Final Design

Key design components

1. Shaft and Collar Assembly

How did we get our design?The two most important objectives stressed by our sponsor were that the helicopter simulates the motion of an actual model helicopter and that it be safe to use by students. Taking these things into consideration our team focused first on simulating true helicopter motion by ensuring that the motion of our model helicopter would not be restricted by our stand in any way. We focused on finding linear rotary bearings and a precision shaft that would allow us ease of motion in the two degrees of freedom we were modeling. We then looked into methods of counter balancing the mass of the test apparatus, so that the model helicopter would lift only its weight, which led us to the pulley counter balance solution. After solving our motion problem, we then turned our focus to safety. We looked into various rotor blade containment systems ultimately settling on a fan shroud idea that we obtained from the Quanser helicopter. The helicopter model must also be able to rotate freely without any restriction. To accomplish that task, the wires running from the helicopter to the power supply should not twist. Therefore the slip rings were considered to use to prevent wire twisting. Students using the test stand should be taking measurements such as position, velocity and acceleration. Optical encoders were installed for each degree of freedom for taking measurements. To be more precise in measurements, counts per degree in encoder was increased with gear ratios which led to higher resolution that the sponsors requested.

a) b)

Figure 1: Shaft and collar a) 3D CAD model and b) Final Product

The purpose of the shaft and collar is to constrain the helicopter motion to the two desired degrees of freedom. Several alternatives were considered for this purpose: the first design considered was a simple steel or aluminum rod with a bronze/brass sleeve bearing. The second was a hardened stainless steel shaft with linear/rotary ball bearings. The steel rod and sleeve bearing design would have been much lower cost, but much higher friction than the alternative hardened steel with linear/rotary ball bearings. The stand has the potential to be used to test different types of RC helicopters as well as the helicopter designed by our team. As a result, we expected there to be a wide range of reaction forces and friction forces in the collar/bearing assembly. Because this is such a critical component in capturing the motion of the system, it was decided by the team and the sponsors to use the hardened steel and linear/rotary ball bearings. This ensured smooth helicopter motion with a very low friction coefficient in any testing situation.

2. Rotary Optical Encoders

a) b)

Figure 2: 3D CAD model of a) Top encoder assembly and b) bottom encoder assembly

Figure 3: Final product a) Top encoder assembly and b) bottom encoder assembly

Two encoders are used in the test stand, one to measure the translational motion and one to measure the rotational motion. As can be seen from above figure an optical encoder was connected to one of the counterweight pulleys through a transmission system in order to measure the vertical translation, velocity, and acceleration of the helicopter while another encoder was connected to the shaft through a different transmission system in order to measures the rotational components. Rotary encoders were/ chosen instead of linear encoders due to physical limitations of the linear encoder. Linear encoders require more space than rotary encoders and have a fixed length to work with, whereas rotary encoders are adequate for the entire helicopter translation when combined with a transmission system. Lastly, the resolution of the rotary encoders was able to be adjusted using various gear ratios to achieve a desired resolutions resulting in greater accuracy in our measurements. (Please check Final report in Reports section for more details.) This application of transmission systems helped achieve the preferred resolutions of approximately fourteen counts per mm of translation and four counts per degree of rotation.

3. Counterweights

Figure 4: Counterweights

Counterweights were incorporated into the final design to counter the weights of the center shaft and the helicopter mounting assembly as well as provide a method to measure the vertical translation of the helicopter. As the helicopter raises and lowers, the weights attached to the lower plate will move an equal distance, thus creating the opportunity to measure translation through the pulley system attached to the counterweights. Without the weights the stand would require very powerful motors to lift the combined weight of the shaft and helicopter mounting assembly. These more powerful motors would also require a greater power supply and larger propeller blades which would also produce the need for larger propeller shrouds. Furthermore, if the stand suddenly lost power, the helicopter and shaft would fall with a larger amount of acceleration and final velocity without counterweights than with them. Counterweights were included in the final design to eliminate the need for high-powered motors, create a method to measure translation, and to construct a safer environment for the students using the stand.

4. Load cell

Figure 5: Load Cell

In order to implement accurate controls on the system, it was necessary to know all of the system parameters, most importantly, the thrust and rotor torque. This was accomplished by utilizing a load cell with a 111.2 Newton capacity oriented in two different positions. The first position was oriented downwards to allow the support plate to make contact with the load cell, providing us with a measurement of thrust, or the vertical force of the helicopter. The second orientation was sideways in order to pick up the torsion forces generated by the primary rotor and the tail rotor. By utilizing these two different orientations we were able to simplify our design and use only one load cell, as opposed to two, simplifying our cost and our design. The load cell mounting bracket was designed to lock into three positions—an upper position to measure vertical thrust, a side position to measure stabilizer thrust and primary rotor torque, and a lower position to stay clear of stand motion.