iRoboBumper: SONAR-Obstacle Detection System for Robotics Platforms

Licensed under CC BY-SA 4.0

Mechanical Design

This section details the steps to create the rotating housing, as well as the non-rotating base. Acrylonitrile Butadiene Styrene (ABS) plastic was chosen, as it is relatively lightweight, rigid, and can be cut, drilled, and sanded using standard tools. The ABS plastic that was used for this project is textured on one side and flat on the other. This is a drawback in most cases, and ABS plastic that is smooth on all sides is recommended instead.

1. To begin, a rectangular piece of 1/4" ABS plastic was cut with a dimension of 3" x 4". This piece serves as the base of the rotating portion of the system. A diagram of the position and size of these holes are shown in Figure 19, as well as pictures of the ABS plastic base before and after the holes were drilled / cut out.


Figure 19: Housing Base Design and Pictures

2. Next, holes were drilled in the base for the servo, proximity, and PCB standoffs. In addition, rectangular holes were cut in the base for the servo and proximity sensor themselves. The 10 holes were drilled with a 1/8" drill bit and were counter sunk with a 1/4" drill bit on the flat side of the ABS plastic base. Counter sinking these holes allows the screw heads to remain flush with the bottom of the rotating base. This is important because the distance between the bottom of this panel and the top of the system's non-rotating base needs to be minimized to maximize the IR reflected energy of the proximity sensor, which is mounted facing down through the hole that was cut out above.
3. Once the rotating base was made, two pieces of 1/4" ABS plastic were cut with a dimension of 3" x 3.5" to be used as two of the housing sides. Four holes were drilled into each piece to mount the PING))) sensor modules. Figure 20 shows the dimensions of the PING))) sensor from the datasheet, as well as the layout of the holes on the side panel. In addition, a picture of a side with the holes drilled is shown.


Figure 20: System Sides for PING))) Sensors

4. The 2 large holes were drilled with a 5/8" drill bit, which was used to ream out the holes until the PING))) sensor's silver Ultrasonic components could slide through. For the smaller mounting holes, the same 1/8" drill bit was used to drill the holes and a 1/4" drill bit was used to allow the mounting screw to be counter sunk.
5. Next, we need two pieces of 1/4" ABS plastic to mount the toggle switch, which is used to turn the system on and off. First, a rectangular piece was cut that is 1" x 3". This piece does not require any holes to be drilled in it. The second piece is a 1" x 1" square. A 1/4" drill bit was used to drill a hole in the middle of this square and a 1/8" drill bit was used to drill a shallow hole for the mounting plate that came with the toggle switch. Figure 21 shows the two completed pieces.


Figure 21: Toggle Switch Mounting Pieces

6. Lid: To make the lid, three pieces of 1/4" ABS plastic were required:
1. A single 3" x 3" piece was cut.
2. Two pieces were cut with the dimensions of 3" x 4". In one of the 3" x 4" pieces, a 1" x 1" square was cut out of one corner. The piece with the 1" x 1" cut out will be the top of the rotating part of the system and the other two pieces will be the remaining sides.

Figure 22 shows the three lid pieces.



Figure 22: ABS Plastic Pieces for the System's Lid

7. Non-rotating base: This is the part of the system that the servo horn is connected to, and contains the alignment holes which allow the proximity sensor to determine the pan rate and “home” direction.
1. The non-rotating base of the system was made from the final two pieces of 1/4" ABS plastic, cut 5 1/4" square, as shown in Figure 23.


Figure 23: ABS Plastic Pieces for the System's Non-rotating Base

2. The two base pieces were clamped together and a 1/8" hole was drilled through the center, as shown in Figure 24. This hole was just used as a pilot hole and marked a common center in both pieces.


Figure 24: Drilling Center Hole for the System's Non-Rotating Base

3. After the clamps were removed from the two base pieces, a 5/8" drill bit was used to enlarge the center hole in one of the base pieces for it to be used as the top of the non-rotating base. This 5/8" hole allows the center of the servo horn to fit through.
4. Bottom of the base: In the other ABS plastic piece for the non-rotating base, a 1-1/2" hole saw was used to create a large opening in the center. This second piece is the bottom of the base and the 1-1/2" opening allows the entire servo horn to fit inside. The servo horn secures to the top piece of the base; this will be discussed and shown in more detail below.
5. Figure 25 shows the bottom piece of the base with the hole drilled out. A scrap piece of ABS plastic was clamped to the bottom while the hole was drilled to minimize the chance of the 1-1/2" hole saw damaging the ABS plastic as it cut through.


Figure 25: Bottom Piece of the System's Non-Rotating Base

8. Assembly:
1. With all of the pieces of ABS plastic required for the rotating housing and non-rotating base now cut and drilled, it is time to put them together with epoxy. J-B Weld ClearWeld Quick-Setting Epoxy (Figure 26) was used, which can be found in most hardware stores. It creates a strong bond between the pieces of ABS plastic.


Figure 26: Epoxy Used to Join ABS Plastic

2. The first pieces to be glued together two sides that hold the PING))) sensors to the rotating base. Epoxy was mixed and placed on the narrow edges of the rotating base. Then, the sides are centered onto this base and securely clamped in place to allow the epoxy to dry. One of the 3"x 4" pieces for the lid was used to keep everything square during clamping. Figure 27 shows the sides and base clamped together during this process.


