Unique Solutions in Material Handling

Moving products around is mechanical work.  When the work is done by a control system and actuators its mechatronics.  Mechanical work, whether by humans, by horses, by hydraulics, electrics or whatever, is still work.  Figuring out what technology approach will be the most cost effective way to get the work done is the challenge.

Many of the constraints of the work are environmental.  If work is being done outdoors, then temperature and humidity are a factor.  Felling trees and in the forest requires extremely high forces due to the work needed to cut through a tree and drag it to a truck to be hauled off for processing.  Processing trees, even in a plant environment, requires some serious hardware, 125 horsepower band saws are not unusual.

Doing work on a ship or oil rig has additional constraints because of the presence of explosive fumes and fuels.  Often the need to avoid any possibility of igniting a combustible atmosphere causes engineers to apply pneumatic control systems.  Yes, there is still a compressor somewhere to generate the compressed air supply, but that is usually remote or contained to avoid exposure to the volatile atmosphere.

Environmental constraints come in many forms.  Extremely high temperatures push the limits of what is possible.  Making glass, semiconductors, and primary metal processing are all high temperature environments where engineers have developed whole technologies in order to bring us the materials we use in everyday life.

The simplest action of rolling or sliding becomes a real challenge when environmental constraints are added to the work statement.  Sawdust becomes a potential abrasive in woodworking environments that can introduce severe wear in moving parts.  Corrosive and explosion proof atmospheres as well as food industry applications introduce all sorts of chemical compatibility problems that require special materials and processes in order to meet strict guidelines for safety.

As always, resourceful engineers have worked out solutions for all of these difficult applications.  One family of solutions to rolling applications is the use of all ceramic bearings.  No steel, no lubrication.  None is needed because the ceramics are extremely high purity to start with and have extremely high precision surfaces eliminating the need for lubrication.  No outgassing or contamination to worry about.

Other solutions take the form of air bearings and non-contact material handling devices.  Air bearings have become more readily available for conventional applications, but are particularly compelling in large machinery applications where precision is required.  Large flat screen display glass  presents unique challenges that successfully addressed using a combination of air bearing regions and vacuum regions to move the glass without actual contact and with overall flatness measured in millionths of an inch.

A unique solution in pneumatic material handling takes compressed air driven into a funnel shaped recess and creates a vacuum in the center and an air cushion at the edges where the air is exiting.  This creates a vacuum pickup that never quite comes in contact with the part, leaving no marks.  Perfect for solar cell and some food and beverage applications.

Engineers continue to meet the unique challenges of industry and create commerce at the same time.  And that’s what it should be about.

Energy Saving and Automation

In an era where energy costs have become a focus of attention, many people have authored articles with reducing energy as their theme.  Saving money is always a good thing.   Perhaps we can gain a little clarity on where the real savings are.

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.

Servo Tuning

There are many opinions about servo tuning.  Every engineer who has ever worked with servo motors has experienced the difficult process of tuning the motor.  The difficulty is in the fact that the rules about tuning are hard to apply, because every situation is a little different.

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.

More Measurement and Motion

For all the measurement technology we have available there are some elements of motion control that are generally missing.  We have laser interferometer measuring tools that are accurate to a fraction of 1 micron.  There are rotary position sensors that can divide a circle into a million digital positions. Many of the semiconductor industry’s processes would not be possible without the incredible advances of measurement technology.

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.

Motion, Measurement and Control

Motion control is all about control.  But you cannot control what you cannot measure.  So there is an important measurement component to the control of moving systems.  The difficulty lies in knowing what to measure, how to measure and what to do about things you can’t measure.

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.

Complete Control

Torque is equal to current when it comes to electric motors.  When sizing the motor and drive circuit, the discussion usually revolves (pun intended) around the torque of acceleration.  This is because the torque required to accelerate the load is generally the largest component of the load requirement.  However, there are two aspects to the torque requirement that should be considered.

First, the total torque required is really made up of three components.  Torque of acceleration, frictional torque and torque required to overcome the inertia at constant speed.  In most cases the frictional torque is small and is often ignored.  The torque required to keep the inertia mass moving at constant speed is often a fraction of the torque of acceleration and is taken into account in servo sizing software, so it is not considered separately.

Second, what makes the torque of acceleration so important is that the formula is divided by time.  So as the time allowed for the load to move decreases, which is usually what we’re trying to do in motion control, the torque required to accelerate goes up arithmetically.  This is why the torque of acceleration dominates the discussion when evaluating motor requirements.

Everything in the control system is oriented as a PID controlled velocity loop, and the other major control loop, current, is being ignored from the programming standpoint.  Of course current regulation is performed in the drive circuit between the power electronics and the motor winding.  This is required to prevent damage to the drive circuit. But current control has no place in the programming of the trajectory.  This is an oversight that needs correction.

