A lot of it is precisely ‘doing more with less’.  How do we make the same quality of parts at prices low enough to compete with foreign competition?  It’s not easy when the typical pay scale for manufacturing labor is $8 a day in some parts of the world.

There are a couple of obvious components to price competition that don’t get a lot of attention.  Scrap rates and delivered cost.  When a US company buys parts from offshore, any defective parts are very costly.  The direct shipping cost, duties and processing fees are additional and can be 10-15%.

During my years at Rockwell Automation, we investigated the cost of selling US products in different parts of the world.  Depending on where in the world we are talking about, the shipping and logistics can accumulate between 25 and 40% additional cost to the product being sold.

So the cost of scraps and logistics are the minimum cost hurdles for companies seeking to export their products to the US.  Low cost producers have to make parts cheaply enough that the landed cost and scrap rates cost out less than the price of producing them in the US.

Doing more manufacturing in the US requires finding creative ways to lower costs.  That is the second area that is undergoing change.  American manufacturing technology is finding ways to reduce machinery and process costs.  And this area of effort may provide key strategies that will help the US gain back ground in the pursuit of more world class manufacturing.

Innovation processes like additive manufacturing allow fabrication of metal parts with no machining.  For higher levels of precision there are new machine tools that can do final machining to less than 0.001″ accuracy and the costs of machine tools are lower than ever.  These are the keys to producing high quality parts at lower costs.

There are unique mechatronic solutions that can improve machinery performance across a wide range of applications.  The Acme screw which is very inexpensive, has limited accuracy but plenty of torque handling capability.  What happens if you can add a very inexpensive linear feedback technology to the simple low cost Acme screw?  You get a really high resolution linear motion system that is very inexpensive.

The great news is that these products are currently available.  And that means that making better machines that make better parts at lower cost is practical, achievable and there are no technical challenges.  Common off the shelf parts will get it done.

Some people define servo’s as closed loop versus drives which are open loop.  The term servo does require that there is a feedback device to provide the loop closure.  But there are many AC drive vendors making closed loop inverters to enhance the performance of the motor.  AC drives with feedback are generally used where positioning is required.  So the feedback element is not the determining factor for defining if an application is AC or DC.

The overall power level may define one versus the other, but not always.  Brushless servo motors are generally limited to 7″ or 8″ diameter and an equivalent horsepower rating of 20-30 horsepower.  There are frameless motors with even higher horsepower ratings.  But  the size and power rating are strictly a function of manufacturing and marketing constraints.  For a major manufacturer, the question is really, how many motors of a given size are we going to sell?  Based on the high cost of Neodymium permanent magnets, a larger servomotor is going to be very expensive.

But overall power ratings are not limited when you consider products from specialty companies like Powertec.  Powertec takes standard AC motor frame designs and increases the power density by adding embedded permanent magnets on the rotor.  Since the magnets are Ferrite, which aren’t as expensive, they are much more economical and allow designs as big as 400HP.  So power level by itself doesn’t determine what technology to use.

The real answer is in the load conditions.  What is the dynamic response required for the target application?  The rate of change of the load is the key.  Most AC drives are specified in terms of the frequency response or dynamic response of the power electronics.  This important parameter is expressed in Hertz.

Dynamic response is the ability of the drive to regulate speed when the load varies.  The load torque can change significantly, usually 90-100%, and the drive will recover the set speed within the time defined by the dynamic response.  Typically, an open loop AC drive has a 10 hertz dynamic response, which means that it will regulate to 1/10th of a second.

AC drive technology has improved to the point where dynamic response can reach 200 Hertz when a rotary encoder is added to the motor.  This means the drive can regulate load variations wiithin 5 milliseconds.  Which is pretty fast when the load mass is high enough to require a motor of 25 horsepower or larger.

The basic physics are simply that the bigger the load, the slower the dynamic response.  You just can’t make a ton of rotating mass change speed really quickly.  And that’s how the controls should respond.

