Centralized and decentralized drives the pros and cons

Venerdì, 31 Luglio 2020

www.servotecnica.com

Centralized and decentralized drives the pros and cons

Executive summary

This white paper sets out to identify the pros and cons associated with centralized and decentralized drives for motion control applications. The intention is not to favor one approach over the other, as in many cases each will have applications to which they are best suited. Instead, the idea is simply to make machine builders and engineers aware of the specific advantages and disadvantages that can help bring about project success.


Introduction

Adopting a centralized strategy means that the drive, along with all of the other necessary motion control components, is housed within a cabinet, the likes of which can be seen alongside machines and production lines worldwide. In contrast, taking a decentralized (or distributed) approach sees the drive technology relocated from the control cabinet to within far closer proximity of the motion control process – sometimes even integrated with the actual motor itself.
Decentralization can of course occur on many different levels, from an individual soft starter or drive located at the motor, to an entire decentralized system, which may comprise, for example, a VFD, overload protection, motor disconnect switch, I/O and bus module. In many cases, all of this equipment can be provided as part of a single package from one supplier.

distributed drives example servotecnica

Centralized and decentralized drive architectures have their virtues and drawbacks. Deciding which is best depends very much on the specific application. Indeed, mixed architecture systems are fairly commonplace within industry, especially when drives exhibit some commonality of features, thus demonstrating the co-existence of both approaches.

The following chapters outline some of the most notable pros and cons associated with centralized and decentralized drive strategies, focusing on areas that include cabinet/control panel size, application sizing, options, and modularity.


Cabinet/control panel size

As is often the case in an increasing number of industrial applications, space is a limiting factor. With a centralized approach – if space permits – then all of the drives can be located within one cabinet, simplifying diagnostics and maintenance. Similarly, it might be possible to centralize certain services, such as cooling, power distribution, and security. Clearly, performing visual diagnostics is also far easier when adopting a centralized motion control strategy.
Of course, all of this centralization comes at a price: the footprint or dimensional envelope of the cabinet increases. With many factories and plants compromised for space, machine builders are increasingly less keen on bulky control cabinets. Being able to promote machines with a ‘compact footprint’ has become a major USP for OEMs in recent years. There may also be a size issue when industrial facilities look to upgrade or extend existing machinery, perhaps to eliminate a bottleneck or boost efficiency, for example. In such situations, engineers often find that space is limited or reserved for capacity expansion in the future.
Another key factor here is that the cabinet control panel represents a significant cost in terms of material and labor, as it is typically engineered specifically for the application. Therefore, the cost associated with the design, build, and installation of a centralized system has to be a major consideration in overall machine expenditure.
Housing the drive systems in a cabinet does of course offer full protection from the external environment of the plant or workshop. However, as heat loss is generated centrally, effective cooling is required inside the control cabinet.
When deploying a decentralized drive strategy, the dimensions of the main electrical panel are typically extremely compact, while set-up costs are lower in terms of both material and processing. Another advantage is that distributed drives are wired with cord sets to reduce the possibility of error and shorten commissioning time.
Naturally, a decentralized approach is particularly suited to simple stand-alone motor control, but not exclusively. As the drive can be mounted on or near the machine/motor, the benefits of size reduction are clear to see in comparison with an equivalent centralized system. Reduced costs also result in thanks to eliminating the need for a customized control panel, not to mention the manpower required for system assembly and installation. In addition, wiring lengths are reduced, while further benefits include improved EMC behavior and the widespread distribution of heat loss, reducing the requirement and cost of a centralized climate control system.
While some assume that this type of architecture is insufficiently robust to provide a high degree of safeguarding against the surrounding environment, the contrary is in fact true. Many of the latest decentralized drives offer embedded features and a high level of IP66/NEMA 4X protection to permit installation directly on the motor or nearby. The rugged design of decentralized drives can guard against penetration by dust or jets of water and offer protection for technicians and other personnel against contact with live parts.
There are few drawbacks with decentralized drives in this area, although visual diagnostics and maintenance are sometimes more complex because the drives are often located in places that are difficult to access.

Cabinet/control panel size Centralized Decentralized
Footprint/Dimension  
Centralize service ✓   
Visual diagnostic  
Cost of material     ✓
Cost of labor    ✓
Accessibility   ✓  










Application sizing

A major advantage of centralized solutions in terms of application sizing is that motors are not subject to any derating. To provide a commonly used definition, any adverse operating conditions require that the motor performance be derated. Such conditions can include ambient temperature above 40°C, motor mounting position, drive switching frequency or the drive being oversized for the motor.
Derating is a design process that can make a significant contribution to reliability. With a centralized approach, smaller motor dimensions and lower rotor inertia are typical, while the achievable performance is higher than decentralized solutions. With regard to shortcomings, those opting for a centralized solution need to take longer cable lengths into account, not forgetting that each motor is connected to the cabinet via two wires, one for power and one for feedback.

