A widely practiced computational technique, used for solving various engineering and mathematical problem is the finite element method (FEM), also known as finite element analysis (FEA). Using FEA, it is possible to solve problems in numerous departments such as structural analysis, fluid flow, electromagnetic potential etc. This is achieved by solving partial differential equations where a larger system is separated into simpler parts known as finite elements. By developing a mesh of a part, finite elements can be created by converting an initially infinite number of degrees of freedom problem into a limited degree of freedom problem. A system of algebraic equations is created when boundary conditions are entered into the FEA by approximating the unknown function over the domain. These equations form the finite elements, which are assembled into a bigger arrangement of equations that represents the entire problem. FEA then approximates a solution by using these equations [39].
Model description
To ensure that the designed Smart Plug was able to withstand accidental impacts, simulations were made using the FEA software Abaqus FEA. Dynamic simulations were made in Abaqus/Explicit to analyze the structural behavior of the Smart Plug. The simulations were performed to resemble impact simulations where the Smart Plug was dropped on a rigid- and a wooden surface from a 1 m distance.
All separate parts of the Smart Plug were imported into Abaqus and reassembled into one product. The explicit analysis was performed over a time period of 2 ms by using isotropic linear elasticity as the material model. Mass-scaling was used in order to reduce the computational analyzes time without sacrificing the simulations accuracy. This was implemented by scaling the mass of the elements with a time increment below 2×10-8. The instantaneous velocity the Smart Plug had at one meter free fall was calculated using following equation [40]:
𝑣𝑖 = √2𝑔ℎ (3.3)
Where g is the force of gravity and h is the traveled distance during the fall.
The shape of tetrahedral elements was used as the mesh-shape with an explicit element library using a linear geometric order.
Optimization
An optimization process is generally performed to obtain an attractive product. Optimization can, for example, be performed to reduce weight or size for ergonomic and environmentally aspects. An optimization process can be performed using FEA and an iterative process where the construction is simulated until satisfaction is reached [34].
4 Results
This section presents all produced results, including concept generation and selection, product design, material selection and simulation results. A step-by-step manual is presented that illustrates how to assemble the designed Smart Plug.
4.1 Failure Modes and Effects Analysis
The performed FMEA is presented in table 4.1 and consists of predicted errors and remedies in order to prevent or minimize them before they emerge. A larger format of the FMEA is presented in appendix D.
Table 4.1. Performed FMEA
The concerning conclusions drawn from the FMEA was the user’s risk of getting an electrical shock or burn. Although the minimum requirement of IP-classification for the Smart Plug is IP20, only hindering penetration from objects equal to, or greater than 12.5 mm is an unnecessary risk of potential danger. Therefore, the ability to penetrate foreign objects into the smart Plug should be minimized in the design when it is possible. By minimizing the possibility of penetrating the Smart Plug, fingers and conductive objects ability of coming into contact with current when the Smart Plug is inserted to a socket is minimized. Selecting an insulating material is of great importance, both due to Smart Plugs functionality and personal safety for the use.
in open position 4 Visual Inspection 4 160 10 – Hazardous – Without warning
9 – Hazardous – With warning 8 – Very high Responsible: Nadia Samson FMEA Date (Orig.):
FMEA
Process/Product Name: Smart Plug Prepared By: Nadia Samson
DETECTION (1 - 10) RPN
4.2 Concept development Concept generation
The concept development of the Smart Plug is very limited by the socket design standard DIN 49440:1989-12 and the plug design standard DIN 49441:1972-06, as well as the predeveloped PCB assembly. The Power PCB must be in contact with two terminal pins so the current can flow from a wall socket to the PCBs, see figure 4.1. Due to this, the Power PCB and, at least parts of the Relay PCB, must be submerged in the plug design of the casing. The terminal springs located on the digital PCB, must be penetrated by terminal pins connected to the plug whose appliance we want to give current through. This leads to that the digital PCB should be relatively close to the socket design of the Smart plug since terminal pins are 19±0.5 mm in length according to DIN 49 441:1972-06 [14], [15].
Figure 4.1. Further PCB layout explanation.
