Matching Motor to Load – The first step in getting motion systems right

Motion control solutions are primarily mechanical in nature.  If the mechanism is right for the load, the motion solution can be designed to meet the project objectives without difficulty.  The starting point for this process is the selection of the motor.

In keeping with the mechanical nature of the problem, consider that across all of the possible fields of use for electric motors, very few of them occur at typical motor nameplate speeds.  Even in large fan systems where you would think you need high rpm, 1200 to 1800 RPM is a rarity.  60 miles per hour in a car is really only 800 rpm at the rear tire, depending on the tire diameter.  Most servo systems are designed at 4000 to 8000 rpm in order to increase the energy density of the solution.  Size restrictions abound in many types of equipment, so this approach makes sense in some markets.  But generally, motor speeds and load speeds do not match up very well.

This overall situation in the motor industry make the requirement for mechanical transmission of some type a necessity, and most often it includes gear reduction.  Gear reducers have been engineered over the years in many forms, complex, simple, low and high accuracy, low and high efficiency.  There are also belt and pulley reducers, clutch systems, and recently more exotic systems like magnetic couplings.  All with the intent of matching the motor speed to the required speed of the load.

The most significant contribution of gear reducers is the multiplication of torque output of the motor, or said in reverse, the reduction of the load torque requirement by the ratio of the reducer ( minus efficiency losses ).  Due to the relatively low cost of mechanical solutions, gear reduction is the most inexpensive way to gain torque.  In high performance systems, a torque increase comes at a relatively high price if it has to be derived directly from the motor and drive system.  This is based on the cost of power electronics and permanent magnets.

The other major effect of gear reducers is the reduction in reflected inertia of the load.  The neat thing here is that the inertia is reduced by the square of the ratio.  So a 10:1 gear reducer will reduce inertia by 100 times.  This is a huge advantage especially when high performance velocity regulation, as in multi-control printing, or precise positioning are required.

The feature of “controllability” comes from a combination of dynamic response in the current and voltage regulation between the motor and power electronics and the position feedback device.  The best way to know what “zone” of performance is required is to use the parameter “dynamic response” to gauge the behavior of the load.  This is an unequivocal measure that all manufacturers are able to reference for performance.  Unfortunately, dynamic response is not always the first parameter on a spec sheet, so it takes some digging to get to. But over a number of years of doing motion control projects, it is one of the key variables that can clearly distinguish where an application sits in the “performance continuum”

Servo Tuning

Making an electric motor operate correctly is as varied and complex as the millions of ways we use electric motors. Most of the time it’s just a matter of turning the motor on with a switch and letting nature take its course.  Nature in this case as defined by Maxwell and Faraday.

But when the application requires some level of precision it can be more complex.  And that’s where tuning comes in.  The difficulty with tuning is that there are almost as many different descriptions of what servo tuning is, as there are suppliers of servo motors and drive amplifiers.

One of the most significant aspects of electric motors is understanding the performance requirement in terms of time.  How quickly must the motor and control system respond to changes in the load?  This property of motion is called dynamic response.

As an example, if a conveyor is running unloaded using a gearmotor to control belt speed, whenever a product is added to the conveyor, the belt speed will momentarily decrease.  The specification for how much time will be allowed for the motor to recover its speed is the dynamic response.  The inverse of the time period of the load is the frequency response in Hertz is the value of dynamic response required for stable control.

Dynamic response is also important in defining what technology of motor and control is required for an application. Variable frequency drives have a basic frequency response of 10 Hertz.  So that sets the bar for a range of performance.

Variable frequency drives are also capable of greatly improved performance as the technology has migrated over the last 30 years.  By adding a feedback device, the close loop performance of a frequency drive can be as high as 200 Hertz.  Which is 50 milliseconds.

The time domain of frequency response can be translated to the position domain as following error.  When a closed loop system uses an encoder to measure motor or load position, time and position can be interchangeable measures of performance.

In this way, defining a certain number of encoder counts as an allowable error becomes a significant way to define when the system is properly tuned.  And since no system is “perfect” defining the following error as zero or one is likely going to lead to constant control system faults for exceeding the following error limit.

When it comes to “tuning” the performance of the motor and control, the motor and the load must be controlled together because the motor and control are required to convert electricity to torque.  So the “tuning” of the system is much more complicated.

