SMART SUCTION CUP
The clamping device is one of the most important parts of the ROMERIN project as it is a climbing robot, the mechanism used will be vacuum suction cups to allow the robot to move through building facades. Currently, there are a lot of climbing robots that use suction force as a adhesion method. Applying the principle of vacuum generation, it is possible to generate adhesion forces through the pressure difference, allowing robots to scale on arbitrary surfaces, made of different types of materials and with different methods of application.
During the life of the ROMERIN project different versions of the suction cup have been made, each better than the previous one. The original design of the suction cup was composed of three motors, which can be seen in the first design of the image and was characterized by having a diameter of 8 centimeters. However, it was decided to make a series of modifications to improve the functioning without losing range of movement within the degrees of freedom available to it, achieving the result of the center of the image.
Each of the three versions is described below along with the most relevant features and improvements of each suction cup.
This version was developed as a modification of the first robot of the project, the commercial model XYZ Bolide Y-01, so that it could move through vertical walls. The plan for this is to attach a suction cup at the end of the robot legs. In each suction cup there is a rotor, the turbine, which rotates at a high velocity to create a vacuum in the cup.
In these suction cups a partial vacuum is created so that the cups will stay attached to the surface, in order to do that every suction cup is equipped with its own turbine and motor. The turbine as well as the stator around it is printed with a 3D printer.
The first design of the geometry of the turbine was done intuitively from looking at existing design of other vacuum applications. Finally a first turbine was created in Inventor, it has a diameter of 40 mm and a total height of 10.8 mm, the ten blades are curved backwards and they are also curved going from the front plate to the back plate.
Because every suction cup needs his own turbine, motor and enough battery capacity to power all of them, the weight of the entire robot will increase, making it harder to walk on a vertical wall. This is why the turbine should be designed as efficiently as possible, with the help of a software program such as Ansys Fluent will be very easy to change the design of the turbine and run simulations with it in order to find the best design.
The simulations carried out were intended to find the design that would be able to reduce the pressure as much as possible, for which tests are carried out on the height of the turbine, the amount and shape of the blades, the insertion of a motor, etc. The most relevant tests according to the data obtained were the following:
Height of the turbine: . The initial design has a total height of 10,8 mm with 9,2 mm being the height of the blades, two more turbines with higher blades were created to see if the height of the blades changes the pressure that is created in the suction cup.
The simulation shows, in the image below, that exists a linear relation between the height of the turbine and the pressure. Increasing the height of the turbine will result in a lower pressure in the suction cup, which is a favorable outcome for the project. The only drawback of this solution is the extra power required to rotate bigger turbines.
Shape of the blade: The original design had curved fan blades but changing these to a straight blade design improved the pressure as well. When the blade is curved, the airflow is pushed against the back plate and that diminishes the velocity. This is not the case when the blades are straight; the streamlines concentrate in the middle of the turbine where they exit towards the stator.
The best turbine for the project would be a turbine with straight blades and the dimensions should be increased until either the physical limitations are met or the power required to rotate the motors gets too high. Both properties are the best improvements that can be made to decrease the pressure even more.
Finally, after studying the results of the simulations, the following 3D model was designed and manufactured.
After the manufacture of the first version, certain disadvantages were encountered during the tests: high vibrations and a weak grip on the engine clamping part. The main objective of this version was the optimization of the design and subsequent construction of the adhesion mechanism composed of a centrifugal turbine, with its respective rotor and stator, and the sensor circuit.
Following the validation of the previous prototype, the design of the rotor and the stator is optimized, relying on all the analyses performed. Experimental tests consisted of measurements with a stator and a rotor connected to a brushless motor with a gradually increasing voltage. The parameters to be measured were: the atmospheric pressure, the revolutions per minute of the rotor and the consumption in amperes of the engine.
After having analyzed all the results, it has been observed that the rotor that gives better results in both performances and vacuum achieved is the original double blade rotor, centered at the entrance. For the stator a more robust design was made, where a more robust grip for the engine would also be sought.
Once reached an optimal point in the mechanical system, starts the design of a sensory circuit. Before starting the design, different sensors of each type were investigated, in order to choose the most suitable sensors to the needs of our system in terms of tolerances, measurement ranges, supply voltages, etc. The two types of sensors used were:
- Infrared sensor: Used to measure the distance to the plane or support surface and indicate when the suction cup is properly supported. The model chosen was the TCRT5000.
- Barometric sensor: Used to measure atmospheric pressure at all times and verify that the vacuum is done correctly. The model chosen was the BMP280, welded into an integrated circuit for protection. A linear regulator is also included so that both sensors have the same power voltage.
The other part of the electronics consists of a Brushless motor, Turnigy Multistar model , and its corresponding variator, responsible for generating the vacuum inside the suction cup. The microcontroller used is the PRO TRINKET, it was chosen for its size and for adapting exactly to the signals needed. Its main function is to control the 6 engines, using the PWM signals sent to the respective ESC speed controllers, and to read the information received by the sensors placed on each of the legs.
Following with the improvement process, another version of the Smart Suction Cup is developed. The goal of this prototype is to develop an efficient, modular, compact and energy-efficient vacuum generation system.
The first stage is based on a first design of the suction system based on the results obtained by the work previously developed and based on centrifugal rotor design methodologies applying theoretical fluid mechanics developments.
It was manufactured using 3D printing methods for the rotor, the suction cup coupling and the housing cover, and laser cutting methods for coupling the motor to the housing. 3D printed parts are made of ABS plastic and the motor coupling is made of wood.
Once the design and manufacture of the initial model is completed, the evaluation of the prototype is carried out through experimental tests related to the amount of vacuum generated and electricity consumption.
After analyzing the results, it is observed that the system is capable of generating large vacuum quantities for relatively low rotation speeds. However, the electrical power consumption to achieve these values is very high, causing the efficiency of the system to be low.
In orde to validate the results obtained experimentally, a computational model is developed using ANSYS simulation software. The main purpose is to perform a review of the aerodynamic behavior to observe the development of the air flow, the resulting pressures and speeds in different areas of the preliminary system.
After analyzing the results obtained for the preliminary model, an optimization process is performed divided into 2 stages:
- Determine the geometrical parameters of the rotor for generating acceptable vacuum quantities for low energy consumption. Parameters related to the number of blades and the height of the rotor are studied.
- Redesign the suction system due to the high power consumption obtained in the preliminary model. First the diameter of the prototype is determined to achieve greater efficiency, from here a redesign of the suction system is made, shown in the image below, adapting the different parts to the new size of the rotor.
Finally, adding the suction system to the suction cup we get the following 3D model, shown sectioned to show what is inside.
After the design a review of the flow behavior within the new optimized system is performed, the procedure is the same as that performed for the preliminary system.
Once the analyses and studies of the optimized mechanism have been completed, the results obtained by the two developed systems are compared:
- In terms of the amount of power consumed as a function of the amount of vacuum generated: the optimized model shows a 69.5% reduction in energy consumption and generates 4.4% more vacuum.
- Relative to the vacuum generated by the rotor rotation speed: the preliminary model needs a lower turning speed while the optimized model must rotate at a higher revolution to generate the same vacuum.
It should be noted that this is the latest version of the adhesion system and is being used in ROMERIN++, the most advanced model of the ROMERIN project.