Due to the mentioned standards that must be fulfilled, and the layout of the PCBs, the design of the Smart Plug shape became limited. With the diameters of the plug and socket design predetermined by standards, the casing in between the socket and the plug design were designed alongside the shape of the PCBs, creating an as small Smart Plug as possible as material usage and size was minimized, see figure 4.2.
Antenna PCB
Power PCB Relay PCB
Terminal springs
Must be in contact with the terminal pins connected to the Smart Plug
Digital PCB
Figure 4.2. Sketch of the intended Smart Plug.
Alterable parts of the concept solution were developed. The alterable parts of this concept were the plug-section where the compatible socket types can vary depending on the design, and the mounting of the assembly in order to hinder the Smart Plug from becoming an accessible part.
4.2.1.1 The plug
- Alt 1.1: Plug of type F, see figure 4.3.
Figure 4.3. Sketch of plug type F.
- Alt 1.2: The plug must be compatible with a socket of type F according to requirements.
However, by designing a plug compatible with both type F and E sockets (and type C since they are not grounded), the Smart Plug can be used in more sockets and thereby will be a more useful product in certain countries, see figure 4.4.
Figure 4.4. Sketch of plug type F, compatible with sockets of type E.
4.2.1.2 Mounting of the assembly
- Alt 2.1: The plug and socket are glued together, ensuring that the Smart Plug cannot be mounted apart without harming the Smart Plug.
- Alt 2.2: The plug and socket are attached together by fitting and screwed together to secure the Smart Plug from being mounted apart without the use of a tool.
Concept selection
The altering parts were evaluated in an elimination matrix that can be seen in table 4.2.
Table 4.2. Elimination matrix
The alterable parts 1.2 and 2.2 passed the screening. The smart plug will therefore be designed so disassembling is a possibility so that separation of plastic and metal can be done before disposal of the product. By making waste separation a possibility, the Smart Plug’s impact on the environment when disposed of can be reduced if conducted correctly. The Smart Plug will also be designed with a plug compatible with both type E and F sockets so the possibility of utilizing the Smart Plug is maximized.
4.3 Manufacturing method
The Smart Plug is presumed to be produced in large quantities, making it favorable to use a manufacturing technique with high tooling cost rather than a high part cost. The geometry of the Smart plug is assumed to become relatively complex and surface finishing is necessary for the design. Since the Smart Plug is intended to be sold as a consumer product, the company that contracted Sigma Connectivity to design a Smart Plug will have their company’s name displayed on the Smart Plug. However, since this client is not to be disclosed during this degree project, the surface finishing required will not to be visible on the results for detailed design.
The surface finishing is nevertheless still taken into consideration when determining the manufacture method.
Due to above-mentioned factors, injection molding was determined as the optimal manufacturing technique. The total cost for injection molding is mostly determined by the cost of the mold, making the process cost efficient when large number of products are manufactured.
The technique can handle complex geometry but when constructing a 3D-model of the Smart Plug, the geometry should, if possible, be kept as simple as possible to keep the manufacturing cost as low as possible. This since an increase in size and complexity of the mold increases the cost to create the mold.
Injection molding
Due to common complications such as cracking, warping, sinking, shrinking and breaking when manufacturing a product, the manufacturing method must be kept in mind when designing a product in order to avoid such problems. Important parameters and their design guidelines that should be regarded when injection molding is the chosen manufacturing method can be seen below [41].
4.3.1.1 Wall thickness
Minimizing the wall thickness is preferred since thick walls can lead to long cycle times and poor mechanical properties. The wall thickness should be as uniformed as possible to simplify the flow pattern and minimize variations in shrinkage that can lead to warpage. Shrinkage can appear when the thicker wall sections cools, and thereby increasing the stress level between the two thicknesses. When non-uniform walls are required, its beneficial to transition the wall thickness by e.g. gradually adjusting the wall thickness so stress concentrations and other potential issues are reduced [41].
Stress concentration areas that could reduce the part’s impact strength can also be created where there are abrupt changes in wall thickness. This should therefore also be avoided when designing a part [41].
4.3.1.2 Ribs
By adding ribs to a part, the part’s bending stiffness increases without increasing the wall thickness of the part. To avoid sinking, the maximum thickness of the ribs should be 50-75%
of the nominal wall thickness. Furthermore, to avoid large variation in wall thickness, the height of the ribs should not be greater than three times the nominal thickness. To avoid non-uniform shrinking than can lead to warping, ribs should be designed on both sides of the nominal wall if possible [41].
