider leg design to the 3 main parts: the coxa, femur, and tibia. This is the case with most hexapod designs that can be found online.

Three joints with a single motion each will mean each leg will have three degrees of freedom. This reduction will provide smooth movement for Spiderbot while walking and enough support for the spider to stand when stationary.

Keeping the reduction of joints in mind, the following image is my initial design of the spider’s tibia, which was created in the program SolidWorks.

One of the main issues with last semester’s design was maintaining balance while stationary. To avoid this, each tibia portion will have a tibia “plate” on opposite sides of the servo motor. To keep the whole tibia portion from collapsing on itself, each pair of tibia plates will be supported by metal spacers or standoffs.

The tibia will measure 7 inches in height as a response to the project requirement of clearing an obstacle of 4 inches tall, located in the planned route for Spiderbot.

For the body portion of Spiderbot, I plan to use a circle-based design since its shape will alow for uniform weight distribution. This way, the tibias will be evenly spaced out and, assuming the hardware will be placed in the middle of it, the center of gravity will remain at the middle of the Spiderbot.

The image below shows the design of the base plate. The radius of the center portion is 3 inches to accommodate the power source, the microcontroller, the servo breakout boards, and the Android© based cellular phone.

The extensions of the circle have a radius of 4.4 inches to accommodate the servos and their proper positioning (see rectangular cutouts).  The rest of the cutouts are for weight saving and heat ventilation. Seeing as a solid plate would have unwanted and unnecessary weight, it would be best to make cut-outs of the plate while at the same time leaving enough mass and surface area for the other components.