Concept #5

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A design concept I’ve been working on lately.

The nice thing about CAD software is you can see how different ideas play out before you spend a fortune to discover they don’t work. The past month or so I’ve been drawing up robot mower designs with little luck. The model above is the only one I feel even moderately good about.

Most of the concepts I’ve put together so far feature a sheet metal chassis shaped around my batteries and motors. The upside with these previous designs is that it allows for the smallest vehicle footprint. The downside is that they involve a lot of welding and waterjet parts, and it’s difficult to come up with a simple mechanism to adjust the deck height because the motors mount directly to the chassis.

This new concept has the mower deck hanging beneath the robot frame. Separating the deck from the robot frame allows both to be simplified greatly. To adjust the deck height I plan on putting a few turnbuckles between the robot frame and the mower deck. An added bonus is that I can disassemble it and throw the robot in the trunk of my small Hyundai sedan for field testing.

One potential downside I anticipate with the design above is that it will be top heavy. The four batteries have to sit on top of the robot frame. We can box them in and secure the to the frame, but I’m not sure what the vehicle center of gravity will look. Guess we’ll have to do some more modeling to find out.

The Emergency Stop Switch

This weekend I had some time to install my emergency stop switch. At first I thought I would keep things simple and just mount the emergency stop switch on top of the control enclosure and route one of the battery wires straight through the safety switch. Sounds simple, right? This method, however, presents two big issues:

  1. Hitting the emergency stop switch shuts power off to the entire rover.
  2. A high current carrying wire has to run through the control enclosure.

The first item above is an issue because we want to be able to communicate an emergency shut down state to the ground control laptop. If all the power is shut off to the rover, a power failure and an emergency shut down will appear identical from the ground control’s perspective.

The second item is an issue because it defeats the purpose of separating the power and control electronics. The constraint here is that the emergency stop switch has to be mounted on top of the control enclosure so it is visible, accessible, and in a safe location, but we can’t have a high power wire running through it. We want to keep those noisy high current wires away from sensitive electronics.

The solution? A relay! Or more specifically, a set of relays. We’ll have the emergency switch trigger a relay that cuts power to the drive motors only in an emergency state. We’ll also keep all the high current wires contained to the power enclosure.

Safety System
The wiring diagram for the emergency stop system.

The FIT0156 emergency stop switch has two integral switches triggered by the big red button. One is NC and the other is NO. Our system uses the NC switch. When the button is pressed, the switch opens and prevents 5V from flowing to the CH1 and CH2 pins on the relay module. This opens the motor circuit, immobilizing the rover.

The IM120525001 2 channel relay was only $3, but I wasn’t sure if it would be large enough to conduct the current needed by the motors. I decided to take a gamble, and I’m glad I did. The module works very well. The spec sheets say it can conduct up to 30A at 24VDC. The only drawback is that the screw terminals on the module aren’t big enough for 14 gage wire. I had to use 16 gage wire instead.

I measured current flow between the 5V output on the Mauch BEC and VCC on the IM120525001 module and my ammeter said it consumes 170mA, a little bit high for my liking, but manageable. The Mauch BEC is rated for 3A, so it shouldn’t be a big issue.

I oversized both enclosures knowing there would be additional things I add later, and I am glad that I did. The emergency stop switch and relay module both fit nicely in my enclosures.

Relay
The power enclosure with the relay module mounted nicely in the lower right. Both of the motors have one wire routed through the relay module before connecting to the Sabertooth motor controller.

I used one of my remaining 8 pins on the DB15 breakout board to route the emergency stop switch down to the power box. This wire goes to both the CH1 and Ch2 pins on the relay module. When you hit the button both motor circuits are opened.

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The big red button installed on the rover. Be sure to include large, clear signage stating that this is how you shut it off!

