2010 University of Kwa-Zulu Natal - Automatic Goalkeeper Coach

2.2 Design of the Fluidic Muscle and Spring

Once the manner in which the ball was to be fired was determined various methods to compress the spring were conceptualized and analysed, for example CMS electric cylinders. Festo manufactures a product called the Fluidic Muscle. The muscle operates by using compressed air to expand a rubber hose in its peripheral direction. Due to the conservation of volume, the increase in diameter causes the muscles length to decrease and therefore generate a tensile force. The fluidic muscle offers up to ten times the force than a standard cylinder of the same diameter and has a good dynamic response, even at high loads. The fluidic muscle would be used to compress the spring.
The electric cylinder had a weight in the region of 9 Kilograms, while a fluidic muscle to operate to the same specifications weighed 1.245 Kilograms. The reduction in mass affected the size of the components used to move the “ram” as the moment of inertia would decrease and the forces experienced also decreased. The use of an electromagnet in the electric cylinder configuration would add to the weight of The Ram and increase the moment of inertia. Furthermore by selecting the fluidic muscle, a number of components, such as the electromagnet, could be reduced and thereby save money.
Design of the spring was conducted through design analysis equations outlined in Fundamentals of Machine Component Design, by Robert C. Juvinall. As the fluidic muscle was to be contained within the spring and used to deflect the spring by 60 mm, the spring had to have a minimum inner diameter. This was determined by finding the diameter of the muscle after it had been compressed by 60 mm.
The fluidic muscle was designed with the force needed to propel the ball as one of the parameters. A deflection at which the muscle was to operate was used as the second defining parameter. The muscle was to deflect the spring by 60 mm. The spring was designed to have a force of 1800 N at this deflection. The fluidic muscle was designed on MuscleSim, which is a design program supplied by Festo for selection of a muscle.

2.3 Positioning System

The physics of the soccer ball was analysed with the effect that spin has on the ball’s motion being utilised to control and vary the trajectory of the ball in the vertical plane. Horizontal positioning of the ball was aimed at not introducing spin. The ball was to be hit along the centre axis in the direction that the ball should go. This would eliminate curvature within the horizontal plane. The ball was to be moved vertically by means of an electric drive and the distance away from The Ram altered by varying the lengths of an electric driver shaft. This combination is discussed under the title of “Vertical Positioning System”. The Ram was to be moved about the ball to get the desired horizontal trajectory. This system is discussed under the heading of “Horizontal Positioning System”. The Ram is composed of the firing mechanism, referred to as the cannon and the shoe which is a component fastened to the end of the spring which will make contact with the ball.

2.3.1 Vertical Positioning System The vertical positioning system is responsible for the accurate location of the soccer ball to The Ram. The vertical positioning system is broken up into two sub-systems that provide this accuracy. The first sub-system that was implemented was responsible for maintaining a specified distance between the soccer ball and the front end of The Ram at all times.

The second sub-system that was implemented was responsible for the various vertical offset displacements between the centres of the soccer ball and The Ram. The offset allows for the ball to be projected along the height of the goal post. The two sub-systems are integrated together, where the distance corrector is mounted to the slide of the electric linear drive.

2.3.2 Horizontal Positioning System   The horizontal positioning system was split into two sections, one which governed the linear motion and the other defining the rotational movement. Various products were selected from both SEW and Festo to achieve the desired movements. The moment of inertia of The Ram was determined (see Appendix B6- Mass Moment of Inertia of The Ram) and its influence on the selection of the components analysed.   2.3.3 Linear Motion of The Ram To achieve the required horizontal motion for The Ram, a synchronous linear motor was selected. The Ram is connected to a servo motor which aligns The Ram to the centre of the ball after the linear motor has moved The Ram to an assigned position. The synchronous linear motors are able to traverse high loads at vast speeds and have extremely high forces of attraction that assist in high impact load forces.

The SEW SL2 synchronous linear motor is made up of two parts, namely: the primary and the secondary. The drive operates as a rotary motor with the core as the primary and the rotor as the secondary. This configuration of the primary and secondary serves as an electromagnet that meets the requirements to linearly traverse The Ram with the mass that is applied to it. The linear motor is capable of reaching speeds from 1m/s to 6m/s however the horizontal system does not require such a high speed and can be controlled. The Ram will need to traverse at a total distance of 253.8 mm between the two end positions of The Rams in order to stay in range of the goal post. Synchronous Linear motors are powered and controlled with a MOVIDRIVE® drive inverter.

