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Step 4: Controlling the Outrunner: Sensorless or Sensored?

Unlike a regular old DC motor, brushless motors require an electronic controller to commutate the motor. The methods by which they do this differ, but it commonly boils down to whether or not the motor has Hall Effect sensors placed strategically in the windings to sense the magnet rotor's position so the controller knows which coils to switch. Other strategies include position encoder wheels, but the Hall sensor is by far the most prevalent in small motors. 

The vast majority of model motors are sensorless

That's the first rule to keep in mind. Because the load experienced by an aircraft increases predictably with speed, and is zero at zero speed, pretty much all aircraft motors (especially outrunners) are just spinning rings of magnets on a stick. That is why they can be made so cheaply. Industrial motors for general motion control like servo systems and robots and whatnot need to deal with constantly varying and transient loads, so they therefore have some kind of feedback built in already.

Usually, the only model motors that come with sensors built in are small R/C car and truck motors like this .

Sensored control, the fundamental method

In a sensored motor (and associated controller), there are generally three sensors positioned at 120 or 60 electrical degrees apart in the motor that output a 3-bit Gray-like position code. The placement of these sensors depends on the winding configuration, number of slots, and number of magnet poles. For more information on how and why this works, see my hub motor instructable .

The advantage of sensored commutation is that the controller always knows where the motor is. Therefore, sensored motors usually have low-speed and stall characteristics more like a classic DC motor. Sensored commutation has its downsides - sensor failure or glitchiness could mean the motor controller stops running the motor. Most inexpensive industrial and commercial controllers, even EV controllers, are just governed by a state table whose input is the 3 Hall sensors, and so sensor failure would mean an erroneous output and non-running motor.  There is also the issue of finding the correct combination of Hall sensor leads and motor phase outputs - up to 12 possible ways of matching them together, if your motor doesn't come with a controller.

Finicky details aside, sensored commutation is the way to achieve consistent stall- and low-speed behavior, and "real" vehicle controllers are usually sensor-commutated.

Sensorless control, i.e. R/C speed controllers

A sensorless controller has to detect the position of the rotor through some other means. The most common method is to randomly bump the motor (drive two phases) and observe the back-EMF, or generated voltage, profile on the third phase. The slope of the BEMF dictates which direction the motor moved in, and therefore which coils to switch next. Sensorless commutation uses more sophisticated position state estimators which have to have some speed feedback to "pick up". In other words, sensorless motors can't technically move without being already moving. This clearly poses a quandary for vehicles and other inertial loads. If the initial "bump" is not strong enough, the motor will not move enough to generate a meaningful voltage pulse.

For viscous loads like a propeller (load proportional to speed), this is not a problem at all. Therefore, the vast majority of R/C controllers are sensorless.

Sensored ones do exist - again, for cars and trucks, which as you might guess, are just like tiny versions of EVs you personally ride, an inertial load. Sensorless controls for cars and trucks are also commonplace, since modern motors are powerful enough that a pulse on the phases is enough to move a small model.

The biggest implication of sensorless control is the dubious availability of stall torque. This makes the torque equation presented two pages ago a little misleading. You will generally not be able to stand on a vehicle and challenge someone to a drag race. Sensorless vehicles, especially those using R/C parts, will need to be "kicked off" or push started .

Sensor Augmentation

The only real difference between a sensored motor an a sensorless motor is... sensors . You can actually take any common R/C outrunner and add Hall Effect sensors to use them with EV traction controllers. Numerous ways exist to append sensor feedback onto your motor: I cover installing sensors within the windings (inside the motor) on my hub motor Instructable; and here are twoways people have used externally mounted sensors. You would end up with an additional 5 wire harness coming out of the motor, comprising three Hall sensor outputs, logic power, and logic ground.

Hall sensors are typically "open collector" i.e. they can only sink current. Controllers have internal pull-up resistors built in, so there's generally no need to internally pull the Hall sensor output to the logic rail. However, power supply decoupling capacitors placed right at the Hall sensor leads seem to be helpful. The Hall cable should not be routed parallel or immediately next to the phase conductors, since the high switching currents in the phase wires can cause induced noise problems with the sensor cable.