One of the best descriptions is by George Ellis of Kollmorgen corporation.  His paper on tuning and autotuning is the most coherent explanation of all the unique elements of tuning a servomotor.  I highly recommend a thorough study of this paper in order to become familiar with tuning and its application.

Optimizing Motion

Optimizing a motion control system is not easy.  There are many tradeoffs that need to be considered.  The strategy needed for each situation tends to be unique based on the problem that is being considered.

One strategy involves speed, torque and time.  These three variables are a connected system that is defined as the mechanical work to be performed.  Speed torque and time must be optimized according to the priority of each variable.  The difficulty is in exploring three variables simultaneously.

Two variable tradeoffs are no problem.   So having a two variable strategy for three variable systems would be really handy.  And as it turns out you can consider any one variable as a constant for the purpose of optimization.  So its easy enough to consider time as constant and optimize torque and speed.

This strategy turns out to work really well.  The speed and torque requirement for a given actuator system was very well defined and had been prototyped with good performance.  But the customer needed to optimize the cost of the design.

The cost of servo systems primarily follows the power requirement.  The more torque needed, the more expensive the servo system.  So the first place to work on cost optimization is the motor drive combination.  If the torque requirement can be reduced, the cost goes down.

So how do we reduce torque? You can either trade off time, or look for ways to reduce the load inertia. If time is a priority, then reducing the load inertia is the way to go.  And reducing the load is most easily dealt with by material substitution.  Steel is pretty strong, but may not be needed.  Aluminum may be a suitable substitute at one third the density of steel but half the strength.  Strength is absolutely required and reducing the weight is going to generate big cost benefits, titanium may turn out to be the best choice.  I have had applications where titanium was the used, and it was the right choice.

Sometimes the materials substitutions can be significant.  It is possible to use plastics like polycarbonate as an alternate to aluminum.  Again, the material strength is lower, but polycarbonate is so light, that more material can be used volumetrically to achieve the necessary strength, and still end up reducing the weight of the load.  So this is a viable option.

In the project I am currently working on, we have reduced the weight of the actuator system by about 25%.   What was really amazing, and unexpected, was that the weight reduction and rearrangement of the load structure resulted in a 56% reduction in the torque required.  The motor and drive has been reduced from a Nema 34 motor to a Nema 23 motor.   And further improvements are expected.

Which is why I like my job.

Changing Landscape

Over the last few years there have been a number of changes in the cost of technology that are impacting the motion control marketplace.

The first is the cost of microcontroller technology that is dedicated to electric motor applications.  Up until recently, the Digital Signal Processor was the “de facto” standard for motor control.  Not because it was the the ideal solution for motor control, but because it was the only processor with sufficient bandwidth to handle the analog input and output requirements representing 3 phase motor voltages and currents and math calculations needed to regulate the motor as needed.

Doing motor control is one of the toughest applications for a variety of reasons.  The rate of change of motor data is 16 milleseconds at 60 hertz.  If the motor has 3 phases that are staggered at 120 degrees from each other, then three channels of 12 bit analog waveform data are being monitored as inputs in order to control a motor and the information must be handled with absolute precision at 5 millesecond timing.  That’s a lot of data before any control calculations are begun.

Recent generations of microcontrollers have emerged with the processing bandwidth, 50 megahertz processor speeds, 8 channel a/d and d/a, dedicated pulse width modulation channels for controlling power semiconductors, quadrature encoder inputs and even families with embedded network communications.  The communications capability does not impact processing speed of the code dedicated to the motor regulation algorithms.  This is because the communications are handled as interrupts and scheduled.  Which is also a weakness with a DSP.  DSP’s do not like to answer requests for information.

And the really good news is that some of the new processor technology is available at the $3 level at 10k pieces for controllers without communications.  Processors with communications are typically in the $5 to $6 level for comparable volumes.

At the same time power semiconductor prices are declining.  Power mosfets and IGBTs have dropped to half the price of five years ago.  The performance specifications have improved as well.  Typical peak currents are 200% of continuous rating.  So the overall performance is excellent compared to the power semiconductors in the past.

Thermal management is also getting good attention.  Some of the newer mosfets include thermal pads on both the top and bottom of the chip.  This can potentially double the thermal performance of the fets in a motor control application.

