Small Design Changes

mower 1-6-19
The robot mower design as of this weekend. Some wiring is completed.

I’ve decided to change a few things on the robot mower design. I have a tendency to get hung up on really small things by spending way too much time thinking about them. The deck height adjustment was turning into one of those things.

I’ve changed the design to include four simple clevis pins. They’re $0.50 a piece. You don’t need to adjust the deck height that often, and spending $100 on parts to do that in a fancy way is a waste of effort and money.

I modeled up a discharge chute because I think the mower will need one to look professional. I intended to 3D print it, but it turns out the dimensions are big enough that most hobby websites won’t take it. I know of a place nearby that does industrial 3D printing and they quoted me $290 for the part out of ABS. Yikes. Maybe I’ll try to adapt an off the shelf chute instead. That’ll be more reverse engineering but it will probably work better. And cost a few hundred less.

xt60
Some simple XT60 connectors for the motors. They’re rated for 60A and should be appropriate for this application.

I’ve started modeling up some wires and coming up with a way to neatly connect the motors was more challenging than I thought it would be. I found some “wall mount” XT60 connectors that will hopefully will work well. I’ll have some mounting plates NC machined and then attach the XT60 wall mounts to the plate.

Progress

combine
The robot lawn mower design.

I’ve started doing some wiring diagrams for the robot lawn mower, as the mechanical portion is fairly well defined. However, there are a few things I’m still not sure about:

  1. Will four 12V 35Ah SLA batteries will provide the energy needed to run the mower?
  2. Should the battery bays be replaced with some off the shelf enclosures?
  3. Is there a better way to do the deck height adjustment mechanism?
  4. Are the motors sized appropriately, both for speed and torque?
  5. Is a pulley quick disconnect bushing really the best way to attach the cutting blade to the motor shaft?
  6. The sheet metal used on the weldments is 0.1875in thick aluminum. That is expensive and probably too stout. It should be changed to 0.125in thick.

I’m chasing my tail with these questions above, so I will take some time off from modeling the mechanical side of the mower and work on wiring for a while. Hopefully things will be more clear when I revisit these issues later.

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.

IMG_3956
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.

IMG_3874
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.

IMG_3763 (1)
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…