User:Medelen8/ENES100/APW Design

Problem Statement
The purpose of this project is to eventually turn a Fisher Price Power Wheel into a completely automated car that can move and steer on its own without running into obstructions using sensors such as cameras. Our specific team goal was to design the gas pedal system and determine what force will be needed as well as what type of motor and how the pedal assembly will be implemented.

Requirements for each element or component derived from system level goals and requirements
The requirements that we set for ourself was to create a design for a mechanism that would control the movement of the Power Wheel through an Arduino. This mechanism requires enough force to push the pedal to move the Power Wheel and take the the force off the pedal to stop the Power Wheel's movement. Additional requirements are listed below:


 * 1) The motor must be powered by a 12 volt source and under 30amps to work with the monster motor shield
 * 2) The motor must be able to operate in the forward and reverse direction
 * 3) The motor needs at least 0.10 N lbs of force to push the pedal
 * 4) The motor shouldn't continuously be on when the pedal is depressed.
 * 5) The power wheel needs to be able to stop quickly (Release pedal within 2 seconds of receiving the signal)
 * 6) The device that connects to the pedal needs to be durable for multiple cycles (30 cycles a minute, at least 50 hours of total use)
 * 7) The motor needs to cost under $30.00.

All of these requirements apply to the motor and are designed around the constraints of the motor shield and our power source.

Alternatives in design

 * 1) A stepper motor with a rubber stopper that would rotate into the Power Wheel's pedal.
 * 2) A linear actuator that would push the Power Wheel's pedal.
 * 3) A motor attached to a lever that would in turn push the Power Wheel's pedal.
 * 4) When a motor spins, it would spin a nut, attached to an extension, horizontally into the Power Wheel's pedal.

The initial design
Our first design was to have a simple stepper motor with a rubber stopper that would rotate into the Power Wheel's pedal to make it move. To make the Power Wheel stop, the stepper motor would simply rotate in the opposite direction to remove pressure from the pedal.

Experimental prototypes and testing conducted during design
Only one prototype was made through our design process. This design was a lever with a longer side that through torque, should have been able to push the pedal if the motor pulled on it. Unfortunately, with our dry run of just pulling the string ourselves in place of the motor, we were unable to push the pedal.

Appropriate optimization in the presence of constraints
We decided to use the screw system over the other options because of cost, effectiveness, power consumption, and the amount of force it is able to apply to the pedal. Our top choice was the linear actuator but due to the cost and the constant power requirement, it wasn't feasible for our design. The stepper motor wasn't able to provide enough force for the pedal. The screw system meets all of our requirements.





Technical and scientific knowledge
Motor Specification

In order to determine the specifications of the motor, we first needed to figure out how much force was required to depress the pedal. To do this, we positioned the power-wheel so that the pedal was parallel to the floor and placed weights on it until enough the pedal was fully pressed down. We added weights until we reached 2KG. To figure out how much force this would require we used the equation:


 * $$Nm = 2 kg * 9.80665 = 19.6133 Nm$$

Then, we took the measurements of our design with the axle / pully and found the lengths of $$ l1 $$ and $$l2 $$. The following formula is how we found tension in the string:
 * $$ T = \frac{l1}{l2} F = \frac{7}{14} 20 = 10 N $$

After finding the tension in the string connected to the pully, we needed to find the force a gear would apply to it. To do this, we multiplied


 * $$T$$ by the radius ( $$ r $$ ) as follows:


 * $$ T_M= rT = (0.2m)(10N) = 2.0 Nm $$

Once finding this, we needed to find a motor with an output of at least .2Nm. We had a list of 3 motors with extremely different specifications, but around the same price. To find out the power, or wattage, of each motor we used the formula:


 * $$ P = I^ 2R = I^2 \frac{v}{I} = IV $$ to first find

The motor we ended up choosing has 12vdc and varies between 1.5amps and 17amps. Using the lowest amperage without load (1.5amps) we found it had a wattage of :$$12vdc * 1.5amps = 18watts$$

After finding the wattage of the motor, we needed to see how much force it could handle. To find the force, we used the equation
 * $$T=\frac{P}{x}=\frac{18w}{179\frac{rpms}{min} \frac{2\pi}{1rpm}\frac{1min}{60seconds}} = 1NM$$

From this equation we can see that the motor has a minimum force of 1NM without load at 1.5amps. Under full load the motor ramps up to 17amps and can handle a max of 11NM as shown below:
 * $$12vdc * 17amps = 204watts$$


 * $$T=\frac{P}{x}=\frac{204w}{179\frac{rpms}{min} \frac{2\pi}{1rpm}\frac{1min}{60seconds}} = 10.88NM$$

Since we only need 2NM of force to push the pedal, this motor will be able to handle this load without stressing itself. And since its designed to handle more power, efficiency will be higher and reliability will be prolonged as well.