The other big cost factor in motors and controls is the number of connectors needed. Brushless servos require power, hall effect sensors and feedback devices.  This puts a huge cost burden on the system, sometimes as much as 10 to 20% of the total price. Which has lead to a significant number of motor, drive combinations which eliminate the cabling costs.  The tradeoff is the overall peak torque, but for many applications, this is fine.

More choices, better prices mean more options for the motion control enthusiast.

Fast, Faster, Fastest – Or Not?

Speed is relative.  Especially in the world of industrial control.  1 millisecond look ahead features in the machine control world used be considered “cutting edge” (pun intended).

The programmable controller, the standard of industrial control, has a speed of execution metric.  It is generally specified in thousands of instructions per millisecond.  At 1000 instructions every 2 milliseconds, for example, it is easy to assume that there are no applications that will be a problem.

Programmable controllers, PLC’s, are the very essence of dependability.  In fact, they are one of the few controller technologies recognized by all the major safety agencies.  PLC code execution is very robust, and in recent years, has even migrated to fault tolerant systems that are the next level for high reliability.

As these systems have migrated to faster and less costly processors, users have benefited from falling prices and increasing performance.  Enhanced features like Ethernet communications, math functions, importing values from Excel spreadsheet, and even motion control has made their way to the PLC platform as a common industrial hardware solution.

However, when you add motion control to any control system application, you must ask and answer the question, how fast is fast.  How “real time” does my solution have to be in order to behave correctly?

This actually gets down to the level of understanding digital sampling and analog to digital conversion.  You can look at a 12 bit  analog command signal and think that 4096 increments of the command voltage to the drive is plenty of resolution.  But you could also be wrong.

If the drive is a servo, it is entirely possible for the 2 kilohertz sampling rate of the drive to catch the little steps between output values and actually try to follow the step function, which was never intended.  This situation can cause current inrushes to occur which will either cause nuisance trips in the drive, or gradually cause the drive to shut down from overheating.

Think its far fetched?  Not at all.  It actually happened to me during a project on a PC board plating line at Hewlett Packard some years ago.  The fix for this might be output smoothing of the analog command, but that function doesn’t exist in PLCs.

But one of the critical aspects of insuring the reliable performance of the PLC also creates some problems for time sensitive  applications.  PLCs update all of their inputs first, execute all of their logic and then set all of their outputs. So latency is built into the process intentionally to protect the application from certain types of failure.

If you have to read a value from a sensor, the analog value has to be stored in a register. Then it has to go through a read cycle in order to be retrieved and operated on by the math calculation in the program.  So there can be several clock cycle of difference between when the data was read, when the program calculates a value, and when the output value is sent to the output to be updated.

These little timing anomalies creep into the execution and can appear entirely random and very difficult to de-bug.  And often you don’t know ahead of time that the problem is going to affect your project.  Until it’s too late.

Industrial PCs, as an alternative, have migrated from the old 25 megahertz 486 to the current 1.8 gigahertz Celeron chips.  These systems run hundreds if not thousands of times faster than PLCs.  Often for industrial application the Operating System can be Linux to increase reliability.

So ask yourself how fast your system really needs to be to meet its requirement.  Sometimes the fastest PLC on the block isn’t going to be the right choice.

Motion Feedback, Still Changing

The only constant in the world of motion control seems to be change.  One of the most crucial aspects of motion control is the feedback mechanism.  After all, if we can’t measure where we are, we cannot control it.

The first order of business is to estimate what feedback resolution will be needed for the application.  More resolution than needed can create serious instability issues in the control system.  High resolution feedback also requires greater bandwidth in the controller, which will cost extra.  This makes it worth your while to be sure what that you are buying just what you need for the application.

A subtle aspect of the feedback device is the minimum pulse width of the square wave.  When you get to 250 kilohertz feedback frequency, the minimum pulse width of the signal is 4 microseconds.  Which is kind of quick.  So another parameter to check on the controller is the minimum pulse width of the input so you don’t end up in missing pulses accidentally when a certain speed is reached.

Most applications depend on the feedback attached to the servo motor as the reference for position.  This is not always the best case.  Even precision gearboxes have some backlash in them.  So resolution requirements should dictate where the feedback is applied physically.  Many rotary applications require a second feedback device to be attached directly to the load.  Linear applications get a real boost from the use of a linear feedback sensor that can provide an independent reference for the linear axis.  Again, this requires the “axis and a half” input to be supported on the controller.