So the total cost of our emergency system:

  1. Emergency stop switch, $6
  2. Relay module, $3
  3. Emergency stop sticker, $2
  4. Miscellaneous wire, $1

Tack on a few dollars for shipping and sales tax for those items and you’re still easily under $20. Not too shabby.

The Control Enclosure

The control enclosure went together even faster than the power enclosure. I intended to use adhesive backed Velcro but had a handful of command strips laying around and decided to use those instead.

There weren’t any holes that had to be drilled in the metal plate inside the enclosure, so I pretty much eyeballed the location of everything, slapped a command strip down to place everything.

Ardupilot recently started supporting GPS blending and I had an old NEO 6M GPS receiver on hand and thought it would be worth trying out. I had to re-crimp the serial connector from a 4 pin DF13 to a 5 pin because the 6M was for an APM module I had purchased a while back. We’ll see how it turns out.

Finished Control Enclosure
The two GPS receivers mounted on top. The one on the right is a U-Blox NEO M8N, the left is a NEO 6M.

To mount the telemetry radio, I made sure to drill a hole in the thinner portion of the Polycase enclosure wall. These enclosures are injection molded (I think) and the walls are tapered so they can be removed from the mold easily. I mounted the radio by sandwiching the wall with the radio and antenna.

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The control enclosure.

All that’s left is to put everything together on the wheel chair now.

The Power Enclosure

Drawing wires in my CAD model was difficult work, but the effort paid off in spades when it came time to start wiring my enclosures. Doing this provided a huge advantage: I knew exactly how much wire I needed, not just the size, but down to the length, insulation color, even how much to strip off the ends.

Because of this, putting the power electronics enclosure together went quickly. I think I spent more time stripping and crimping wires than I did assembling components in the box. Below is a picture part way through construction.

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The power electronics components. Things are starting to take shape.

I missed the fact that the Mauch current sensor only comes with 10cm long power wire leads. Because of this I had to use one of my 10 gage Posi-Locks to splice another wire so it could reach the toggle switch. Even a well thought out CAD model won’t prevent every mistake.

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The completed power electronics enclosure mounted on the wheelchair chassis.

I was able to salvage quite a few bits and pieces from the old wheelchair wiring harness like the battery terminal boots and some nice pieces of 10 gage wire. I also ended up using the 16 gage Posi-Locks to connect to the motors. The wires were smaller than I modeled them.

I was a little bit paranoid about getting all of my connections right, so before I connected the wires to the battery terminals, I did a lot of bench testing with my multimeter. Having modeled up my wiring in the CAD program I had a good idea of what things should have continuity between them and what things shouldn’t. Once I was satisfied that everything was wired up properly I connected the battery terminal wires.

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The wiring to the battery terminals and motors. The Posi-Locks make for an excellent connection, but you have to strip the wires to just the right length.

With the wires connected and in place, I put some zip ties around them to clean things up. There were some convenient holes in the wheelchair chassis that I used to secure them to.

At this point, all that was left was to make sure the Sabertooth and Mauch BEC power up properly. So I flipped the switch up, and to my surprise everything seemed to work.

The Sabertooth lit up and the fan came on briefly. The Sabertooth BEC measured 5V as it should. A blue LED on the Mauch current sensor came on. The voltage across the Mauch BEC measured 5.30V on the money.

With the power enclosure complete, all that’s left is the control electronics enclosure…

Paper Plots: You Don’t Need an NC Machine

Polycase (the guys I bought my enclosures from) advertises their epic machining capabilities on their website. They sent me a sample free of charge and I was quite impressed. So before I ordered my enclosures, I used their quote wizard to see what it would cost to machine the enclosure and the mounting panel.

The numbers I got back were astronomical, partly because I was only buying two enclosures, both with different cutouts. The cost shoots up fast for each additional side of the enclosure you need to machine. I had four sides I needed machined on one box, so the quote came back in the neighborhood of $200. For $200, I can afford to drill my own holes and make my own cutouts.

