Controlling BLDC Motors: Simple is Usually Better

Other Parts Discussed in Post: INSTASPIN-BLDC

Dave Wilson, Motion Products Evangelist, Texas Instruments

I hate tax season!  I not only hate the fact that I have to pay the taxes, but also that I must document the process.  I’ve noticed over the years that my taxes have become increasingly more difficult to do.  Granted, my income diversity has grown, which complicates the process.  But still, tax laws have gotten to the point of being totally incomprehensible.  In fact, it is SO complicated that I can’t even do my taxes anymore!  I have to pay a tax professional to do them for me.

Which brings me to the subject of motor control.  As technology progresses, the techniques we use to control motors are also growing steadily more complex.  Unlike taxes, this is generally a good thing, as it allows our motors to boldly go where no motor has gone before.  But every now and then, I think it is healthy to step back and try to see the forest for the trees.  Does your application really need super speedy torque response and layers upon layers of observers to get the job done?  Just like my taxes, do you really need to use a technique that is so complicated that you have to hire a motor control professional to do it for you?  Wouldn’t you rather use a technique that you can completely wrap your mind around?  Sometimes, simpler is just better!  And that is why I am so excited about InstaSPIN-BLDC™!

Just so you know, “InstaSPIN” is a whole new category of motor control solutions from TI.  The first of these solutions to be announced is “InstaSPIN-BLDC,” which performs sensorless commutated control of brushless-DC motors.  TI has just made available four new kits powered by InstaSPIN-BLDC software running on three different TI MCU architectures: one for Stellaris™ microcontrollers, one for MSP430™ microcontrollers, and two for Hercules safety microcontrollers.  You can find out more information on each of these kits here.  These kits join our existing InstaSPIN-BLDC kit which is powered by our C2000TM microcontroller family. 

Now before you say “been there …done that,” let me tell you why InstaSPIN-BLDC is probably superior to the sensorless commutated solution you are currently using:

1.        Easier setup.  Unlike that new high-def TV you just bought, everything you need (including the connectors) is inside the box.  Just wire it up, and fire it up.

2.        Insanely simple to tune.  There is only one tuning parameter that you need to adjust, which is the flux threshold.  But even with NO tuning at all, we have found that the default setting used in the GUI interface will start about 90 percent of the motors we connected to it on the first try!

3.        Low speed stability.  This is a big enhancement over systems which simply use the back-EMF signal for commutation.  Instead, InstaSPIN-BLDC uses flux!  Flux is the integral of back-EMF which provides two distinct advantages at low speeds; a signal with higher amplitude, and a signal with better filtering.  Both of these together mean a commutation signal with much better SNR, allowing sensorless operation at full torque at much lower speeds.

4.        Speed invariant performance.  This blows the doors off of any technique which uses back-EMF zero-cross timing!  Imagine trying to drive your car forward by only looking in your rearview mirror!  Sure, everything will be fine as long as you’re driving on a straight road with no surprises.  But what happens if you unexpectedly encounter a sharp curve?  I know this sounds crazy, but that’s EXACTLY what we are doing with most sensorless commutation techniques used today.  If you are simply timing the instances of when the back-EMF signals have crossed zero to schedule your next commutation event, you are essentially trying to predict the future by looking at the past.  As long as the velocity is unchanged, everything is fine.  But if you accelerate or decelerate too quickly, you can miscommutate or even stall your motor.

With InstaSPIN-BLDC, the back-EMF signal is continuously monitored.  In other words, InstaSPIN-BLDC uses up-to-date real-time data to determine when to commutate, instead of just using past data. We are essentially counting the flux lines cutting through the coil in real time.  When enough flux has been accumulated, we figure that the rotor must be in such-and-such a position, and it must be time to commutate the machine!  The speed of the motor is virtually irrelevant to this accumulation process, and only affects how long it will take.  When I saw this for the first time, I thought to myself, “of course this is the way you would want to do it!  It just makes perfect sense!”

5.        Incredibly robust.  I think this is my FAVORITE feature!  One of the reasons it is so robust is because it is so simple!  I remember early in my career working on a sophisticated medical product where the calculated Mean-Time-Between-Failure was around 15 minutes!  Seriously!  With InstaSPIN-BLDC, there’s just not a lot that can go wrong, which means it just keeps going, …and going …and going…

To better understand how InstaSPIN works, check out an animation of the commutation process here.  If you want to learn how to fire up your own InstaSPIN-BLDC kit, you can watch my video here.

So if you like motors, and if you like SIMPLE, then you’re going to LOVE InstaSPIN-BLDC!  Take it out for a “spin” yourself, and experience the joy that comes from the simple things in life!  In the meantime…

Keep Those Motors Spinning.  :-)

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