Mechatronic Top Ten – Hard Disk Drives

One of the mechatronic Top Ten applications has to be the hard disk drive.  Strangely, it is not an application that you hear much about.  That’s probably because unless you work on hard disk drive design, you pretty much take for granted that little black box that stores all your information. So the group that is actually pushing the design frontier of hard disk drive technology is a very finite group.  There are only a dozen companies actually making disk drives these days, after consolidation in the market has resulted from acquisitions and mergers over the last decade.

Worldwide consumption of hard disk drives is in the tens of millions per year, and like all things electronic and high volume, the industry produces ever more memory at ever lower prices.  The absolute value of hard disk technology is one of the most incredible bargains in the world.  The current state of the art is about 10 cents per gigabyte which is quite a bargain compared to the 1.5 Megabytes for the old 3.5″ mini floppy disk.  With seek times in the low milliseconds, memory is almost instantly available due to 7200 RPM platter speeds.  The 7200 RPM speed is the equivalent of 75 miles per hour at the edge of the platter.  Higher speeds have been delivered, but the thin aluminum platter is subject to “flutter” which can cause a head crash.

The spindle motor is a 3 phase dc brushless motor that is designed to accelerate the memory platter to the 7200 RPM running speed in just 2 or 3 milliseconds.  This is an incredible feat considering that the power available is limited to a small lithium battery.  Further, the spindle motor must coordinate it’s motion with a linear actuator to place the drive’s read head a few millionths of an inch above the platter surface at the exact target sector on the disk.  So, just getting the platter to spin, which is hard enough given the time constraints, is further complicated by the extreme challenge of coordinating the rotational motion with the linear motion of the read head.

What makes this all even more astounding is that the budget for the motor can only be a few dollars, given a retail selling price of $60 for the whole package including the memory.  I don’t know how these guys come up with the solutions, but they consistently do and they consistently do it at lower prices.  The last thing I remember reading about was the elimination of bearings in favor of fluidized bearings.  At 60 million units, saving money on bearings adds up to a lot of money.

One of the many ironies of the hard disk drive is that it is at the root of many improvements in industrial motion control.  The venerable 33035 controller chip from Motorola was developed specifically to run hard disk drives.  It later appeared in a number of industrial servo amplifier designs delivering precise control of higher current power to a variety of brushless dc servo motors.

You never know where the breakthroughs are going to come from, but we keep them coming.  Keep up the good work!

Energy Equivalence or Not

Among the many issues facing us today is the cost of personal transportation.  To a large extent, modern American manufacturing was built largely on making cars and all the steel, carpet, glass and all the other products that are required in a car.  Interestingly, the electronics sector of our economy, far bigger than automotive, has increased it’s contribution to the modern automobile, but that’s another topic.

There is a lot of material being published about our use of cars and our dependence on foreign oil.  Oil and Gas companies made the decision some years ago that it was cheaper to send tankers of gasoline refined on foreign soil than to ship the crude oil and refine it here.  That was the beginning of the current problem.  Now after many years of disuse, our refining capacity has been mothballed.  What you don’t hear much about is the fact that a lot of that capacity can be brought back within a  year by recomissioning old plants.  Yes, new plants are needed.  Yes, in the short term we need to drill for oil.

But the really strange discussion is around the energy equivalency of various conservation techniques, and the number of barrels of foreign oil that it will save.  Most of the time, these equivalencies are purely theoretical.  The only thing that will save barrels of foreign oil is more fuel efficient cars and driving less.  And by the way, American consumers have been demanding higher efficiency cars since the first Oil Embargo in 1974 when I bought my first Moped and my wife and I went to school and back on a gallon of gasoline a week. Anything else is a political statement, and one that should really be ignored.

It is dis-information to say that using compact flourescent light bulbs is the equivalent of so many barrels of crude oil.  Yes, there is an energy equivalence, but there is no direct connection between the two because light bulbs consume generated electricity.  So there might be a valid statement about how many pounds of carbon dioxide the compact flourescent saves in our national energy picture based on emissions from coal fired power plants.  But even that’s difficult to measure, what percentage of our national energy supply is nuclear?  Doesn’t that mix require that we calculate the CFL bulb CO2 savings as a percentage of the fuel mix that goes into the national power supply?

