Control, Motors and Efficiency

I was talking with some friends about control technology and made the observation that over the last decade the progress in the control field has been really amazing.  Particularly, the processor technology that is available for controlling electric motors is operating 1000 times faster than the control platforms of a decade ago.  We look at events in nanoseconds, not microseconds.

Increasing the control system’s frequency response is not signficant in itself.  But it does mean that software can be applied to problems that are more subtle in the operation of a particular system.  Observation of the phase relationship between the rotor and stator in an electric motor is now commonplace in 3 phase systems.  Algorithms for optimizing this relationship dynamically are also commonplace to adjust the power factor or reduce energy consumption in inertial loads like fans.

But this is not where the big energy gains will come from.  These improvements are smaller and more incremental.

Variable speed motors are systems that are made up of electric motors and power electronic systems.  Both are subject to losses in the form of heat.  In the motor bulk magnetizing of the stator, phase loss due to load, and copper losses due to the construction methods used are common.

Better metallurgy is needed to reduce losses associated with magnetizing the stator core.  The steel industry has attempted to address this issue, but the high cost of exotic alloy laminations prevents the advanced materials from becoming widely used.

Copper loss is improved in the segmented stator, but this manufacturing technique is most often found in more expensive servo motors, even though analysis suggests the cost is lower.  This may have to do with scale effect, since the servo motor world runs at much lower volumes than the AC motor world.

The other major dependency in the speed control is the power semiconductor.  The costs for power devices are falling and performance is improving.

So where are the big efficiency gains going to come from?

The control system strategy.  If the application is not well regulated you might be able to get a big increase in efficiency by measuring things more carefully.  In a cooling tower changing from a +/- 10 degree thermostat to a +/- 1 degree thermostat allowed me to implement a control system that reduced the energy consumption sufficiently to pay for the equipment in less than two years.

No new technology motor, nothing special about the variable frequency drive.  Just what was available at the time.  The big difference was the strategy.  Measuring what was important and organizing everything in the control system to achieve our objective.

New EPSON RS3 Robot

Rosemont, IL — EPSON Robots introduces the new EPSON RS3 Robot, featuring all the benefits of a typical SCARA plus more. The unique new design of the EPSON RS3 clearly puts it ahead of other robots in its class with superior cycle times and larger work envelope access thus opening up new application possibilities.

“Unique to the EPSON RS3 is our new work space design which maximizes work envelope usage” stated Michael Ferrara, Director of EPSON Robots. “No other robot vendor offers a 350mm SCARA arm featuring the largest working quadrangle greater than that of a typical 750mm SCARA arm. Since there is no dead space in the center of the work envelope, the EPSON RS3’s largest working quadrangle is 494mm x 494mm, which up till now has only been possible with a much larger SCARA robot. With the ability to maneuver back under itself for the shortest movements possible instead of having to move around itself, the EPSON RS3 delivers superior cycle rates. This means more parts processed in less time, while using a fraction of floor space which results in more profits for our customers.”

The EPSON RS3 is literally a zero footprint robot, thus saving our customers valuable floor space. It is also capable of easy integration into compact assembly cells. Furthermore, the unique work envelope allows for unprecedented design flexibility with over 360 degrees of axis rotation for omni directional access. All these exclusive features make the EPSON RS3 robot the most versatile and unique SCARA available in the market today.

The EPSON RS3 is perfect for lab automation and other process heavy applications where large quantities of parts are presented to process or testing stations.

EPSON Robots
www.robots.epson.com/rs-series.htm

Energy

Everyone has an opinion about Energy Policy.  Just ask.  They’ll tell you!  And I am glad for the fact that there is a lot of discussion taking place.  We need good dialog and good information.