Features of Nippon Pulse’s Green Drive include:

• Acceleration (peak) force of up to 600N for 40 seconds
• Effective stroke lengths between 10mm and 1540mm
• Cooling systems that can increase rated force up to 20%
• Rated force between 13N and 150N
• Position repeatability of ±0.05mm
• T-slots for easy and quick integration into applications
• Position sensors, temperature sensors, interpolation electronics
• Four different feedback output types: analogue SIN/COS, Digital Bus BISS-C, Digital A/B TTL Linedrive Incremental, and Absolute SSI
• Color coded quick connectors
• High-performance slide bearings

The Green Drive currently is available in two sizes, the G16x series and G25x series. The G16x series features a shaft (magnets) with a 16mm diameter and the G25x a 25mm shaft diameter. The G16x series is 66mm wide and high, while the G25x series is 88mm wide and high. Each has varying lengths depending on the required effective stroke.

Nippon Pulse will be highlighting the Green Drive at the ATX West tradeshow in Anaheim, CA in mid-February. Those interested in the Green Drive can visit booth #4348 to learn more about the actuator.

Nippon Pulse America, Inc.
www.nipponpulse.com

“We are very pleased to add Kaman Industrial’s selling capability and the value added approach they brings to their customers to our already strong distribution channel.” said John Hegel, President of Minarik Drives.  “Kaman’s penetration into the user and OEM markets will open doors for us that had been previously inaccessible and will help us serve a greater cross section of business across the U.S.”

Minarik Drives is an independent company that specializes in low to medium power electric drive and power applications.  It has been a standard, and a leader, in the DC drive business for almost 60 years.  With design engineering and manufacturing headquartered in S. Beloit, Illinois, it provides standard and customized solutions at a globally competitive price.  More information about Minarik Drives is available at www.minarikdrives.com or by calling 815-624-5959.

The traditional metal cutting machine tool has been around since the 1800′s and was entirely manually operated.  Since the machines were manually operated, the dexterity of the operator became a major factor in accuracy and repeatability of part manufacturing.  Because of the skill required, we still have the term “master machinist” in circulation, even though most machining today is automated.

During the Second World War, the Air Force was confronting the difficulty of manufacturing airplane parts.  Through the work of John Parsons and MIT, the first “punch card” controlled machine tool was built.  Parsons’ company was using early punch card computers to generate a larger number of points along the curve of a wing brace.  The numerical information was then used directly by machinists as a look up table for manually positioning a milling tool.  Parsons realized that if they could motorized the manual process, it could greatly increase the speed of the machining process, lowering costs dramatically and increasing accuracy at the same time.

Gordon Brown’s Servomechanisms group at MIT has recently been working on early forms of closed loop dc motor control for the gun turret on B-29 bombers.  By combining these recent technologies to numerical punch card calculation approach the first Computer Numerical Controlled Machine Tool was demonstrated.

The rest, as they say, is history.  The lessons learned in computer numerical control have been instrumental in every major field of manufacturing.  Cars, electronics, robotics, would not be feasible or cost effective without the underlying control technology of CNC.

Which brings me to a 2 major points as we contemplate the next generation of manufacturing.

Additive manufacturing is maturing rapidly with a wide range of materials, steels and titanium are now available, and precision is improving at the same time.   The surface finish requirements for a large number of parts cannot be achieved with a strictly additive process.  The new wave of additive manufacturing requires a complementary subtractive technology at complementary prices.

Secondly, while there are an increasing number of machine tools at low cost, they are not CNC.  This will likely be the next “breakout” technology.  There are a number of technical hurdles that have to be addressed in terms of reducing the cost to a level comparable with the Makerbot.  With the current generation of dedicated motion controller chips, lower cost step motors and low cost feedback technology, this should be a slam dunk.

Get your pencils out and get after it!  There’s some serious money to be made here.

