The Manta Ray – Part 1

This robot platform is easy to build, inexpensive and flexible enough to support a number of options for the control board, motor drive, power and sensors. It also looks really cool and moves very well. With so many options available, this could very well be your next robot platform! This is part 1 of 2 of this article. In the first part we’ll assemble the hardware and discuss options. In part 2 we’ll actually provide some example applications for some of the accessories, such as line-following and obstacle avoidance.

History and Evolution

Parallax Inc. where I work as an Engineering Tech, used to sell a robot platform called the Stingray (Figure 1). Vern Graner did an excellent article on the Stingray in the December 2009 issue of Nuts and Volts. Many of you may remember this robot which had an aluminum chassis, motor mount and sensor plates and all kinds of flexibility and options.

Eventually the Stingray was discontinued, but had a large group of fans that would have liked to see the Stingray evolve and continue on. As fate would have it, that did not happen. My personal belief is that this was because the Stingray Robot, which was originally conceived by me, went from a personal project to a committee project and where there’s too many cooks in the kitchen, well, you know how it goes. The original concept for the Stingray robot came from the robot used in this GPS Navigation article I wrote. Originally codenamed the mid-size robot, the idea was to design a robot that was smaller than the Arlo Robot, but larger and faster than a BoE-Bot. I originally wanted a skid-steering robot, but the powers that be wanted a tail-dragger.

Fresh Ideas

At some point one of the former Tech Support guys at Parallax came up with a similar platform which used a caster wheel instead of the Omni-Wheel used by the Stingray. This prototype platform was originally made from aluminum. Kenneth Glass, the CNC Machinist at Parallax was talking to me about this platform and a version he made by laser-cutting 1/4” acrylic. I took a look at his prototype and saw an immediate resemblance to the Stingray robot, as well as an opportunity to try and get it right this time.

This beta prototype (Figure 2) has been marked up for changes that I wanted to make to the design. As you can see it used the older motors from the Stingray and even had a BASIC Stamp controller board.

Revising the Prototype

I saw some potential in Kenneth’s prototype. The overall design looked really cool; it just needed to be practical as well. Originally there were slots in the top deck, presumably for adjustable sensors. These were removed and the position of the two outer PING))) sensors was changed. Holes were added in the bottom deck to handle an H-Bridge driver, line follower module and Lithium Ion power board from Parallax. A new top and bottom deck were laser cut from the available DXF file and the result is shown below (Figure 3).

In discussing what to call this new platform we decided on The Manta Ray. It has a similar look to the Stingray, but has its own personality. I heard the name Bat-Bot was considered and ultimately rejected.

Build Options

The Manta Ray platform was designed to be flexible in the sense that you can load the DXF file into a CAD program and make changes to the existing features, remove them or add your own. The file includes an upper and lower deck; however, it is possible to build a fully working robot with just the upper deck.

That said, the current file is set up to use readily available parts from Parallax such as the Motor, Bracket & Wheel Kit (#570-00080), Caster Wheel Kit (#28961), HB-25 Motor Controller w/Mounting Bracket (#910-29144), PING))) Protector Stand (#725-28015), PING))) Ultrasonic Distance Sensor (#28015), Infrared Line Follower Kit (#28034) and Li-ion Power Pack Full Kit (#28989). The control board mounting holes are compatible with most BASIC Stamp and Propeller chip boards.

Assembly Time

The accessories used have their own individual instructions for assembly so we’re not going to get into that here, but rather focus on how to attach each accessory to the Manta Ray. First we’ll start with the Motor, Bracket & Wheel Kit. The kit is shown here unassembled and assembled (Figure 4a / Figure 4b).

The kit does not come with the screws needed to mount to your particular chassis and the length required can vary with your chassis. In this case I used 3/4” x #8-32 screws and #8-32 nuts as shown in Figure 5.

Next, we’ll install the Caster Wheel Kit. It’s worth noting that when the prototype platform was designed, the Caster Wheel Kit Rev A was the same height as the Motor, Bracket & Wheel Kit. The Caster Wheel Kit was recently revised to make adjustments for height on the Arlo Robot Platform and is now slightly taller than the Motor, Bracket & Wheel Kit causing the robot to tip ever so slightly forward. You could install spacers between the brackets and the upper deck to even things out, but it doesn’t seem to be a problem. Here is a side-view with the caster wheel attached.