Figure 27: Epoxying Sides to the Rotating Base

3. Next, epoxy was used to secure the two sides of the lid to the top of the system. This time the base of the system with the sides that hold the PING))) sensors was used to help keep everything aligned while the epoxy was setting. In addition, scraps of ABS plastic were used to keep the edges flush. Figure 28 shows the lid of the system clamped together while the epoxy was setting.


Figure 28: Epoxying the Lid of the System

4. Once the epoxy for the lid had set, the clamps were removed and the lid was set aside.
5. Next, the pieces of ABS plastic to mount the toggle switch were attached to the rotating base.
   1. First, the 1" x 3" piece was secured to the side using epoxy and clamped into place.
   2. Once the epoxy had set, the 1" x 1" piece was secured to the top using epoxy and allowed to dry.

The base with each of these pieces securely mounted is shown in Figure 29.



Figure 29: Mounting Pieces for Toggle Switch

This completes the construction of the base and lid for the rotating part of the system.

6. Creating the Non-rotating Base of the System
1. To create the non-rotating base of the system, the rough sides of the two base pieces of ABS plastic described earlier (Figures 23-25) were secured together using epoxy. The epoxy was applied to the rough side of one piece and both clamped together, making sure their sides were aligned. These were set aside and allowed to set up.
2. Next, a few holes were drilled in the non-rotating base:
   1. First, the servo horn was removed from the servo and mounted to the top of the base, through the 5/8" hole using a #4-40 screw. A scrap piece of ABS, washer, and a nut were used on the opposite side to hold the server horn securely centered in the 5/8" hole.
   2. Once secure, four holes were drilled through the servo horn and the top piece of ABS plastic using a 1/8" drill bit. Note that the next-to-furthest hole on the servo horn was used as a pilot hole for these. The ABS plastic, with the servo horn secured to it and 1/8" holes drilled is shown in Figure 30.


Figure 30: Drill Holes to Attach Servo Horn to the Non-Rotating Base

3. Once the 1/8" holes were drilled, the servo horn was removed and a 1/4" drill bit was used to allow the screws to be counter-sunk. In addition, a 1-¼” diameter circle was drawn about the center of the base.
4. Next, a 3/8" inch drill bit was used to drill three alignment holes around this circle with 90 degrees between them, leaving one half of the circle without a hole. The position of these can be seen in the left picture in Figure 31.
5. Finally, four 1/4" holes were drilled 3/4" inch from each corner and a 5/8" drill bit was used to allow the screws to be counter sunk. These four holes provide a mechanism for the system to be secured onto the host platform.

Figure 31 shows the completed top of the non-rotating base with all holes drilled and the bottom of this base with the servo horn installed.



Figure 31: Completed Non-Rotating System Base

7. Mounting Brackets

To finish the mechanical design aspect of the system, we need to create the mounting brackets for the pair of 2xAA battery holders and perform the wiring of the two battery holders, toggle switch, and power connector.

1. A pair of 2xAA battery holders was chosen instead of a single 4xAA holder, as the pair would allow the weight of the batteries to be distributed more evenly and better access to the battery and electronics.


Figure 32: Battery Holder Mounting Brackets

2. The brackets were created out of 1/16" thick, 1 1/2" x 1 1/2" aluminum angle iron using a Dremel tool with a chop saw blade to cut them to size and a 1/8" drill bit to drill all of the holes.
3. Two holes were drilled to allow the battery holders to be securely attached using #4-40 x 1/4" pan head screws and nuts.
4. Two additional holes were drilled to allow the bracket to be attached to the base of the rotating part of the system. Once these last two holes were drilled, the brackets were used to mark where holes need to be drilled in the base.
5. A 1/8" drill bit was then used to drill the holes in the base and, like before, a 1/4" drill bit was used to allow the screws to be counter-sunk.

Figure 32 shows the brackets. In addition, images further down in this System Build Up section show the mounting holes in the base and position of the battery holders.

8. The Power Wiring Harness
1. Next, we need to connect the positive (i.e. red) wire of one battery holder to the ground (i.e. black) wire of the other battery holder to put all four AA batteries in series. This will give us the ~6-V input system voltage.
2. The remaining red wire is connected to one input leg of the toggle switch. The center leg of the toggle switch and the black wire from the other battery holder are then used for the power connector. Figure 33 shows the completed power wiring harness.


Figure 33: Battery Holder / Toggle Switch Wiring Harness

3. Finally, we need to create the jumper wires (Figure 34) to connect the two PING))) sensors and the Proximity Sensor PCB to the Main System PCB. These are each 3-wire cables roughly 2.5 – 3 inches long.
   1. TE Connectivity wire-to-board connectors (Mouser PN: 571-3-644564-3) were used to create these cables as they mate to the TE Connectivity 3 pin headers (Mouser PN: 571-6404563) that were used on the two PCBs.
   2. The PING))) sensors have a 3-pin 0.1” male header, which the TE Connectivity wire-to-board connectors (Mouser PN: 571-3-644564-3) will also mate to. These standard 0.1” male headers do not have a friction locking mechanism, but in this application we do not need it.


Figure 34: Battery Holder / Toggle Switch Wiring Harness

4. Before the system was built up further, the housing was sanded, primed, and painted. This is completely optional and you can customize it to your taste. However, if you want to paint your housing, this is a good point in the build process to do it.
↑ Top





Previous
Read the Hardware Design

Next
Read about the System Build Up
Close Project

We would love to hear what you think about this project; please tell us in the comments section below.

Log In below to leave a comment.