In the wind energy arena the biggest change has been the shift to direct drive permanent magnet generators.  By eliminating the gear “increaser” to convert the low RPM of the propeller system to a high RPM for a standard high power generator.  This is crucial step in bringing the cost of wind power down. Current systems are weighing in at 100 tons and have to be suspended above water or land 165 feet in order to pick up sufficient wind currents to be economically practical.

There is no single solution that is ideal for wind applications.  One supplier has a generator that is made up of 4 smaller units on a single large ring gear.  This system seems to have significant advantages in reducing the size and weight of the generator and makes maintenance more simple in the event of a failure.

Among the major mechatronic challenges driving change in the motor industry, electric vehicle applications are continually pushing the boundary for energy density and efficiency.  The performance demands of electric vehicles and other mobility applications make every percentage point of efficiency crucial to the range of the target vehicle.  This has led to a rash of new motor and drivetrain designs with a variety performance capabilities.

Each new innovation seeks to organize the basic materials of the electric motor in a new way to improve some aspect of performance.  Electric motors are copper conductors, “soft” magnetic steels and many times, permanent magnets.  The basic costs for copper wire at $5-6 a pound, commodity strip steel is about $.50 per pound but has to be punched in precise shapes, coated with insulation and stacked into larger assemblies, and $16. per pound for permanent magnets.  Complex processes associated with motor manufacturing make motor costs considerable.

In a recent development teams in academia in Australia and the US have developed simple low RPM motor structures based on polymer actuators referred to as “artificial muscle”.  While this development is in its early phases, the simplicity and low cost are significant and very appealing.  A demonstration of the new technology can be seen on YouTube at;  www.youtube.com/watch?v=ZcCPNJR5PCMand it is very much worth the watch.

The only sure thing is that we continue to meet the challenge of new market needs with innovation.

This was the case when a friend of mine was designing a material handling system for newspaper bundles.  A very exotic conveyor system with 8 servo driven belts and a design that involved 10 pages of hand calculations of inertia.  We shipped the servos and sent out a field engineer to start up the project only to find out that the motors and drives were too small.  The designer had forgotten to account for friction.  In this case the frictional load was 3 times the mechanical load due to the unique belt and roller configuration.

So the first lesson is; don’t forget to look at friction as 1 of 3 components of the torque load.  The three being; steady state torque, torque of acceleration and friction.

Then there is the fanciful wishing that there wasn’t any friction to worry about.  Kind of like doing experiments in the space station and having no gravity.  It’s fun to think about, but there are few real world situations where this is likely to work.  The only exception is air bearings.  Of which there are a few.

If you have ever played air hockey, air bearings are like that.  Parts in motion tend to stay in motion when there is no friction to worry about.  And that would be good in a lot of applications.  No friction will generally result in smaller servos, so there are savings in the hardware requirement.  No friction means no mechanical wear, nothing to service as the machine runs up cycles.  No friction also means high speed motion is a lot easier to achieve.

Cars coast to a stop because of friction.  That’s a good thing.  Without friction, parts would end up flying off the conveyor instead of going where you want them to go.  In conveyor belt applications there is usually a lot of friction and that helps the system slow down and stop.

So the second lesson is; friction can be your friend.

In between systems with friction and systems with no friction, there are rolling bearings.  Systems like the Bishop Wisecarver “Vee Guide” are among many products on the market are examples of this.  Rolling element bearings have very low coefficients of friction, so losses are low and therefore the energy needed to overcome them is very low.  This also results in very low wear, so maintenance on this type of mechanism is also low.

The are dozens of linear actuators on the market and each vendor has developed unique bearing solutions, whether sliding or rolling, that perform well at varying price points.  There are no universal rules for selection.  The typical criteria are move speed, positioning accuracy, life expectancy and cost.

A few weeks ago, I had the opportunity to write a couple of small “C” programs in a training class and re-discovered why I don’t like to write control software.  I don’t have much background in C programming.  It’s not that C programming is inherently good or bad, it’s just another language.  What is difficult to deal with is each controller having it’s own library of C language instructions.