But best of all, manufacturers are proliferating some excellent choices in the feedback arena.  Optical encoders have been the staple technology in the past.  But now there are Hall effect magnetic feedback systems from numerous vendors.  In addition there are now capacitive feedback devices from companies like CUI Stack and others which offer programmable feedback features and superior environmental resistance.

Linear feedback devices are proliferating as well.  Linear feedback devices were primarily available for high resolution semiconductor applications.  The high resolution millionth of an inch feedback, required by the semiconductor industry was simply overkill in most industrial situations.  And controller that could handle the 20 megahertz data rates that were generated by high speed linear motors were rare and again, expensive.

Recent product introductions from vendors such as Turck make linear tape scale feedback devices available with resolutions from .004″ to .0001″.  This is ideal because the predominant feedback requirement in general purpose motion control is  in this exact range.

So progress, while sometimes slow, has been made.  There are a lot more choices available to the controls and mechatronics engineer.   Let’s take advantage of that and try to make the best choices for the best solutions we can come up with.

Motion Control or Mechatronics, What’s in a Name?

Are we talking about Motion Control? or are we talking about Mechatronics?

Words are important in describing reality and communicating with others.  The term Mechatronics was coined a few years ago, possibly from the frustration that the term Motion Control does not adequately cover the subject.  It is, however, a made up word. And as a result, its real meaning is somewhat controversial.

The complexity is in the “mecha” part.  The idea is that the we are trying to include all things mechanical.  And that’s where it opens the door to everything and anything.  Mechanical engineering can include hydraulics, pneumatics, bearings, cams, gears, springs, shafts, materials, you name it.  The fact that an almost endless list of technologies makes up the field of “mechatronics” dilutes the meaning of the term.

So I would like to assert my original definition based on the old term ” Motion Control”.  And, by the way, I am OK with the use of Mechatronics interchangeably with Motion Control, but I will also suggest that the two should be identical.  What we need is more clarity about the subject.

Motion Control is the combination of three core subjects, mechanics, electronics and electrical, and control.  And while the topic incorporates all disciplines, the relationships between the three are extremely important to keep in mind.  The mechanism that is to be controlled is exclusively mechanical in nature, regardless of the means of motive power. And the goal is exert control over the mechanism.  The means of that control is usually electronic and electrical in nature.

The mechanical system sets the boundary conditions for what is possible from the perspective that the physics of mass and inertia (F=ma) cannot be manipulated by the control system.  There is, however, a separate relationship between time, torque and inertia in which various tradeoffs can be explored to improve a given system.  More on this in another post.

John Eidson is attributed with an insight regarding the nature of real time control, although it was in the context of communications.  Everything you need to know about the system you are trying to control is contained in the description of the system to be controlled.  To this I will add, that in order to control something, it is necessary to describe it sufficiently well that the control system can achieve its objectives.  The “magic” in the control system is in how well or poorly the mechanism is represented. If you can’t describe it, you can’t control it.

In the case of motion control or mechatronics, the ultimate goal is performance or behavior.  This means that the control system will be the ultimate determinant of how the system will operate.  But it also means that when control system strategy is developed, an in-depth understanding of the desired behavior is required.  All of the behavior being modeled is based on mechanical equipment, gears, cams, common power takeoff shafts, rotary indexing features, are all based on real world mechanisms.  And with a better understanding of the mechanical context of mechatronics, better solutions are will result. And better solutions benefit everyone.

Accuride Expands Heavy-Duty Slide Options

Accuride presents models 7950 and 7957, two new slides designed to expand the company’s heavyduty slide line-up and provide an intermediate load rating choice between the 3600 and 9300 products. The 7900 slides offer full extension, a load rating up to 350 pounds, and accommodate drawers up to 42″ wide. For added versatility, the slides may be flat mounted at a lower load capacity of up to 150 lbs. per pair.

Designed for applications such as mobile toolboxes and utility vehicle storage, the 7900 slides are a good drop-in alternative for .75″ side space projects, giving fabricators additional slide options for midrange loads, while increasing the aesthetics of the finished product.

Model 7950 does not offer a disconnect method, while Model 7957 has a handed lever disconnect. Both models have an enhanced ball retainer design, which mitigates retainer migration. Two special order models in a smaller cross section of .724″ are also available: models 7930 (non-disconnect) and 7937 (handed lever disconnect). All products are available in lengths of 12″ to 36″ in a clear zinc finish.