The immediate problem is that we don’t have a good rule base to apply current control to trajectory planning.  This is however, a great opportunity to improve how motion control is done by the entire industry.  Some simple rules come to mind that might demonstrate beneficial results.

What would be the impact of knowing that the trajectory can be divided by the sign of the acceleration?  Simply knowing that acceleration is positive, negative or zero would permit better regulation of the load.  Knowing that the acceleration is increasing or decreasing has similar potential benefit.  If the acceleration of the load leads to a period of constant velocity, then as the acceleration is performed, there is an inflection point where the acceleration force starts decreasing to reach the torque that is required to maintain constant velocity.  This approach suggests that acceleration could be dynamically controlled through current and achieve a move profile with little or no overshoot using no gains whatsoever in the control system.

The control system of the future will achieve superior performance because the control model makes use of both speed and an torque to move the load.  A more complete model should lead to more realistic control with better performance.  That’s what this industry is all about.

Torque and Motion

Torque isn’t just torque when we’re talking about motion control.  It’s another one of those subtleties of the field of mechatronics that requires consideration when you are doing a new project.   With retrofits there are different rules. It is possible to use the motor as a sensor by monitoring the current over time which will reliably tell what is going on at the load.

The most important aspect of torque is the proper description of torque from the mechanical perspective.  Torque has three mechanical aspects, the torque needed to overcome the friction of the load, the torque needed to keep the inertia mass moving, and the torque of acceleration.

Each one of these has to be considered.  They also have to be considered in terms of their relationship to the total amount of torque required from the motor and drive combination.  Generally we consider the frictional torque to be a small fraction of the requirement and sometimes we can ignore it altogether.  But this is a mistake.  I have had some complex conveyor and material handling fixtures in which the friction was the most significant part of the load.  And it was ignored and caused all of the servo sizing to have to be increased significantly.  A very expensive mistake.

What makes the torque requirement so critical is that the torque needed to accelerate the  load is a complex calculation that has the change in the time in the denominator of the formula.  This means that as the time required for move decreases, the torque required increases arithmetically.  This is why acceleration is generally the main consideration in sizing servo systems.

The electrical component of torque is that torque is current.  So the amount of current that is required from the drive amplifier must be correctly sized or there won’t be enough torque available to power the load.  The calculation uses the torque constant of the motor which is expressed as ounce inches of torque per ampere of current.  But this calculation doesn’t consider the rate at which torque needs to be applied to the load in order to achieve the desired move times.

Coincidentally, the rate at which power can be added to the load is also the breakdown condition of the power transistors that are used for drive amplifiers.  In very high speed applications, this value also has to be considered in terms of the thermodynamic implications.   Using a bigger amplifier will allow you to push more current through the motor, i.e. more torque, but you have to have dwell time for the motor to dissipate the heat that is generated.  So there’s no free lunch here.

Putting the “Mecha” into Mechatronics

Motion control, machine automation, mechatronics, actuator technology, whatever you want to call it, the thing you have to keep in mind is that in motion control, tasking is mechanically bounded.  The boundary conditions of the system are defined by the mechanical solution.  It is generally impossible for the control system and power electronics to cause a large inertia mass to behave like a small inertia mass with high acceleration rate and high frequency dynamic response.

There is a lot that can be done from the control system to improve overall performance.  Some of that improvement takes the form of sophisticated control algorithms.  Some improvement may take the form of increased power electronics to provide added acceleration and deceleration capability.  And sometimes projects get scoped to do things that are marginally impossible.  This is because there is gap between our understanding of the difference between mechanical and electrical properties of the application.

The gap in understanding is partly due to the educational system that teaches mechanics and electronics separately.  With the newer mechatronic programs that are available at many schools this gap is decreasing.

But there is a deeper issue.  The issue is context.  Mechatronics combines electronics and mechanics.  The missing context is that everything we do in the electrtronics, is an  analog for something mechanical.  If you don’t know how the mechanical relationships work, it’s unlikely that you will get the best results in controlling the system.

There are all sorts of examples.  A belt and pulley reducer is a mechanical system that turns a high speed input to a lower speed output.  The analogy in motion control is electronic gearing.  We can arbitrarily program any given motor to follow a speed reference from an encoder, tachometer or other motor, and follow the input signal at a programmed speed or ratio.

Electronic line shafting is a similar application that seeks to operate several independent motors and loads as if they were connected on the same mechanical shaft.  This application requires very high angular precision between the following loads which requires high speed regulation between the motor and associated electronics, but mathematically is very simple.

From the mechanical standpoint moving something from point a to point b seems pretty simple.  But as the time requirement for the motion decreases, the forces acting on the system become very significant.  As the load is accelerated from rest, it gains momentum.  Overshoot at end of the acceleration profile is a property of the momentum or kinetic energy of the system.  Tuning the system properly involves understanding the mechanical properties of the load and specifically how the control systems relates to the mechanics.