It’s not that any particular language difficult, it’s that every language is iterated on different controllers and the instruction set and programming quirks have to be learned on each platform.  Ladder Logic instructions have become largely standardized and the difference from one platform to another are becoming less and less significant.  Turning discrete inputs and outputs on and off is pretty straightforward.  Reading analog signals, doing some mathematical operations and setting analog outputs is also fairly straightforward.   Even when there a lot of I/O to deal with, the knowledge base required to understand the applications of the technology are ultimately very repeatable.

The variations of how to do motion control on different platforms are very significant.  Each controller company has to come up with a complete programming environment that defines how to command the controller to execute motion tasks.  The creation of commands and processor executables requires coding and testing the code over man years of development.  This is a complex form of knowledge capture and there are a lot of nuances as programmers come up the learning curve before good effective programming environments can be created.

This is part of the reason why the motion control field hasn’t progressed as much as other control disciplines.  There is no agreement on any standard programming methods past trapezoidal move profiles.  The situation becomes more complex because each motion control vendor develops its own programming environment based on the selection of processor platforms and what its programmers come up with for the programming suite.  This creates a barrier to entry for new companies, and makes improved code solutions problematic.

Many of the PLC programming suites include dialog boxes that provide scripting for the motion commands in the ladder logic program.  The technology is readily available to make a high level motion programming suite that is processor independent and capable of addressing 80-90% of all motion control applications.  This will make motion more accessible to a wider audience and simplify the programming aspects of motion and machine control.

We need to bring the industry into the 21st century and make everyone’s lives a little easier.

Control systems can be described as “open loop” or “closed loop” depending on the whether or not the system being controlled is well characterized.  Many forms of motor control seek to be “open loop”, that is, without the use of a feedback device.   However, this notion should be modified to open loop meaning without an explicit feedback device. This is because great effort is expended to “infer” what is going on in the motor through various means. The most common of which is current.

In the world of electric motors, the alternating current motor of Nicola Tesla is well understood, and rarely requires a feedback device.  Motor speed is derived from the frequency of the power being supplied minus losses depending on the details of rotor construction and how a specific load affects the motor.  The standard ac motor has a small amount of rotor “slip” from 1800 rpm to 1750 rpm which reflects the magnetizing current losses in the motor and magnetic features in the rotor that would be needed to maintain perfect synchronism with the line frequency.

Load variations can be measured by sensing the current in the line going to the motor.  So there is a feedback element available from which a great deal of information can be derived.  This is where the ambiguity about feedback comes in.  The current needed to run the motor with no load is fixed value, so more current read on the motor leads is load, until the motor reaches locked rotor current or stall.

In brushless dc systems a similar approach is used.  Detecting the zero crossing point of the phase current establishes precise timing of the rotor speed and is used to regulate timing of current pulses to all three phases of the motor.  In this way even the brushless dc motor can be operated without an explicit feedback sensor.    However the tradeoff here is very poor low speed regulation of the motor which makes this approach unsuitable for many applications.

From a control system standpoint, feedbacks are the last, slowest loop in the control scheme of the motor.  This makes sense in the context of position control as it is normally executed in a PLC or motion controller.  However, this makes load regulation more of a challenge since the actual error detection of the control system is being done a level removed from the actual load.

A host of mathematical tools from the signal processing domain have traditionally been employed to characterize the lag created by the control system and the interaction of the controls at varying speeds. All of which works well, but has also lead to “rules of thumb” that are not very clearly understood and which are sometimes misleading.

Start with the big loads.  Plant air handling, building HVAC and lighting are generally a lot more significant in total Watts or equivalent horsepower.  1 Horsepower is equal to 746 Watts.  If you are located in the northern states, winter heating uses a lot more energy than summer air conditioning.  In the southern states, it’s the opposite.  There is one study that puts the northern thermal cycle at a much higher overall cost, so everybody needs to move their manufacturing to the south.