Accuride International
www.accuride.com



Out Of The Gait: Robot Sets Untethered ‘Walking’ Record

The loneliness of the long-distance robot: A Cornell University robot named Ranger walked 14.3 miles in about 11 hours, setting an unofficial world record at Cornell’s Barton Hall early on July 6. A human – armed with nothing more than a standard remote control for toys – steered the untethered robot. Ranger navigated 108.5 times around the indoor track in Cornell’s Barton Hall – about 212 meters per lap, and made about 70,000 steps before it had to stop and recharge its battery. The 14.3-mile record beats the former world record set by Boston Dynamics’ BigDog, which had claimed the record at 12.8 miles.

A group of engineering students, led by Andy Ruina, Cornell professor of theoretical and applied mechanics, announced the robotic record at the Dynamic Walking 2010 meeting on July 9, in Cambridge, Mass.  Ruina leads the Biorobotics and Locomotion Laboratory at Cornell. The National Science Foundation funds this research.

Previously, students in Ruina’s lab set a record for an untethered walking robot in April 2008, when Ranger strode about 5.6 miles around the Barton Hall. Boston Dynamics’ BigDog subsequently beat that record.

One goal for robotic research is to show off the machine’s energy efficiency. Unlike other walking robots that use motors to control every movement, the Ranger appears more relaxed and in a way emulates human walking, using gravity and momentum to help swing its legs forward.

Standing still, the robot looks a bit like a tall sawhorse and its gait suggests a human on crutches, alternately swinging forward two outside legs and then two inside ones. There are no knees, but its feet can flip up – and out of the way, while it swings its legs – so that the robot can finish its step.

Ruina says that this record not only advances robotics, but helps undergraduate students learn about the mechanics of walking. The information could be applied to rehabilitation, prosthetics for humans and improving athletic performance.

Cornell University
www.cornell.edu

Igus Develops A Simpler Robotic Bionic Joint

When it comes to robotic joints, engineers have had to put together complex custom configurations out of multiple components, which involved considerable development time with the mechanisms. This time requirement often reduced the amount of time artificial-intelligence programmers had with the system. So, two goals of robotics developers were to enable the programmers to be involved with the process earlier and develop a straightforward modular system. The realization of these goals is closer, thanks to a recently introduced innovative robot joint module system from igus. The company is currently looking for beta testers for this new product.

The robot unit, known as Robolink, was primarily designed for robot developers and laboratories that work with humanoid systems, as well as with lightweight engineering systems for handling and automation. The design was inspired by Dr. Rudolf Bannasch, Managing Director at the Berlin-based company EvoLogics, a high-tech company working in the field of bionics and humanoid robots. He provided both the motivation and developmental support behind this Robolink component.

It consists of a drive-and-control unit, joints in different lengths, and arms in different sizes, including a duct for additional control cables. The jointed arms are made from carbon fiber reinforced plastic and other lightweight materials. At the end of the jointed system is the option to connect to different types of tools.

The drive-and-control unit was purposely designed as a black box. Robot developers have the option to work with pneumatics, electro technology, or hydraulics.

The bionic core of the robot’s skeletal parts is the injection-molded plastic joints. They are controlled through cable pulls that transfer tensile forces, similar to the way tendons function in humans. The cable sheath is held and the inner cable moved. This way, the gripper, shovel, hook—or whichever tool the developer chooses—is moved and operated.

All data cables are routed safely through the jointed arms. The cable pulls are routed through from one joint to the next—just as joints are connected in humans. Only four cables are required for each plastic joint to rotate and swivel freely. These cables convey images, acoustics and forces, which are the artificial senses of humanoid robots.

The cables themselves are made from technical synthetic fibers. The fibers are extremely strong, hardly stretch at all, are resistant to chemicals, and are lubrication-free and wear resistant. When compared to steel, their lighter weight also makes them much more energy efficient.

Since the system is modular, it can be constructed with all kinds of humanoid robot configurations. This ranges from jointed arms, moving ‘digger’ arms, through to four-legged ‘creatures.’ The joints can be easily combined as required.

igus’ development objective was to keep the moving mass as low as possible, so that the actuators can be separated from functioning tools, such as grippers, hands, suction cups, and so on. Particular attention was given to enable quick assembly, as well as the use of tribo-optimized plastics to reduce lubrication needs and weight.

igus
www.igus.com

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