2.3.4 Rotation of The Ram

To rotate The Ram along the horizontal plane, a rotary device is required. The device needs to be accurate and precise. The DSMI Swivel Module (part number 192271) would have been ideal for the required application but due to its maximum working mass moment of inertia being 1200 kgcm2 when being used in conjunction with Axis Position Controller SPC 200 (part number 170522), the DSMI Swivel Module cannot be used.
Many motors and gear box combinations have sufficient torque to drive and stop The Ram to the required firing position but lack the required driving and stopping mass moment of inertia. The Ram requires to be moved a maximum angle that is small enough to allow it to be moved at a low rotational speed. High speed movement of The Ram is not required.

2.3.5 Motor Housing The motor used to swivel The Ram is to be housed on a mounting structure with The Ram in the centre. The height of the structure was determined by finding the combined heights of the bearings, swivel shafts and adding it to the outer diameter of the outer sleeve of The Ram. A height of 200 mm was sufficient. The width of the structure was calculated by finding the angle that The Ram needed to be rotated. This was done by trigonometric methods as seen in Appendix B8 – Motor Housing Calculations. The length of the goal posts and the perpendicular distance of the centre of the ball to the goal posts were known. For a housing length of 100 mm on either side of the point of rotation of The Ram the distance that the centre-line of The Ram will rotate will be 33.27 mm. This distance was then summed with the outer radius of the outer sleeve and a width of 90.3 mm was to be accounted for. A clearance of 20 mm between The Ram and the mounting structure’s square tube support was incorporated.

2.4 Ball Handling via Suction Cups

To lift and hold the soccer ball, vacuum technology from Festo was used as it proved to be the best and simplest means available. A suction cup was to be used to lift the ball from the retrieval system to the positioning system by means of a linear drive from Festo. This will be referred to as “Suction Cup 1”. The ball would be lifted to another suction cup, which would then activate and take the weight of the ball. This will be referred to as “Suction Cup 2”. Suction cup 1 would then be switched off and lowered to the retrieval system, where it would wait for another ball.
Suction cup 1 would be providing stability so that the ball would not roll off the lifting device due to wind or some external force. It was assumed that a lateral force (with respect to a vertically positioned suction cup) of 11.5 N would be sufficient to hold the ball in place as forces due acceleration would act vertically on the lifting device and the device supported the ball from movement in three directions. The height of the suction cup and holder must be as small as possible as it would be positioned below the retrieval system and influence the angle of the ramp.
Suction Cup 2 is only to hold the weight of the soccer ball, which is 4.37N. A suction cup with a diameter of 15 mm had a holding force of 8.5N, although this is sufficient. To ensure that inertial forces do not cause the ball to separate from the suction cup a diameter of 20 mm was selected. An axial holder with a height compensator up to 6 mm was selected as the suction cup had to be flush on the surface of the soccer ball or else a vacuum would not be created. The lifting device would lift the ball a distance of 5 mm beyond the level of the suction cup. This ensured that its accuracy and repeatability in attaining the height of the suction cup would not result in a gap between the suction cup and the ball’s surface. A filter was to be used to prevent contaminants like dirt from damaging the venture tube. Polyurethane was used as the suction cup material due to operating conditions.

2.4.1 Suction Cup Vacuum Generator

The vacuum generator for the suction cups works off the venturi principle, where air flows through a tube with a changing cross sectional area. The velocity of the air changes as the area varies due to the conservation of mass. Bernoulli’s equation states that the total head of the flow must remain constant.

3.1 Lifting Device

A linear drive from Festo was employed in the design to transport the soccer ball from the retrieval system via the bucket to the vertical positioning system. The bucket allowed for a greater traversing time for the soccer ball to be transported to the vertical positioning system as mentioned in section 5.3-Ball Handling. The linear drive was designed to operate at a fast speed due to the stability that the bucket offered. Decreasing the time in which the system is effectively non operational is beneficial as the cycle time decreases. An end position controller was selected as it offered a further decrease in the cycle time and allowed for two intermediate positions to be programmed. The linear drive must have an encoder for the end position controller to determine the position of the slide along the track and send the necessary signals to the proportional valve. It was decided that the DGCI linear drive which has an integrated displacement encoder would be used as it prevents the slide from overshooting the designated stops. The end position of the DNCI linear drive was designed on Softstop [26], which is a design program supplied by Festo for selection of the linear drive.

