Selecting a Motor

As I mentioned previously, I need to find a motor manufacturer that posts more than a picture of their product on an electric bike. I need torque curves and CAD models. I am surprised at how few of these manufacturers exist, or at least have websites that I can find. Maybe I just don’t know how to search for them.

Part of the problem is that I’m looking for a motor that has power output on the level of a small riding lawn mower. Finding smaller BLDC motors has been pretty easy, but finding something that operates efficiently at lawn mower speeds while still outputting decent torque has been challenging. Also, it has to cost no more than a few hundred dollars, because I’m cheap and I need some money left over for an RTK GPS module down the road.

So I’m starting out from behind the 8-ball to begin with. I wasn’t sure such a motor existed until I found the folks at Golden Motor. These guys are a little sketchy, and they need to hire a web designer from the 21st century. But they appear to have a wide selection of motors in some pretty epic sizes, with fairly comprehensive performance data, too.

But before we go looking at individual motors, let’s review our requirements.

Motor Requirements

At the moment I am leaning toward a single mower blade design. I’ve learned the hard way that complexity is not your friend, and simple designs are generally more robust.

To maximize the deck width, I am considering a 30in long blade. A 30in blade needs to rotate no faster than 2400RPM to be safe. Riding mowers with one single 30in cutting blade are equipped with engines that output about 10ft-lbf of torque.

The plan currently is to have four 12V 35Ah sealed lead acid batteries wired in series to power the motor. So it must be rated for 48V.

Evaluating the BLDC-108 Motor

This is a 1500W motor that Golden Motor makes. It’s also the only one that doesn’t have  a nice chart for all the relevant performance parameters, but they did give an excel spreadsheet with several data points. I graphed the data and here’s what it looks like.

BLDC-108 speed torque power
The speed and power versus torque curves for the BLDC-108 motor. The blue curve is speed and the red curve is power. Excel only lets you plot two separate y-axes, which is very frustrating. Get it together Microsoft, this is 2018!

The efficiency of the motor is greater than 85% between 1 to 3.5ft-lbf torque with a maximum efficiency of 88% at 3325RPM and 2.6ft-lbf torque. The motor consumes about 40A of current at the torque shown on the chart.

To achieve the maximum safe blade rotation speed of 2400RPM, we need to use two pulleys of different diameters on the motor shaft and the blade spindle. Taking the maximum speed the motor operates at of 4072RPM and dividing it by 2400RPM gives us the pulley ratio we’ll need, which in this case is 1.7.

If we use the motor with a pulley ratio of 1.7, we could obtain 5.9ft-lbf of torque at 1800RPM. Reducing the speed increases the torque.

Some thoughts about this motor:

  1. The torque is a little on the low side, even after you factor in the pulley ratio. I suppose you could go with a higher pulley ratio, but that reduces your speed even further, which I’m told affects the quality of your grass cut.
  2. I like that the current draw is only 40A. With four 35Ah batteries, you could in theory get 3.5 hours of operating time, neglecting current consumption from other electronics and motors. I like that number.
  3. The picture on the website is nothing like the CAD model you can download. So that’s something I’ll need to investigate if this is the motor I choose.

The motor is priced at $142 with $60 for shipping. BLDC motors require a separate motor controller, so that will be another $95 with $30 for shipping. So a system built around this motor will cost a total of $327. Not horrible. I wish I knew why the shipping was so high…

Next time I’ll evaluate the 3000W HPM3000B motor.

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.

Getting the Wheels to Turn the Right Way

One of the painful experiences I had on rover V1 was getting the motors to turn the correct way. It sounds simple enough, but there are several different variables that control motor rotation direction and behavior:

  1. Motor wiring polarity. How you connect the two wire leads on the motor into the Sabertooth motor controller affects rotation direction.
  2. DIP switches on the Sabertooth motor controller. There are 6 on-off switches on the Sabertooth that determines how it interprets RC input from the Pixhawk.
  3. Servo output from the Pixhawk. There are 8 servo outputs on the Pixhawk. These must be assigned to the correct output function.
  4. PPM encoder wiring. My RC receiver has 6 RC outputs, but there is only one RC input on the Pixhawk. I use a PPM encoder to mix the six channels into one. Depending on the wiring configuration into the PPM encoder affects which RC channel gets associated with yaw, roll, pitch, etc.
  5. Mission Planner servo reversing. You can reverse the RC input in Mission Planner.
  6. Skid Steer settings. This rover turns by varying the rotation speed of the left and right motors relative to each other. There are a settings related to this behavior.
  7. RC mapping. Which stick on the RC transmitter should control throttle or steering? There’s settings for that, too.

Pretty straightforward, right? This post is for my own personal recollection of this process in case I lose my settings or have to rebuild the rover for some reason. But hopefully I can also shed some light on how to wade through this mess of options and get your rover behaving as desired.

Step 1: Put your rover up on chocks

Seriously, get those wheels off the ground. We’re doing this exercise because the wheels don’t turn the right way. In the wise words of Robby, “your shins will thank you“.

Step 2: Preliminary Wiring

You gotta start somewhere, so don’t worry too much about getting all the wires set up correctly on the first try. Something is going to be backwards or configured incorrectly in Mission Planner. Here’s my checklist for preliminary wiring:

  1. RC receiver is plugged into the RC slot of the Pixhawk servo rail.
  2. Channels 1 and 3 of the Pixhawk output have servo wire running to the RC input terminal block on the Sabertooth (these are the defaults in Mission Planner).
  3. Motors are wired into the motor output terminals on the Sabertooth.
  4. Batteries and BEC are all wired appropriately.