Similarly it is a common to talk about the energy equivalence of a battery’s energy storage capacity compared to the energy density of gasoline.  This too, while appearing to be very scientific and logical, is very lopsided.   It ignores the fact that we are really talking about transportation.  The proper context would be that an electric car has an efficiency of 80% to 95% of input energy converted to output of the moving vehicle and an internal combustion engine is only 25-40% efficient in converting gasoline’s stored energy to mechanical motion of the car.  So comparing the energy density of gasoline with the energy density of batteries is out of context and misleading.   And yes, batteries are still not where we need them to be.  But the Lithium technology is a good first step, and it’s being aggressively engineered to improve density even further and bring costs down at the same time.

If the IRS allows 50.5 cents per mile, and the emerging electric cars cost .04 cents a mile to operate, that’s the real cost of technology comparison that counts.

Linear Motors

The linear motor has been a relatively recent addition to the electric motor world, considering the age of the ac motor is just about 100 years.  Linear motors have grown up primarily in the semiconductor industry where extraordinary precision and speed is required.  And as with all systems that offer the ultimate in performance, they have traditionally been very expensive.

But linear motors properties are quite unique and where many motion systems can achieve extreme precision, the tradoff is usually speed.  It’s hard to do both, and do them both well.  So the linear motor has carved it’s unique niche in the motion world.

But with time and applications, linear motors have become more cost effective, to the benefit of many new applications.  In addition to the sub-micron position accuracy, the technology has extraordinary speed and acceleration capability.

I had the opportunity to commission a linear motor for a unique requirement a few years ago.  We had some very tough constraints to deal with.  I did some calculations, and found that we were pulling 16 G’s of acceleration during portions of the motion cycle.

As with all systems, there are trade offs even with the most exotic systems.  There are several with linear motors as well.  They generate a great deal of heat.  High cycle rates and extreme acceleration profiles will often push the linear motors to their limits, and in response vendors have offered air and liquid cooling systems to offset the  thermal limit.

In some multi-axis applications, particularly Cartesian motion, the moving mass of one linear motor axis becomes part of the payload of the other axis.  This is very significant since motors are primarily iron cores with Neodymium Iron Boron magnets, all very dense materials.  This will cause a huge increase in the moving mass, increasing the power requirements dramatically.

And as with all linear motion, there are bearing considerations that must be accounted for.  Linear bearings are an integral part of the motor, necessary to maintain air gap between the stator and forcer, and ultimately attaching to the load.

Linear motors can be adapted to some very unique applications as has recently been shown through the use of curved actuators making hemispherical manipulators that can operate in large cylindrical envelopes.

Recent advances in linear motor systems include integrated off-the-shelf solutions from many vendors.  Since linear actuators are a combination of bearings, motors, feedback devices, amplifiers, etc., this complex system requires quite a bit of effort to integrate.  So making standard offering actuators helps control costs and makes integration for the user much quicker and more straightforward.  This creates a great opportunity for the many suppliers of linear motor technology to continue the trend forward and innovate great solutions.

As with all of electric motor history, every unique requirement leads to unique problem solving.  American innovation continues, at its finest.

Peak versus Continuous Power

Another aspect of applying electric motors to power mechanical systems is the relationship between peak power and continuous power.  In mechanical systems the forces required to start a load may have no relationship to the power required to keep the system running.  Further, the  ideal demand for mechanical power may occur at a speed that has no relationship to the electric motor speed.

AC motors operate at fixed speeds unless they are controlled by a frequency inverter.  So matching the electric motor to the demand for mechanical power requires some electrical sophistication.  The most important factor in most energy conservation applications for inverters and AC motors is creating the right control strategy to match the demand for power to the to electric motor.  (we’ve done some articles on this subject so I won’t repeat the comments here.