4.3.1.3 Draft angle
A minimum of 0.5-degree draft angle per side is required in order to easier eject the part from the mold. The draft angle should tilt on all walls parallel to parting directions [41].
4.3.1.4 Radius
Stress concentrations are created where sharp corners are present, and they should therefore be avoided. The inside radius should be designed to be at least 50 % of the nominal wall thickness, making the outside radius 150 % of the nominal wall since uniform walls are desired [41].
4.3.1.5 Bosses
Bosses are generally designed to secure fasteners, aid in assemblies or be used as a detector for a mating pin. To strengthen the structural parts, connecting ribs should if possible be designed with bosses where the connection ribs should be 0.6 times the nominal wall thickness at their base to avoid sink. Bosses that are designed as stand-alone bosses should be designed following the design guidelines for ribs [41].
4.4 Detailed design 3D-modeling
The final design of the Smart Plug can be seen from different views in figure 4.5 and figure 4.6.
The design was constructed so all requirements were fulfilled, and the Smart Plug can therefore be used safely when manufactured with the right materials.
Figure 4.5. Final design of the Smart Plug displayed in different angles.
Figure 4.6. Top and bottom view of the Smart Plug.
The Smart Plug is in total 70,47 mm high, 52,2 in length and 45,2 mm in width. The casing of the Smart plug is 51,47 mm high. The Smart Plug consists of 14 parts, see figure 4.7 for a full assembly description.
Figure 4.7. Assembly of the Smart Plug.
1x socket 1x spring
1x shutter casing
1x female ground spring
2x screws M2x8
1x button
1x
light guide1x PCBs
1x plug
1x male ground spring
2x terminal pins 1x screw 2.5x10 1x shutter
1x isolation
sheet
Figure 4.8 and 4.9 displays the center of the Smart Plug from different views.
Figure 4.8. Smart Plug viewed by trimming the section by the x-plane.
Figure 4.9. Smart plug viewed by trimming the section by the y-plane.
4.4.1.1 Shutter assembly
The shutter is mounted on the rear of the socket and consists of three parts; a shutter which can move in both the horizontal and vertical direction, a spring which ensures the shutter regains its original position when a plug is withdrawn, and a shutter casing. The shutter casing was designed so it performs two functions. The casing was designed to ensure that the shutter remains mounted correctly, and the ribbons on the rear side was designed to support the digital PCB. When terminal pins are inserted into the socket, the shutter will rise in the vertical direction while simultaneously slide in the horizontal direction, see figure 4.10.
Figure 4.10. Shutter displayed in different positions.
If an object only presses the shutter through one of the holes, or presses the shutter with dissimilar forces, the shutter will tilt and will therefore not open to let the object penetrate the Smart Plug. See figure 4.11 for clarification.
Figure 4.11. Shutter fail-safe.
4.4.1.2 Button
The button’s upper part is attached to the plug and locked in place by the socket while the button’s lower part is locked in place by components on the digital PCB. When the button is pressed, the upper part of the button’s design will flex. This will press the switch on the PCB, creating a functional button, see figure 4.12.
Figure 4.12. Button viewed in different angles.
4.4.1.3 Socket
The socket is designed to fulfill standard DIN 49440:1989-12 [14]. Furthermore, around the openings for the female ground spring, protective ribbons were designed to prevent children from inserting foreign objects into the Smart Plug, see figure 4.13. This was implemented since foreign objects could cause damage to the PCB, causing the Smart Plug to malfunction or, in worst case scenario, give an electric shock to the child.
Figure 4.13. Socket with bottom view.
It should not be possible to open the Smart Plug without the use of a tool, according to Annex V in standard IEC 62368-1:2018 [20]. Due to this, and the possibility of waste separation, the plug and socket is assembled together by a self-tapping screw. The alternative to a self-tapping screw was to mold in a threaded insert in the boss when manufacturing the socket. This alternative however was omitted since the possibility of separating the threaded insert from the socket is minimized.
4.4.1.4 Plug
The plug is designed to be compatible with type F sockets as required, but also type E sockets, see figure 4.15. This was implemented so the plug can be used in as many countries as possible.