The caster wheel kit comes with screws designed for the Eddie / Arlo platform, so I used three (3) 5/8” x #6-32 flathead screws to secure the caster wheel to the upper deck. As you can see in Figure 6, there is a slight tilt forward due to the height difference between the two wheel kits.

We’ll wrap up the build of the upper deck by installing our control board (Figure 7). For the assembly we’ll install a Propeller Project Board, which we can add servo connectors and other parts to later once we define the sensors and accessories we’ll be connecting. To install the Propeller Project Board to the upper deck we’ll use four (4) 3/4” standoffs with #4-40 threading and use four (4) 3/8” x #4-40 screws to secure the standoffs to the upper deck and four (4) 1/4” x #4-40 screws to secure the Propeller Project Board to the standoffs. You can always use whatever is in your parts bin if it works.

The Lower Deck

We’ll start the lower deck assembly by installing the standoffs that secure the lower deck to the upper deck (Figure 8). This also helps to visualize available space on the lower deck. The standoffs used are 2” with #4-40 threading and use 1/2” x #4-40 screws on both ends.

The HB-25 Motor Controllers are attached using two (2) 1/2” x #6-32 panhead screws each as shown in Figure 9.

Next, we will install the Li-ion Power Pack using four (4) 3/4” standoffs with #4-40 threading and use four (4) 3/8” x #4-40 screws to secure the standoffs to the lower deck and four (4) 1/4” x #4-40 screws to secure the power pack to the standoffs (Figure 10).

Sensing the Environment

Okay, now it’s time to add some sensors. The Manta Ray currently has options for three PING))) Protector Stands and an Infrared Line Follower Kit. We can always add more mounting options later or modify the options we have, but for now we’ll mount the sensors the platform is set up for to give you an idea of how everything will look mounted. We’ll start with the line follower (Figure 11) since that will be easier to mount without the PING))) Protector Stand in the way. The mounting hardware that comes with the line follower kit can be used with the Manta Ray platform. Just use the 1/2” spacers with the 7/8” x #4-40 screws and the #4-40 nuts. The 1” standoffs and 1/4” x #4-40 screws won’t be used. Note that on this release of the Manta Ray DXF file there are several sets of mounting holes for the Infrared Line Following Kit. This is so you can adjust the position of the line follower underneath and account for how the robot turns. Since we haven’t yet developed the final line following code we’ve allowed some room for adjustment until it has been finalized. At this point the extra mounting holes may be removed.

Finally, we can install the three PING))) sensors on the protector stands. The protector stands come with the hardware needed to mount the PING))) sensors to them. It is important to have the nylon washer between the screw head and the back of the PING))) sensor. See Figure 12.

The hardware needed to mount the stands to the Manta Ray platform is not included so I used six (6) 1/2” x #4-40 panhead screws to mount the protector stands to the lower deck (Figure 13). The holes are such that you do not need a nut. You should be able to thread the screw right into the acrylic. However if you prefer using a screw and nut you can use3/4” screws and place a nut on the underside of the lower deck where the screw comes through.

Bringing It All Together

It’s finally time to attach the lower deck to the upper deck. All that is required is the six (6) 1/2” x #4-40 screws that go to the 2” standoffs from earlier. Once the lower deck is attached you have approximately 1-5/8” of clearance for the lower deck and approximately 5/8” clearance for the line follower sensor if you use it. Figure 14 and Figure 15 show the completed assembly.

Wrapping Things Up

No wiring was done during this build. Before the second part of this article I will be defining which sensors will have code examples and for each application I will include a schematic and description to accompany the code. For now you have time to download the DXF file, make any adjustments and have it cut for you. You also have time to obtain the accessories and options you need to assemble the robot as well as any hardware you don’t have.

I had all the hardware not provided in my parts cabinets. If you don’t, any hardware store or mail order source like McMaster-Carr would have the hardware you need. Stay tuned and we’ll see you next time for some real-world applications with various sensors. If you would like to ask questions or need any additional help please visit the project page listed in the resources and I would be more than happy to help you any way that I can.

Resources

The Manta Ray – Part 2

Discuss this project on Savage///Chats

This project was published in the June 2014 issue of Servo Magazine