We might be a little lacking on the information side.  Nuclear power for generating electricity is not a popular topic, but worse yet, no one seems to want to talk about pebble bed reactors.  Pebble bed reactors have been around for over 25 years and represent the most stable path for producing electricity without burning fuel.  Small spheres of an enriched radioactive material are encapsulated in a ceramic insulator so that the nuclear fuel cannot accidentally achieve critical mass.  The same property of the geometry causes the “pebbles” to achieve high enough temperature to heat steam and generate electricity, but reaches thermal equilibrium at 800 degrees remaining stable without coolant.  So there can’t be a meltdown.

This makes atomic energy safe enough to locate in a major city without fear of a metldown or a chain reaction, the two weaknesses of conventional nuclear powerplants.  The fuel is encapsulated in carbide and graphite materials with processes that are very difficult to circumvent.  And because of the simplicity of the design, these reactors are lower cost than the water cooled reactors.  Could we save the environment and satisfy our energy needs at the same time?  Maybe so.

But this conversation is not part of the energy plan for the US.  Neither is drilling off the US coastlines and putting American workers back in the business of supplying our oil and gas needs in the US.  That makes no sense. The oil industry chose to import gasoline directly from the middle east 30 years ago because it was cheaper.  But we have done nothing to update our supply chain since then, and now we have to buy oil from countries that don’t like the US.

The logic seems to be about reducing our energy consumption instead of increasing our energy production.  Using less is fine until it cuts into our ability to produce necessary goods like electronics.  We don’t need to hobble the largest sector of the economy by telling semiconductor companies that we have to turn off electricity to their plants during the summer months.  They will have no choice but to locate to other countries.

You can’t “save” your way out of a recession.  You can’t save enough money to keep a company in business if it stops selling it’s products.  That’s all there is to it. And our policy leaders need to understand and apply that logic to the current situation.  The best thing to “stimulate” the US economy is to get it’s businesses producing.  Produce more energy with the resources that we have.

And when the car companies can make a competitive electric or hybrid vehicle, we will produce less gasoline and make more electricity.   There are plenty of opportunities to sell new cars to stimulate that industry too!

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.

Top 10 Mechatronic Challenges

I recently wrote on the mechatronic challenge of wind power.  Converting wind into mechanical power that can be harnessed for man’s use has been going on since the 9th Century according to Persian historians.  Certainly wind powered grinding of grains has been around in Europe for several centuries and, lest we forget, wind power pumping of water in the United States.  So there is some irony to the cultural “buzz” about wind power at home and abroad, as if the technology were entirely new.  There’s a lot of history, we’re just updating the technology to produce energy in the age of electricity.

Water has been used for power generation as well.  Following a similar path, we learned during the early part of the industrial revolution how to locate manufacturing plants near waterways so we could convert water flow into mechanical power using the water wheel.  This is, in fact the root of all modern motion control.  All the belts and pulleys, cams, gear reduction systems follow from the work done in mechanical engineering from this period of time.  All of the electronic analogs of the mechanical behaviors found in mechanical systems are the functions which we refer to in mechatronics today.

Wind power and water power gave way in the 1800’s to steam power as the improved steam engine of Watt became the standard of energy efficiency, or should I say “cost effectiveness”.  Because the absolute value of technology is in its cost effectiveness.

Still, wind energy poses a huge technology challenge, as witnessed by the number of vairations that exist and new versions that are emerging.  And hopefully improvements will continue to come from the creativity and  imagination of engineers and inventors all over the world.

But what are the other big mechatronic challenges that come to mind?

Transportation certainly ranks in the top 10.  We have seen hydraulic, pneumatic and electric vehicle solutions touted for a variety of uses, personal transport, delivery vehicles etc.  Ballard Energy and General Motors have both been building hybrid and pure electric buses for city transportation systems for several years with some success.  Interestingly, the electric bus is easier to engineer, which seems unreasonable, but the bus has more interior space to put things like batteries and a methanol converter for generating hydrogen for fuel cells.