The plug has several ribs serving different purposes, see figure 4.14. The ribs were designed to support the power PCB, the digital PCB and the female ground spring, as well as hindering the digital PCB from rotation.
Protection against penetration from foreign objects
Boss
Figure 4.15. Bottom view of plug.
4.4.1.5 Light guide
The light guide is used to distribute light from the LED located on the digital PCB to the design’s exterior. The light guide is a Bivar PLP2-100 component, made from a 94V-0 polycarbonate material, see figure 4.16. The light guide is 2 mm in diameter and 2.54 mm in length [42].
Figure 4.16. Bivar PLP2-100 light guide.
4.4.1.6 Isolation sheet
The isolation sheet consists of a 0.5 mm thick plastic insulation and a 0.05 mm thick adhesive tape. Its primary function is to ensure that the two tabs from the male ground spring does not come in contact with the PCB or its component since it may cause a short circuit, see figure 4.17.
Figure 4.17. Isolation sheet protecting the Smart Plug from short circuit.
4.4.1.7 Terminal pins
The terminal pins were designed to be in direct contact with the assembled PCBs to further ensure current flows through from the wall socket, see figure 4.18. They are manufactured with a H59 Copper material with Nickel surface plating.
Figure 4.18. Terminal pin.
4.4.1.8 Female and male ground spring
The female ground spring was designed to manage the flex requirement provided in standard DIN 49440:1989-12, as well as fulfilling the width requirements [14]. An immersion at both the outer edges was designed to ensure that the weld between the female and male ground spring had enough surface area to be completed on when taking tolerances into account, see figure 4.19. The female ground spring’s middle part was not immerged to ensure that the female ground spring, which is conductive, does not come to close to the components on the digital PCB since it could cause a short circuit.
Power PCB
Terminal pin
Terminal pin in direct contact to the power PCB
The male ground spring is ordered as a completed part and was thereby not designed in this degree project. Figure 4.19 displays both the female and male ground spring and how they are connected in the assembly.
Figure 4.19. Female and male ground spring assembly.
Both the female and male ground spring are manufactured with a H62 Copper material with Nickel surface plating.
4.4.1.9 Antenna PCB
Apart from all standards that were required to be fulfilled, the part that had the most design requirements was the design around the antenna PCB. Since a predeveloped assembly of the PCBs was used for the Smart Plug, all requirements for the components on the PCBs in the near-by area of the antenna were presumed to be fulfilled. No ground is designed to be directly below the antenna, or in close proximity of the antenna and the casing around the antenna is an insulating plastic material.
The design of the Smart Plug was constructed so the closest plastic casing to the antenna is 1 mm away from the antenna’s bottom side, see figure 4.21. The closest insulating plastic to the antenna’s sides is the plastic on the socket. The outlet has a minimum distance of 1.23 mm away from the antenna, see figure 4.20. These distances are enough to not affect the antenna, and the antenna will therefore not pick up a higher effective dielectric.
Weld
Figure 4.20. Antenna PCB.
Figure 4.21. Antenna PCB.
Smart Plug assembly
The assembly of the Smart Plug can be divided into three parts; assembly of the plug, assembly of the socket and the final assembly.
Antenna
1.23 mm
1 mm
4.4.2.1 Assembly of the plug
The terminal pins are molded to the plug during the manufacturing process, i.e. when the plug is injection molded.
1. Insert the male ground spring from the bottom of the plug, see figure 4.22.
Figure 4.22. Insertion of male spring.
2. Bend down the two tabs on the male ground spring and attach the isolation sheet. Note!
The taps in figure 4.23 is not bent in the CAD-figure.
Figure 4.23. Illustrates view of taps.
3. Insert the button and light guide according to figure 4.24. The buttons top part, as well as the light guide is press fitted.
Figure 4.24. Illustrates view of button and light guide.
4. Insert the assembled PCB into the plug. The digital PCB will be supported by four ribs and two guiding features from the plug, see figure 4.25 and 4.26. The power PCB will be in direct contact with the two terminal pins while also supported from the plug casing.
4. Insert the assembled PCB into the plug. The digital PCB will be supported by four ribs and two guiding features from the plug, see figure 4.25 and 4.26. The power PCB will be in direct contact with the two terminal pins while also supported from the plug casing.