Check all the integral horsepower motors in the plant.  A recent DOE study shows that over time, many motors get replaced with whatever is readily available in the next larger frame size.  This is in reaction to plant failures where the exact replacement motor is not handy or on the shelf.  The result is that the plant power and power factor can be very poor because there is a lot of excess capacity that is not being used efficiently.

Industrial plants also suffer from peak demand billing practices.  The utility company agrees to provide power, but large users get billed extra when they have peaks above their average usage.  Again, look at the large loads, and see if some or all can be put on soft starters or inverters with longer starting profiles.  AC motors try to get to full running speed and spend several seconds at poor power factor and huge inrush currents during starting.  Most motors require at least 4 seconds to get to speed.  So, is there a savings opportunity if you can get by with a 6 to 10 second starting period?  Yes, there absolutely is.

The smaller loads like individual plant floor machines are a little harder to regulate.  Some production machines consist of dozens of individual motors and sub-systems.  In large conveyor installations, newer control system turns off whole zones of equipment if there is no traffic for that section.  Use the same strategy in production equipment.  If there is nothing coming into the machine, turn off as much stuff as possible.

Again, look for the largest loads.  In CNC machines, the spindle is usually the dominant load.  Turning off a 10kW spindle motor will save lots more money than turning off 400 Watt positioning axes.  However, don’t pass up an opportunity if one exists.  If there are a large number of individual axes of motion that have low duty cycles, it may be cost effective to put brakes on the load and turn the motors off when they are not in use.

Prudent planning can be turned into real cash savings.

There is a complex interaction between the two that is often not expressed.  The hardware has “firmware” that defines the exact capabilities of the hardware.  The software is a tool for users to create programs that the hardware executes.  These programs are the embodiment of the useful behavior that the process or machine is intended to accomplish.

So is machine control hardware or software?  It’s both.  The hardware is only capable of executing instructions that we built into it by it’s microprocessor and firmware.  Those instructions are merely a library of possible functions.  The user program calls those firmware functions in an organized manner to accomplish some beneficial result.

There are a couple of really significant issues that are often overlooked.  One is experience.  A lot of experience is required to make good product selections.  The application of control systems involves understanding the application requirements and matching those requirements to specific hardware.  Motion systems that do “high speed registration” for example, require very specific hardware to capture the input signal to define where the registration target is, and then to turn off so that input noise is filtered out.  This is a very specific feature, and if you don’t have it, you generally can’t create it.

Complex control requirements like coordinated motion are both hardware and software dependent.  The simplest example is to draw a circle with two linear axes.  In order to know how to deal with this application the control system must have a dedicated motion controller either as a stand-alone element or embedded within the control architecture.  Most high end PLC’s offer a 4-Axis dedicated controller card that do this.

After all the wrangling is done to get the application and hardware properly scoped out, after all the software development work is done, there is still an aspect of control that is missing from this discussion.  It is the external wiring of power, power protection, and safety systems.  These circuits are separate from the control system hardware and software, and yet embody elements of control that are sometimes necessitated by the hardware itself.

Variable frequency drives and some servomotor drives require time to charge their capacitors.  Most drives has interlocks that will prevent operation until the caps are charged.  PLC processors require a time delay to insure that the I/O devices are powered before the processor “wakes up”.  If not, the processor will immediately fault.  The wiring of emergency stop circuits are physically separate and frequently use reverse power logic, they are energized when “off”, to all detection of broken wires.

All of these behaviors are part of the control system but generally not considered in the early phases of system design.  Yet all are required in order to make safe, working systems.

This is rather ironic considering the billions of dollars that are spent on control systems across all fields. Is control fundamentally any different if it is inside a car, automating lighting and HVAC in a large building, on an automotive manufacturing plant floor, in a biological resesearch laboratory, or in a giant refinery where chemical products are made.  It’s all control.  And the more we try to define it, the more inclusive we make the definition, the more vague and ambiguous the term becomes.