But there is a great lesson in what appears to be an almost chaotic string of choices in the transportation arena.  One solution will not work for all requirements.  There are many people for whom a 40 mile per day drive cycle is perfect.  The NEV, Neighborhhod Electric Vehicle, is a golf cart type solution that is rated for street usage, and because of its relatively simple performance requirements, is relatively easy to achieve and lowe cost.  As we categorize cars with greater range, the problems get more difficult, and because of the storage limitations of batteries, have only been achievable as hybrids.  But with some hybrid designs reaching 50 and 60 mpg (estimated), these vehicles may be great solutions for other users.  Although, we must consider their cost effectiveness.  If they cannot be introduced at prices well below $50,000 the absolute value of the technology is not very good.

So forget the 15 second soundbyte that will solve the world’s problems.  It doesn’t presently exist.

I would like to hear from any readers about their picks for the Top Ten Mechatronic Challenge.

Linear Feedback Technology (Linear Motion Part 2)

Linear motion is particularly impacted by the choice of feedback.  And for most systems the use of feedback is not an option.  Linear motors, for example, cannot be operated without a feedback device.  And because of the linear motor’s roots in semiconductor manufacturing, the feedback is usually a high resolution linear tape scale.

How much feedback resolution is enough?  Most of the time more resolution is better.  But there is an element of control theory that says if the feedback resolution is ten times greater than the position accuracy that you are trying to measure, the control system can become unstable.  The other side effect of extremely high resolution feedback is the tendency to “jitter” because it is responding to tiny variations in the real world, which the control system will then have to contend with.  So spending extra money for high resolution feedback may cause other problems.

Where should the resolution be put?  Obviously, if you are using a rotary servo motor, just use the feedback on the motor as the linear position reference.  This works when the required resolution is not very high because in all mechanically linked systems, there is lost motion called backlash between the motor and load.  But most motion controllers and many indexing drives contain dual feedback loops, so using an external feedback sensor will produce great benefit in accuracy and repeatability.

The big benefit in using linear feedback is the elimination of mechanical error as part of the control system.  On a project I did a few years ago we were evaluating a special grinding machine that had a 13 foot long lead screw in it.  The customer know the lead screw had wear and error in it, and that was part of the problem that needed to be addressed in rehabilitating the machine.  Instead of replacing or re machining the lead screw, we specified an external linear tape scale feedback.  The results were fantastic.  Accuracy and repeatability were phenomenal and combined with an integrated servomotor system,  led to a 300% increase inthroughput for the customer.  Backlash? What Backlash?

How much distance do we need to sense?  Some linear motors like piezo-electrics  and voice coil motors have very limited stroke lengths.  Similarly, different feedback technologies have scalability parameters such as sensing airgap and length requirements are considered.  Some feedbacks work in the range of 2 to 6 inches in overall stroke length, some are capable of 3 feet, some up to hundreds of meters.

The exception is the stepping motor and leadscrew combination which can be operated without feedback on the assumption that the load is not varying dramatically.  But even the leadscrew and stepping motor needs feedback when the load varies.  Current detection can be used to determine if the motor has stalled, but doesn’t necessarily give you the opportunity to recover position without an external source.  So the extra cost of external feedback is a judgement call based on the accuracy requirement and how “robust” the system needs to be.

The variety of types of linear feedback are equally challenging, and as with most things, must be considered based on cost and performance.  The most popular feedbacks are linear tapescale systems that use reflected infrared beams that are interpolated to achieve very high accuracy.  The classic linear feedback from the machine tool era is the glass scale which uses through beam optics and a grating embedded in glass to product the linear position information.  Check out companies like Renishaw, Heidenhahn and others for details. Information on  Heidenhahn’s latest innovation is featured on the Project Mechatronics website.

Over the last few years there have been a number of magnetic solutions where a magnetized linear scale is interpolated by taking the sinusoidal waveforms produced by Hall sensors or inductors, and digitizing the results.  Integrated circuitry combining Hall effect arrays and functional support to linearize output are now the prevailing state of the art.  Check out NewScale Technologies Tracker product for details on their new offering.

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