The linear drive was selected to have a working stroke length of 1250 mm as this was a sufficient displacement to compensate for the height from the ground to the centre of the goal post. The cylinder operates with un-lubricated compressed air at a pressure of 6 bars. The bucket that contained the ball and the suction cup that was attached to the end of the linear drive had a moving mass of 3.391 kg. See Appendix B11 – DGCI positioning axes selection criteria.

3.2 The Bucket

The method in which the ball will be raised was altered from the initial retrieval system concept due to the limitations from the suction cup design. It was decided that suction technology from Festo would provide the simplest means of lifting the ball. The bucket is to form the back, lower part of the ramp. Once the ball has cleared the initial ramp it will roll down to the bucket due to gravity. The bucket will comprise of a quarter a sphere, which will be manufactured by making a fibre-glass mould of the outside of the soccer ball. The base and sides of the sphere will be moulded to suit the angle of the ramp. A 20 mm hole is to be drilled in the centre of the base, around which the suction cup will be secured. The bucket will be a separate part to the main ramp and will be attached to a linear drive which will lift it vertically. The bucket will transfer the ball in the retrieval system to the vertical positioning system, after which it will rapidly lower back to the ramp and await another ball from the goal keeper.

4. Valve Terminal

Festo produce several types of valve terminals which suite various demands and applications. Some of the pneumatic components used in the Keeper Coach project needed different air pressures other than the 6 bars being supplied. A 44 VTSA type valve terminal was selected as it enabled pressure plates to be mounted below a pneumatic module to effectively lower the output pressure to that being supplied to the valve. The use of a valve terminal simplified the pneumatic circuit as pressure distribution to the working components could be achieved through one compact terminal as opposed to the use of a distribution block(s) and individual solenoid valves. The selection of the valve terminal was conducted by investigating the various pneumatic modules needed for the equipment and then selecting the electronics modules. The valve terminal was configured using the Valve Terminal Configurator. The electrical peripherals were specified according to the demands from the various components.

5. Sequencing and Control

Many components used in the design of the Keeper Coach need to be controlled. One means of control is to send electronic input and output signals via a higher order controller to the unit designated to control the specific device. An example of this would be the electric linear drive which has an integrated controller. This integrated controller can be controlled with I/O signals from another controller. Components without encoders can be controlled pneumatically by sending signals to solenoid valve to either open or shut the flow to the device. A valve terminal with an integrated programmable logic controller will be used as the higher order controller as it can support the necessary electronic modules needed to send and receive signals from the components. The PLC is a CPX – FEC type and is discussed in the valve terminal design, in section 6.2 - Electronic Modules.

6. Components Used

The following table details the parts required for the system:

Suction Cup Components
Suction cup Gripper ESG-20-SU-HB-QS
In-Line Filter VAF-PK-4
Suction cup Gripper ESG-20-SU-HC-QS-F
Vacuum generator with pressure switch VN-05-H-T4-PQ2-VQ2-01-P
Non-return valve H-QS-6
Vacuum generator mounting bracket VN-T4-BP
Connecting Cable NEBU-M8G3-k-2-M8G3

Lifting Device
Linear drive DGCI-18-1250-KF-F-2M-2L
Valve MPYE-5-1/8-LF-010-B
End position controller SPC11-MTS-AIF-2
System manual P.BE-SPC11-SYS-EN
Drive specific supplement P.BE-SPC11-DGCI-EN
Controller cable KMPV-SUB-D-15-5
Valve cable KMPYE-AIF-1-GS-GD-2
Silencer U-1/8

Rotation of the Ram
Stepper motor EMMS-ST-87-S-SEB
Gear unit EMGA-80-P-G5-SST-87
Motor cable NEBM-S1G15-E-5-LE6
Encoder cable NEBM-M12G8-E-5-S1G9
Motor control unit CMMS-ST-C8-7
Power supply SVG-1/230VAC-48VDC-10A
Control cable NEBC-S1G25-K-2.5N-LE26
Programming cable PS1-ZK11-NULLMODEM-1,5M
Encoder plug NECC-S-S1G9-C2M
Documentation for motor controller P.BE-CMMS-ST-HW-EN

Air Preparation
Filter regulator LFR-3/8-D-MINI
On-off valve HE-3/8-D-MINI
Mounting brackets HFOE-D-MINI