Creativity, problem solving, and group decision-making
When faced with a decision to make, we would all voice our ideas and make a list of what we were saying. Then we would go through the list to look for the best solution; sometimes this was done using a decision matrix or just by eliminating options based on constraints like money. We used a lot of creativity when designing our original part to the power wheel which was the the pedal actuator. Our ideas went from hard wiring the arduino right to the wire harness attached to the pedal, all the way to using different lever systems to maximize leverage for the most force. We even designed an actuator that would turn a screw through a fixed nut which would push the pedal in straight.

We did run into problems from the previous groups work. Their were bumpers on the gears of the motor that turned the steering wheel, these bumpers got stuck in the teeth of the two turning gears and the motor was not strong enough to pull out the jam when the wheel was to be turned in the opposite direction. We were able to get the bumpers off and put them back on thicker this time so that they would not be skinny enough to slip under the motors protective housing and get into the gears. The entire steering wheel motor assembly was also bolted to the power wheel with screws that required a screw driver at difficult angles so those were replace with screws that can easily be removed with an allen-wrench. Another big issue we addressed was the power source, the batteries used to power the arduino motor shield were only around 5 volts and not the required 12 volts, by using the power supplier we were able to determine if the motors were receiving their signals and if they worked (which they did). A big problem we faced was what type of actuator to use after using many calculations shown above we found that the force our actuator would need to exert to push the pedal was .2 Newton Meters. This allowed us to eliminate motors that were to weak or even much stronger than necessary.

Prior work in the field, standardization and reuse of designs (including reverse engineering and redesign)
The previous group created a steering mechanism to control the steering wheel of the power-wheel. They used a rear power window motor from a Honda Accord attached to an Arduino monster power shield to control the direction. They also wrote the code for the steering design but were unable to find a portable power source. By the time we received the project, the motor was jammed so we were unable to see it in action. However, we were able to look at the source code and reverse engineer it to get an idea of how it should work and how we can incorporate it into our design. We decided to reuse the motor since it has enough force to move the steering wheel and the assembly to move it in an arc motion was attached. We had to disassemble the power window motor to fix the bumpers and by doing that we were able to get a better idea of the range of motion.

Performance, life cycle cost and value
In terms of performance, we want the motor for the pedal to be responsive and change rotation quickly for safety reasons. We want the power wheel to be able to stop quickly in case a person, animal, or an obstacle that may damage it gets in its path. We also need it to be durable since it will be changing its rotational spin very often. The lifetime cost will be low since parts won't need to be replaced (Except for batteries).

Aesthetics and human factors
Aesthetics are not important for our project since it will not be a commercially available consumer product. The scope of this project is more of a proof of concept that can be built upon later. Since future teams will be working on this power-wheel, we decided to redesign the wooden structure to make it modular and easier to disassembly. We did this by using wing nuts instead of screws for the main support, and using nuts / bolts to attach the power window motor to the wood brace so it's easier to reach and quicker to take apart. The benefit of these improvements also allows for tool-less disassembly, make it more practical to fix things in the field.

Implementation, verification, test and environmental sustainability
The next steps for this project are to order the motor, attach the motor to the pedal, and test the code. Once the motor is attached and powered, the code can be debugged and modified until it accomplishes the desired goal. The only impact our project may have on the environment are the depleted batteries. To be environmental sustainable, a rechargable battery can be used to power the Arduino and motors. The power-wheel already runs off of a rechargeable battery.

Maintainability, reliability, and safety
With regards to maintainability, we had to modify the previous group's work so that we could work on the car easier. The main thing we changed was the way the steering mechanism was secured onto the wooden frame. The previous group used wood screws to secure the mechanism because that is what they had available. The only downside to the screws was that the mechanism got jammed and we had no way of unscrewing the screws, so we had to pretty much rip out the screws. We improved the system by drilling holes and putting in 3 allen bolts that screw into the actual threads on the steering mechanism. This way if we ever need to work on the steering mechanism it is easy to take off and put back on.

For reliability we modified the existing bumpers so that they would not get jammed. This was the old bumper so what we did was modify the bumper so that it would not get stuck

This new system will now not jam anymore and will alow the steering mechanism to functon properly.

Robustness, evolution, product improvement and retirement
Since we are still designing most of the features of this autonomos power wheel the path of evolution of the power wheel would have to be to have a final product built and that looks good as well.