Update June 2013: Outrunner Hall Sensor appendages

I hate to plug myself usually, but in the past few months, I've successfully developed and am currently selling a line of Hall Sensor Boards and Hall Sensor Mounts sized to several of the typical outrunner suspects. Placed externally, they allow a sensored-only motor controller to drive these motors. See the boards and mounts on Equals Zero Designs. I designed these specifically as a "stock solution" for everyone asking me how to add sensors to your outrunners!
<p>This 4 year old instructable is STILL a treasure trove of useful relevant information. Thanks for sharing!</p>
A fun build! Thanks for the awesome instructable!
<p>Actually instructive. Though the term is common I had no idea what a brushless motor was or how it worked, and I am an electrical engineer, an old one. Sixty years ago I was wondering why the brushes of the dead motor that I was rewinding were not semiconductor devices. The transistor was then 10 years old. Life took me in other directions so the idea never matured - and suddenly I know. Thanks! Further I now understand something about the DC/AC drive of the Tesla Auto. My interest is, at the moment, in electric propulsion for a nearly finished Sampan sitting in my back yard. Though off the shelf devices are readily available I am hoping for something better. Your remarks about RC devices and propellers seem to apply. The battery info is also helpful. I am struggling with the tradeoff of a low HP gas outboard, at less than 40lb and a 20lb trolling motor with a 50lb 12 volt lead acid battery. You advice about how to purchase motors, batteries and controllers is most pertinent. - Boatmakertoo</p>
<p>Hi My gfs bike has a 12mm rear axle and she hates the 6kg front hub. Id like to use this motor to drive the back wheel. It looks very similar to my internal motor on a mac 10t geared hub motor ie lots of turns. Could I not just use the infineon sensorless controller? http://www.hobbyking.com/hobbyking/store/__54888__9235_100KV_Turnigy_Multistar_Brushless_Multi_Rotor_Motor.html</p><p>Anyway thanks for the article, Dan</p>
<p>very good article, a great help to all modellers!</p>
<p>Very good, enlightening article !</p>
<p>You Sir, are my hero.</p>
<p>Hi, your<br>article is sooo good!</p><p><a href="http://www.instructables.com/id/The-New-and-Improved-Brushless-Electric-Scooter-Po/all/?lang=es" rel="nofollow">http://www.instructables.com/id/The-New-and-Improv...</a></p><p>I created<br>some links to it at my site <a href="http://www.avdweb.nl" rel="nofollow"> www.avdweb.nl</a></p><p>You've put all<br>topics under each other so they are difficult to localize, it is sooo long now.<br>Can you create a table of contents on the top with links to the separate<br>topics?</p><p>Or create<br>separate items on instructables.</p><p>Kind regards,</p><p>Albert</p>
<p>It looks like the above named A123 ALM_12v7 drop in batteries are available from: <a href="http://www.buya123batteries.com/ALM_12V7_p/400520-201.htm" rel="nofollow">http://www.buya123batteries.com/ALM_12V7_p/400520-...</a></p><p>for $130.00</p>
What is the pinout for the standard, RC esc sensor connector?
Wonderful instructable. Have been giving it some thought for a bit and wondering why you haven't considered building your own ESC. From your descriptions a sensorless controller suitable for our purposes seems very doable, but not available within the R/C airplane realm. We need a custom, self-starting, sensorless ESC. Other's have made them. <br> <br>Once you have the basics together fine-tuning becomes possible. Seems like the perfect solution to a host of problems. Perhaps the springboard to designs otherwise out of reach.
Thanks for your instructable. It's very interesting and complete but I have a, may be, stupid question. I'm planning to put sensors in an outrunner as well and wanted to know how to tie the sensors to the ESC. There seems to be a standard 6 pin connector shared between the different ESC models. How do I wire the sensors to this connector?
This is a pretty sweet little scooter, might make one for my son if he starts behaving again. lol <br> <br>I am legitimately trying to find that gas motor which you showed but did not use. <br> <br>Can you tell me where you found it? I can only find electric models, which are great but not appropriate for my current project. <br> <br>Thanks, <br>-Grey
I found a good place to get thick very flexible 8 and 4 gauge wire is the local car stereo center. Th 8 gauge I used cost $1 per foot. They had 4 gauge but it was much more expensive.