But how do you make sure you drill everything in the right place? The answer: paper plots!

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The paper plot for the power electronics enclosure mounting panel.

If you drew your parts up in a CAD program and made them dimensionally accurate, you can make a paper plot in no time flat. Even if you didn’t, you can draw up something in a 2D CAD program and make one using a similar process:

  1. Make a drawing of your part that contains the features you need to cut.
  2. Include a feature of known length. In my paper plot I put a scale in the corner, but you could use a feature on the part if you know it’s length.
  3. Scale the drawing 1:1.
  4. Print it off.
  5. Physically measure the feature mentioned in (2) and ensure it measures accurately.
  6. Trim the paper plot and tape it to your part, positioning it appropriately.
  7. Use the paper plot to establish your hole locations!

It’s really as simple as that. I held the panel piece over my paper plot and aligned the edges with the lines on the paper. I recommend lots of tape because you don’t want things moving, especially after you’ve drilled a few holes.

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Use lots of tape: you don’t want the paper plot moving after you’ve drilled half your holes!

I recommend overlaying your parts with holes on the paper plot to make sure they actually align the way you expect. The DXF file for the current sensor mount did not align with the actual part I received. Measure twice, cut once is good advice to live by.

Pro-tip: add cross hairs on every hole feature you plan on drilling. This will give you something to align your drill bit to. Another good idea is to actually put a diameter dimension on your holes, so you use the right drill bit size. I had to go back and check my CAD model to see what size I needed.

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The paper plot for one of the enclosure sides.

You can use this process for just about anything. I also made paper plots for the enclosure itself. I could have used a paper plot to drill holes into the wheel chair frame, but if you have the mating part on hand, it’s better to use that, especially for items whose fit is critical.

Machinists: I’m Sorry

I have a confession to make. I’ve been making the machinists I work with drill some large hole diameters through some pretty thick steel. I didn’t think it was a big deal at the time, but I now know better. It’s kind of painful. Literally.

For mounting the power electronics box to the wheel chair chassis, I had to drill two 0.25in diameter holes through a piece of 1.5in X 1.5in steel tube. No big deal, I’ll just start out with a 0.125in drill bit and go from there.

Generally that wouldn’t be too bad of an idea, if it was actually a steel tube. The ends were plugged with some kind of threaded stud to mount the foot rest for the wheel chair. As I started drilling through the tube I quickly realized that this was actually a solid piece of bar stock.

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The first 0.125in diameter pilot hole. It only took 30 minutes!

To make matters worse, I really didn’t have any sharp drill bits, just some old ones I bought at a garage sale years ago. It helps to have sharp bits if you’re going to drill through 1.5in thick steel.

I clamped the mounting plate to the chassis and started my first pilot hole using the plate as a template. Once I had a nice dimple in the frame I removed the plate and kept drilling.

About 5 minutes in my arms were killing me. The wheelchair is pretty heavy, but I still had to brace it with my left arm to keep it from rolling off the blocks that raised it to where I could get the drill on it. And my right arm started aching from pushing the drill into the wheel chair.

I knew I should drill slowly: too many RPMs and you will heat up the drill bit and dull the cutting edge. But even going slowly I wasn’t making much progress. I tried not to apply too much pressure to the drill either, but without it, it wouldn’t make any chips at all.

I did some reading and discovered that using some cutting fluid can allow you to drill at higher RPMs, so I borrowed some non-stick vegetable oil spray from the pantry and sprayed the bit and the hole with it. That seemed to help, but not much.

In the end it took about an hour for each of the two holes, starting with a 0.125in bit, moving to a 0.1875in bit and finishing with a 0.25in bit. I can’t tell you how satisfying it felt when the drill lurched forward as it broke through the back side of the bar stock.

So machinists everywhere: I’m sorry for all the holes I’ve made you drill in steel plate. I will at least have a very good reason the next time I ask you to do it. I understand (in a small way) your pain.