Efforts continue to increase the power of the PLC (programmable logic controller) across many vendors. By increasing processor speed, memory and capability PLC’s are becoming the universal platform of control as a discrete controller, process controller and motion controller.

Simultaneously, motion control specialty companies continue to increase speed, processing power and I/O structures in an effort to expand the dedicated motion controller as a competitive platform to the PLC.  This is a necessary migration to address control applications where an external PLC could be eliminated.

Is there an ideal mix of motion axes and I/O that will help resolve which hardware solution is best?  Not really.  The fact is that the majority of the market is made up of motion control using stand-alone axes that are triggered by logical conditions in the system.  Coordinated axes require the sharing of pulse to pulse position feedback information.  Stand-alone axes do not share data at that low a level in time.  Most PLC controllers are well able to handle stand-alone axes, especially if an intelligent indexer is used.  This off-loads the motion to the servoamplifier and only I/O handshakes are used.

Part of the ambiguity here is that control is the result of hardware and software together.  ’Control’ seeks to generate complex behaviors using digital methods.  The digital methods, processors, depend on programming techniques in order to implement the desired behavior.  So when we talk about Control, we are talking about hardware and software simultaneously.

What matters most to users of automation technology is both logic control and motion control programming exist in a single environment.  It doesn’t matter if the programming environment is a PLC with motion blocks inside it, or a motion controller with logic blocks inside it.  What matters is that all aspects of a control system can be programmed using a single editor.  Controllers from the major electrical companies like Rockwell Automation and others have opted for the logic-centric programming environment with motion blocks in the ladder diagram.

This approach eliminates the complexity of multi-processor solutions, each with their own programming language, that were commonplace a few years ago.  Multiprocessors have their own unique programming environments and a significant amount of programming to create proper interaction between the various platforms.

Missing from this description is the hard wired control that is part of system start up, power management and safety.  More on this in the next installment.

Considering the rules for servo tuning first, the ideas are basically simple.  Based on the use of 0-10V velocity command, the control system is designed to regulate motor speed.  A Proportional, Integral and Derivative gain value is used to “tune” the command signal sensitivity to allow the control system to regulate the motor and load performance.  This strategy has been created over the years and is used by almost all servomotor and drive suppliers in the motion control industry.

The Proportional term is the most important value in this approach.  The proportional value is generally the amount the 0-10V command signal will be increased in response to a following error.  The more gain, the more velocity will be commanded.  This will allow the control system to correct for changes in load conditions.  The proportional gain is how the system responds to current conditions.

The Derivative term controls how quickly the control system can add or subtract energy from the load.  The derivative can take the form of dI/dt or dV/dt depending on the specific controller.  This term has two important purposes.  As stated, it defines how quickly the system responds to changes in the load condition, and it exactly parallels the breakdown limit of the power transistors used in the motor amplifier.

The integral term provides correction on a cumulative basis.  All previous error information is integrated over time to provide the system with correction to the control command that “damps” reaction to disturbances.

Notice that all the gains are directly tied to time.  The faster the motion, the more P and D gain is needed to provide adequate response in the control.  Many motion applications have low enough dynamics that servo tuning needs to be very low performance compared to the capability of the equipment.

For most applications, and depending on the gear you are using, the best thing to do is start with I & D gains set to zero.  Use the amplifier autotuning for the motor without the load.  Sometimes these are default settings that are already loaded into the controller.  Tune the motor and amplifier until the P gain seems the best for a step input.  Then add the load and run the autotune again.  If possible, use a step response input that is similar to the type of move you will do in your actual application.

Then gradually add D gain until the leading edge of the step response has no overshoot.  If there was little overshoot without P gain, the motion dynamics are probably very slow and that’s OK.  With D gain set for the application, slowly add I gain and see if the trailing edge of the step response is improved.

If the axis is “hunting” after the motion stops, there is probably too much play in the mechanical system. Using a gearbox with a lot of backlash or a timing belt that is a little loose will produce just enough mechanical error that the servomotor will detect.