Double check your wiring to make sure your battery or BEC is wired properly and with the rover on chocks, turn on the power.

Step 3: Sabertooth DIP switches

Dimension Engineering has an excellent wizard to help you get your DIP switches configured properly. You’ll obviously need to select what’s appropriate for your situation, but for mine I selected the following:

  1. Lead acid battery chemistry
  2. Simulated RC signal
  3. Independent mode
  4. Exponential control

Regarding (4), you could probably do linear control too, this is a preferential thing I think. The rest of these settings are mandatory as far as I can tell.

The bottom line here is that we want the Pixhawk to mix the steering and throttle signals for skid steering, not the Sabertooth. I’m pretty sure this is why (3) above is set to independent mode. If it wasn’t, you’d have the Pixhawk mixing the signals, and then the Sabertooth mixing them again. Quite the mess, I’d imagine. Probably one of the many things that went wrong on rover V1.

This should result in the following DIP switch configuration:

Sabertooth DIP Switch Settings
The DIP switch settings I used on the Sabertooth 2X60. Switches 2 and 3 are on, the rest are off.

Step 4: Ensure Your Motors Are Responsive

Flip on the RC transmitter, arm the Pixhawk with the push button switch, and move the sticks around. The motors should at least respond to RC input on the transmitter, even if it seems completely jacked up. If not, go back to step 2 and check your wiring. Motors kicking straight into high gear with the sticks in the neutral position shouldn’t be unexpected at this point. This is also why step 1 is very important.

Step 5: Tweak Some Mission Planner Settings

According to the Ardurover documentation, you’ll want to set the following parameters in Mission Planner:

  1. SERVO1_FUNCTION = 73 (This sets servo 1 output to left motor throttle)
  2. SERVO3_FUNCTION = 74 (This sets servo 3 output to right motor throttle)

In recent versions of Ardurover I think this is all you need to do to enable skid steering. As always, refer to the most recent documentation.

Step 6: Get your RC transmitter mapped correctly

Decide how you want your RC transmitter to control your rover. I have an airplane style transmitter and so I wanted steering and throttle both on the right hand stick. There were a few reasons for this:

  1. When you release the stick it returns to the center neutral position, which will stop the rover.
  2. You can go forward and reverse on this stick. The left stick zero position is all the way down as it’s supposed to be throttle for an airplane, so there’s no way to go in reverse if you assign throttle to this stick.
  3. I can control the rover with one hand and use my left hand for something else.

You’ll want to assign throttle and steering to your desired stick motions in Mission Planner using the RC mapping parameters. In my case you’d go with the Ardurover default:

  1. RCMAP_ROLL = 1
  2. RCMAP_PITCH = 3

This sets your roll and pitch transmitter signals to your the appropriate Pixhawk output.

Step 7: For PPM Encoder Users…

So you cheaped out when buying your RC transmitter? You’re among good company. Your setup will probably look something like this:

ppm
The RC receiver is at the bottom. The PPM encoder as at the top.

PPM encoder users have another layer where they can improperly assign RC channels. Roll might be channel 1 on the transmitter and properly assigned on the servo output of the Pixhawk, but PPM encoder users will need to make sure that Mission Planner shows the Pixhawk as interpreting roll as channel 1.

Pull up the radio RC transmitter configuration page under Initial Setup in Mission Planner and verify that the RC mapping is correct. If it isn’t, you’ll need to fiddle with the way the PPM encoder is wired so that the channels map across.

Step 8: Play With Settings

Now you’ll want to watch the motors as you move the RC transmitter sticks. Forward should obviously cause both motors to rotate in the same direction. Right should cause the motors to counter rotate; same thing with moving the stick to the left. Make sure you check all stick positions so that there’s no funky-ness with the full range of motion.

Are the motors still doing something strange? Now is when you can adjust the motor wire lead polarity. Hopefully you can identify one motor that is behaving opposite how it should. If this is the case, flip the motor wires on that motor and see how this affects things.

Step 9: Document Everything

Did you get it working? Save those parameters. And take pictures of everything, the Sabertooth DIP switch settings, the wire runs, the PPM wiring. Everything. Better yet, take a video of it while you explain verbally what you did to get it to work. You’ll thank yourself later.

Still having trouble? Did you reverse any servos? I thought that servo reversing would be a good idea to get things to work, but I really advise against it. You might get things configured to where your rover does run correctly in manual mode, but when it comes time to throw it in auto, that servo reversing can make things very weird. best to switch wires around instead in my opinion. Get everything configured in the hardware and leave the software at the default settings.

One last word about Mission Planner settings: don’t push buttons. Normally I like to push buttons until something works, but Ardupilot is one of those situations where it really doesn’t help and usually makes things worse.

There are literally hundreds of parameters to play with, and even if all of the settings I’ve described here are correct, there could be a phantom parameter somewhere you tweaked that is messing with your setup. If you didn’t heed this advice you may be better off resetting the parameters to their default.

Step 10: Field Testing!

But that’s for another post. Stay tuned!

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.

img_3764-1-e1533090942883.jpg
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.

IMG_3767
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…