Does overshoot even matter?  Not so much if there isn’t a following axis involved.  At the end of travel it may be a concern if the load is positioning under a fixture or a next mechanical operation.  In many systems, there is settling time required for the load to come to a complete stop.  In these applications, the “S Curve” is an ideal solution because it compensates for the mechanical properties of optimizing the acceleration so that little or no overshoot takes place.

Maybe it’s less about the tuning and more about intelligent trajectory planning.

Units of Measure

My first semester of physics was a disaster.  I really didn’t get it.  The basic forces of motion made sense, but I always had difficulty solving the problems.  I was interpreting all sorts of other issues that weren’t really solving the problem in question.

A major lesson that I came away with was imparted during one of my attempts to repeat the course with a passing grade. (It took me 3 tries before I passed)  ”Always pay attention to the units of measure when you are trying to analyze a problem”.  In the problem solving arena, trying to figure out the relationships is sometimes a little easier when you just look at the units of measure.

In energy its really important to understand the units of measure.  Electrical power is measured as Watts.  Watts can be used as a measure of electric light, electric heat or any form of energy which can be directly derived from electricity.   As we seek to conserve power it is important to consider how efficient it is to convert electricity to another form of power.

The often ignored unit of measure is time.  It is especially important in energy measurements because the ratings of equipment can vary dramatically between starting conditions and running conditions.  Most motors and heaters have a significant inrush current when they are first switched on and then settle over time.   The ratings need to be examined more closely to get this information.

So a 100 Watt light bulb left on for 10 hours is one kilowatt hour.  The cost of a kilowatt hour varies around the US, but here in Texas its around 11 cents.  A computer with a high resolution video card can use 400 Watts.  In this case we are exploring the “dollars per kilowatt hour” for various appliances.  A large refrigerator can use 500 Watts, which is substantially better than older units with ratings over 1000 Watts.  And a central air conditioner rated at 5 tons can run as high as 17,585 watts or $2.00 an hour when running at full load.

When comparing electric motors, regardless of type, Wattage and duty cycle are consistent units of measure for the power that can be produced by the motors.  In this arena what is really being measured is the amount of work that can be performed within the thermal limitations of the motor and drive.  This is really important to keep in mind because at the end of the day it’s all about the amount of work required and the cost of the solution.  The units of measure might be $/kW or $/inch pound of torque.

When comparing motor and drive hardware from various suppliers, make sure the units of measure are the same (which they usually aren’t) and the thermodynamic basis for the ratings is similar.  The exact size of the cooling plate, what material its made of and how long the motor is run for thermal testing is all part of the rating system.

Some years ago I was confronted with a torque comparison between a 4″ servo from one vendor that claimed to put out the same torque as a 5″ servo from another vendor.  After considerable effort, an associate came up with the real answer.  The smaller servo was rated at a thermal limit 50 degrees higher than the larger one.  Although ratings are generally not so disparate in today’s market, there is still plenty of variation that has to be considered.  So make sure you know the units of measure.

Priorities Continued

Its tough enough getting a new piece of machinery built and working.  Turning out the same product reliably can be a challenge.  Even something as simple as a V belt becomes a lot more complicated when you have to turn parts out by the thousands or tens of thousands.

Getting it done on time and in budget are basic requirements of the challenge.  Over the years the only thing I have learned is that things always take more time and cost more than what was planned.    Good planning rarely eliminates mistakes or discovering an unforeseen problem late in the project that can jeopardize the whole effort.

The fix, if there is one, is building some slack in the project work plan.  So if it is at all possible, plan for 10 percent of the time and money to be set aside for contingency.  This will allow room for corrections to take place within the project plan and help prevent delays to the project deliverables.

Among many competing priorities in machine development, one that is rarely discussed is life expectancy.  Machine life expectancy is more subjective because component selections are not simple and straightforward.  Many decisions will be based on judgement and experience rather than on an explicit technical basis.

Something simple like a pulley may be perceived to be more durable if made of steel instead of aluminum.  But that component decision will increase the load mass.  The increased load will then require a larger motor and drive to power it.  And there are hundreds of similar choices that have to be made just like this.

Sprocket and chain mechanisms are generally indicated when high loads are being powered.  But in a situation where high life expectancy or reliability are involved, may be preferred over belts and pulleys.

Mechanical components of this type are subject to significant variations in cost versus life expectancy. The basic metallurgy of parts will impacted.  In one project, the balancing of load versus durability lead to the use of titanium.  The increased cost of the material was offset by reduced cost in the motor and drive solution.

Plating and coating of parts can be a significant opportunity for improved life.  Nickel plating has excellent performance in low friction, corrosion resistant coatings.  There are a number of hard Teflon coatings that can be used for reducing friction.

The life expectancy question is also connected to the maintenance cost of equipment.  For new machine designs, the maintenance issues are largely unknown and are part of the learning curve.  But the reliability of any piece of equipment is a huge issue.  And a lot harder to engineer in a systematic way.  But always worth the effort.

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