Anyone thinking that decentralized motion control solutions do not lend themselves to optimized application sizing would be mistaken. For example, in certain cases, it is possible to employ a standard cabinet for basic functions and add optional independent modules (with their electronics on-board) without having to modify the original cabinet.

In some instances, the use of decentralized drive-based control may be necessitated by machine size. It is possible to eradicate long motor cables from a central control cabinet by bringing power to the decentralized drives in a daisy chain, drive-to-drive fashion, or by using a drive with an integrated power supply. Furthermore, decentralized drives can enable even large and complex machines to be more clearly structured, a particular advantage in applications for sectors such as automotive and intralogistics, for example.

There is, however, one potential handicap for decentralized solutions in regard to application sizing. In motors with integrated drives, a derating of the motor due to heat exchange with the drive must be accepted. To counter this issue, for the same T,n performance, the motor with the integrated drive will be bulkier in design (T,n is the relationship between rotating speed and torque). It should be noted, however, that this requirement is not necessary for nearby decentralized drives.

Application sizing Centralized Decentralized 
Derating of Motor ✓ (with nearby)
Motor cable length  





Options

When it comes to options, centralized solutions tend to score best due to the greater customization of modules inside the control cabinet. This type of framework solution grants the potential to add options in the field at a later date. Although the most common options – such as STO (Safe Torque Off), Safety Bus, I/O and real-time Ethernet protocols – are available for decentralized systems, they are limited in comparison with framework solutions due to restricted space. Typically, options are installed by the OEM and it is generally not possible to add or remove them in the field.

Options Centralized Decentralized 
Safety via Bus
Scalability of the options  





Modularity

Centralized systems are seen as less modular than their distributed counterparts, even though there is more flexibility regarding heat dissipation systems based on air or liquid cooling. Generally, the modularity of the control panel is not related to the modularity of the overall system.

In contrast, decentralized configurations are extremely modular. Here, the architecture of the drives can follow the mechanical modularity of the machine or system, with the clear advantages this brings.

It is probably fair to say that today’s machines and factories are increasingly created based on modularity, predominantly because modular systems facilitate reduced development costs and shorter delivery times. Ultimately, it would appear that the industry is looking to realize its options without expanding its control cabinet.

Decentralized drives can be located where they are needed and, thanks to integrated I/O, can solve demanding tasks without adding terminals, saving time and money.

Modularity Centralized Decentralized 
Modularity   ✓ 
Flexibility dissipation system  





Networking capabilities

Many of the latest drives offer optional communications networking and I/O modules that are fast and easy to install, thus allowing adaptation of the standard drive to individual user applications. Plug-and-drive communication via real-time Ethernet networks is also increasingly commonplace with today’s drive technology. The use of plug-in interfaces for protocols such as Profinet and EtherCAT permit the seamless integration of drives into existing communications networks at the end-user site.

For those considering a distributed approach, using a decentralized PLC module alongside decentralized drives reduces the load on the higher-level controller and can even, in certain applications, create the basis for modular machines that are truly free of control cabinets. Such PLC modules will typically feature RTOS (Real-Time Operating Software) to ensure decentralized intelligence with networking capability provided by a selection of communication protocols. The upshot is that design flexibility and suitability for modular machine construction is assured.

Networking capabilities Centralized Decentralized 
Performance of PLC  
Comunication ethernet RT





Investment costs

It is difficult to draw a direct savings comparison between centralized and decentralized drive solutions, not least because every application is different. In certain straightforward, isolated cases the decision between centralized and decentralized can be clear-cut. However, evaluating the options for a production line, where every workflow step is dependent on other devices, means that the decision becomes considerably more complex.

Some who support or promote purely distributed systems argue that 30% or more can potentially be saved against the comparable cost of a centralised solution, which can sometimes entail more engineering time, more components and wiring, larger panels and PLCs, and slower installation and commissioning.

However, always check with a drive technology specialist for specific application advice as each project has its own requirements that can affect decision making for those seeking an optimum result.


Product solutions

Among the extensive number of product solutions available from Servotecnica is the AMK iC/iX series of distributed brushless drives. The decentralized AMKASMART iC servo drive with integrated power supply, for example, is optimized for use in single-axis applications and modular machine structures. Thanks to the integration of a power supply module, the need for a control cabinet is practically eliminated to facilitate a flexible machine solution.