Interestingly, the same problem with continuous and intermittent ratings show up in a lot of situations.   In the alternative energy arena, many systems are specified based on the peak power available from the equipment.  Most of the photovoltaic systems being installed are flat panels which only reach maximum output for a couple of hours a day when the sun is perpendicular to the solar panels.  During the rest of the daylight hours the photovoltaic panels put out considerably less power.  So there’s a big “disconnect” between the cost of the technology and the value it produces.

Photovoltaic pricing is still very expensive.  Residential installations that can produce enough power to take your home off the grid currently cost about $35,000 including installation.  Most state programs and federal tax rebates will pay for about half the cost.  But even at $15 to $20 thousand dollars, it costs more than most people can afford.

In the wind energy arena, the same rating problem exists. Wind power systems are rated at their maximum output.  But that output can only be achieved a certain number of hours out of the year when the wind is blowing in the right speed range.  Not too fast, because it’s hard for the power conversion systems to function, and not too slow or the wind won’t turn the generator.

So these million dollar machines must harvest the wind enough hours to make a profit.  This means it’s all about “location, location, location”.  The game is to find a location where there is enough wind for enough hours to generate electricity and a profit.  And that’s not easy, and it’s not cheap.  Locations that are suitable, like Altamont Pass in California, are remote and hard to get to.  This make installation more expensive and losses from sending the electricity long distances, less efficient.

In general the difference in peak versus continuous rating wouldn’t bother me so much, but it’s systematic in the alternative energy community.  It suggests a bit of misrepresentation as if to create a greater perception of value, when in fact, the systems being built take 8 years before they break even.

We can do better.

Super Size my Motor?

There is an interesting problem with applying electric motors that is a constant source of difficulty, the nature of peak power versus continuous power.  The problem is that few systems operate at a statistical average power demand.  Frequently, this causes equipment designers to oversize the motor for the application.  At the same time, however, this can put the motor in a very low efficiency operating range.

So what’s the right solution?  Right sizing.  Yes, just like Goldilocks and the Three Bears, not too big, not too small, but just right.

There are some great DOE publications on motor sixing that can be very helpful on the AC motor side, so make sure to give those a look.  But the implications of how to deal with varying loads are complex, each requirement having its own unique conditions that need to be considered.  Is an underpowered application actually safer?  Sometimes, yes.  I recently noticed that a particular orbital sander had been designed so that if the unit became momentarily overloaded, it stalled.  Perfectly safe.  In fact, this design is to be preferred because it prevents accidentally damaging a work piece by burying the sander in the wood and removing too much material.  Who’d have thought of it?  Certainly not Tool Time Tim.  More Power!

In fact, most of us view more as better.  More power means more production.  Or does it.  In an increasingly energy conscious community, more power means more cost.  And that’s really what its all about.  The value of the motor is not just in the purchase price, but also in the operating cost.  Especially if the motor is expected to run for 8 years, 24/7.  (That’s what the life expectancy of large AC motors is)

There’s another trick to the power requirement problem.  How much time is spent at full load and how much time is spent at average power, or, what is the duty cycle?  If the system is starting and stopping frequently it puts different constraints on the motor.  If the system is typically starting only once an hour, then we can consider the thermal duty cycle of the motor.  The momentary peak power requirement is insignificant and the vendor can usually tell from their modeling and testing of their products how much impact the peak current will have on the motor’s average temperature.

After all, its Thermodynamics 101 in the final analysis.  Every energy transformation produces heat as a byproduct.  How much heat a given system can tolerate is the key to its operating life.  In electric motors, the key values are the insulation system’s temperature rating, usually in the range of 150 to 180 C and in the case of steppers, brushless dc and permanent magnet dc motors, the magnet’s ability to resist high temperature and high coercive magnetic fields that can be generated in the motor.  Both sets of limits are generally well considered by suppliers when electrically controller motors are shipped as motor/drive combinations.  This can get a little tricky when pairing motors from one vendor with controls from another vendor.