Wow after reading this article now I'm building a scooter. <br>Bad influence I think.. <br>Welding and machining I get but the electrics are not so much my thing :( <br> <br>I bought 2 motors <br> <br>KA63-18L <br>Constant: 259Kv <br>Battery: 10Cell Lipo <br>Operating Current: 25-60A <br>Peak Current: 72A(15sec) <br> <br>Here is the problem.. Do I run one and have a spare or use them both? <br>If both what would be the best way? <br>
If you run both you need a controller for each. Now this may sound easy but you need to be careful to set up each esc the same and use the exact same model of esc and motor or one motor will do more work. Now unless you want to push more than 200 lbs around at 20 mph or more one motor should be sufficient as long as you use the gearing equation from this article as to not burn up your motor. Good luck and feel welcome to ask any questions you want to.
Hobbyking now sells 20C four cell 5 Ah lipo packs from the US warehouse for about $25 per pack: <br>http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18631
Could I maybe use a large pvc pipe to hold the hall effect sensors?
How exactly do you back install hall effect sensors in a motor? I am trying to make an electric motorcycle and want to use a kelly controller. I have a Turnigy 80-100-A 180 KV brushless outrunner (the same class that is on the scooter in this instructable) and have had a really hard time figuring out how to install sensors. I am also having a hard time figuring out how this motor is wound. <br>Any help would be greatly appreciated, thanks in advance.
It is extremely difficult to install *internal* sensors onto a mystery motor (of which you do not know the internal winding pattern). The C80 series is also very difficult to discern because they require so much current. <br><br>Usually, you would run current through two phase wires (&quot;line to line&quot;) and use a test magnet to observe which poles are polarized in which directions - they should always be alternating from tooth to tooth and there should always be 8 of them on in total. Using this and the process of elimination you can separate the 12 teeth of the stator into 3 groups of 4 teeth which you can call &quot;A&quot;, &quot;B&quot;, and &quot;C&quot;, which are associated with one of the three phase wires. Then, the Hall sensors are placed *between* 2 teeth of *different* phases.<br><br>Because the C80s have a very low resistance and few turns, you need either a ton of current or a sensitive magnetometer to determine the direction of the stator field. <br><br>Using external Hall sensors on the C80s is possibly the easiest solution. <br><br>
Thank you for the info, I have tried to run current through the different phases but I could not get enough current to get discernible results. I am thinking of just rewinding the motor so I know where the different phases are. Unfortunately I do not have access to anything to make an external mount for hall effect sensors. Do you know of a website that could make a mount for me? I also do not have any experience using CAD software. Another question I have is do all hall effect sensors have the same wiring scheme? The website I got mine from did not tell me which lead is which.
hi <br>a bit for the sake of accuracy: when you refer to &quot;convection&quot;, you actualy mean free convection(no air speed), as opposed to forced convection. <br>other than that petty note, its a great article and im learning why i shouldnt have bought the esc that i have...
Just wanted to say a quick thanks for this instructable. It's the detail all in the one place that i have been searching for, for years. Thanks for confirmation of R/C parts as well and the technical detail.
Can you use the sensored kelly controllers with a sensorless motor?
Not the KBS - Kelly does have the KSL line which is sensorless, but I have not heard vehicle stories about them nor know what settings they can manipulate (e.g. ramp-up time, initial current, etc.). And they're huge - they're full-size Kelly cases.
Great tips, especially about bore size changing! Thanks for taking time and explaining this!
If I'm going to use LiFePo4 batteries from HK, will I need to include a battery management system when discharging and/or charging? Great instructable, by the way.

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More by teamtestbot:How to Build your Everything Really Really Fast Chibikart: Rapid-Prototyping a Subminiature Electric Go-Kart Using Digital Fabrication and Hobby Components The New and Improved Brushless Electric Scooter Power System Guide 
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