Tuning, like everything in motion control, is as unique as each individual application.  There are more complex analytical techniques that can be applied to the subject of tuning.  But I hold to the theory that the majority of applications can be dealt with using simple techniques.

Sometimes the motion control aspect of a process is not the primary objective of the control system or machine being considered.  The process of clamping or crimping a can lid onto a can body is an example of this situation.  The motion control system must locate the can lid to the can body correctly, but the final process is the crimping or application of a thrust force to cause the parts to form a strong joint.  In this case the real process variable is the pressure that is exerted at the end of the motion.  The pressure is critical to joining the parts, especially when the can is an igniter for an automotive air bag.

Grinding and polisihing is another example.  The motion control application is required to bring the grinder or polisher into contact with the work piece.  The actual grinding or polishing is the amount of friction generated between the grinder motor and part being worked.  This is actually proportional to the current of the grinding motor, which can be measured and regulated.  If too much current is detected the part might be ruined and the control system can be commanded to move the grinder away from the workpiece.

Important physical attributes of motion include inertia, center of mass and momentum.  There are no convenient sensing technologies that help us with these seemingly basic attributes of the mechanical system.  This is probably why they are ignored in the control system.

However, if the machine was designed in a 3D solid modeling environment, then things like center of mass and inertia are directly available.  A momentum profile can be created as a product of the center of mass and the duration of the motion profile.  This gives us mathematical information that can be used to “inform” the control system in spite of the absence of a control signal that directly measures these properties.

With this in mind one can easily imagine a pick and place mechanism made from two linear stages mounted one on top of the other.  When the two axis are making high speed coordinated moves, the reflected forces of the upper axis put loads on the lower axis.  The data from the solid model becomes useful information in providing mathematical “filters” that can improve the motion in ways that are beyond the current technology of motion control.

There are ample opportunities for improvement in the control of mechanical systems.  We should be looking for new strategies that the modeling and simulation environments provide.

The obvious thing to measure is motor speed.  That part is easy.  Servo motors have built in feedback devices. In the old days, the preferred feedback device was a small generator that produced a voltage proportional to the speed.  In the digital age feedback is by quadrature encoder that outputs a digital pulse that is primarily used for position control.  Most control systems are able to easily integrate the pulse train to derive the speed of the motor.

Unfortunately, most applications require relatively low speed.  Most motors are engineered for high speed.  This is in an effort to package more work related power in a smaller physical package.  Often, the motor is connected by pulleys or gear reducers to get the speed of the motor to more closely match the desired speed of the load.

Some of the important attributes of motion cannot be easily measured.  In addition to speed, torque is extremely important to controlling motion.  Torque can be measured directly from the drive electronics, but this is rarely used for control.

Torque and current are direct equivalents with a slight variation due to the temperature of the motor winding. As the temperature of the motor goes up, the resistance goes up and the current required goes up at the same time.  Since high performance motors have fairly high internal temperatures, this swing can be in excess of 100 degrees centigrade, and should be considered in the control scheme.

Most of the emphasis on current control is in terms of protecting the motor and drive electronics.  The first derivative of current over time  is the limiting parameter of the power electronic devices and is an important boundary condition in safe operation of the electronics.

More important information can be derived by considering the region of the motion profile and the current or torque requirements that are presented.  In order to accelerate a load, a lot of current is needed to overcome the mass of the load.  But once the load is moving the torque requirement drops off.  This creates an opportunity to profile the current requirement while using the conventional error detection scheme of the traditional control.

Other variable that are part of the mechanical system are things like momentum and center of mass.  In multi-axis mechanisms, there is usually a dependency of one axis upon another.  The idea that the mass of one axis is changing it’s center of mass and momentum with respect to the other axis is generally ignored.  This too is an opportunity to gain increased stability in the control and possibly improve throughput by having a better model of the application from which to create the ideal control.

Looks to me like there is a lot of room for improvement.  Let me know if you agree or disagree.