The AMKASMART iX decentralized servo drive, just like the iC, is designed for rotary and linear synchronous and asynchronous motors of various kinds. However, in this case, the power supply and communication are looped from module to module.

With AMKASMART drives it is possible to combine the various distributed solutions of AMK on the machine or production line, and therefore have the possibility of choosing between decentralized and centralized solutions, or use the two types together. After all, there is no one-size-fits-all solution.

Further decentralized products available from Servotecnica include the AMKASMART i3X (three decentralized servo controllers in one housing); the AMKASMART iDT5 and iDT7 servo motors with integrated servo drives; and the AMKASMART iSA controller that features an incoming power supply to facilitate automation without the need for a separate PSU.




by Riccardo Francazi
R&D Manager




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Comparison of Slotless and Slotted Motors

Martedì, 21 Luglio 2020

www.servotecnica.com

Comparison of Slotless and Slotted Motors
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What is the IO-Link?

Martedì, 24 Marzo 2020

www.servotecnica.com

What is the IO-Link?

Managing data communication at inputs and outputs level in automated machinery systems. That’s what IO-Link is for. It is economical, simple and efficient. Well… this being said… readers expect to know more. There is a lot of documentation on the use of IO-Link for the management of Input/Output traffic, but very little yet on its use in the motion control field. Not forgetting its actual limits in that field, this article gives some highlights about motion control applications of IO-Link.

Talk to IO-Link’s users and ask them about its key points. Consistently, you will hear about the fact that this is a simple and robust solution, economical all over the life-cycle of the machine, easy to integrate, that it is simplifying installation, and commissioning, as well as usage and operation, that it is enhancing maintenance… By the way, IO-Link is very suitable for simple motion applications, and this is particularly what we want to mention here. But first, let’s see what is the IO-Link.

Not a fieldbus… Universal… Ready for Industry4.0…

In reality, IO-Link is a standard – the IEC 61131-9 – specified by the IEC international organization. It specifies a Single-drop Digital Communication Interface – SDCI – for small sensors and actuators. Whatever the controller (and the industrial-data communication protocol – fieldbus) which is used for the automation of the system, this “part 9” of the “IEC 61131” standard specifies a unique and universal SDCI technology suitable for applications using small sensors and actuators (which are so largely used in machinery). Yes, the IO-Link organization is a member of the PROFINET consortium, but the users do not have to adapt their sensors/actuators system (connectors, cables, hardware devices and software) to any specific fieldbus protocol. As often heard, “IO-Link is a universal cross-protocol solution” and fits into any architecture based on Modbus, PROFIBUS, EtherNet/IP, AS-I, etc.

Its goal is simple and clear: extend the traditional digital input and output interfaces towards a point-to-point communication solution (whatever the fieldbus which is involved at the PLC level).

On the field, over the last meters running through the heart of the machine (down at the sensors and actuators level), this technology supports bi-directional transmission of process-data, service-data and events. For both masters and devices, it is based on a protocol specified in accordance with the ISO/OSI reference model (physical layer, data link layer and application layer). It enables the transfer of control data and parameters down to devices, as well as the delivery of process data and diagnostic information from the devices up to the automation system.

Physically, IO-Link is based on a simple, robust and proven-for-long technology: the classical 3-wire connection used for the simple sensors and actuators, without any additional requirements regarding cabling. In the words of its promoters, it is “the further development of the existing, tried-and-tested connection technology for sensors and actuators.” And it does not cover the need for communication interfaces and systems based on multiple point or multiple drop linkages. This said, IO-Link is by essence used in factory automation, and it is largely used with simple sensors and actuators, in applications which include small and cost-effective microcontrollers.

In other words, one of the key benefits of the standard is that the sensors/actuators (inputs/outputs) level can remain the same, whatever the higher-level controller of the machine and/or the controller of the complete automation system. This leads to less design and engineering hours, to lower number of repair and spare-parts to keep in stock, to less “knots in the brain” and more peace of mind. The fact is this standard is universal and easily enters many applications!

As some users say, “IO-Link is revolutionizing communication at the field level.” Data from all levels of the machine and of the complete system is made available, entirely in line with Industry4.0 strategies. Being Industry4.0 ready brings today – and will definitely bring in future – the potential for implementing improved (and even completely new) machinery functions. Better and more economical production technologies are already anticipated for future with IO-Link. The fact is that this standard provides and guarantees long-term investments!

Economical… Easy to integrate, implement, use, diagnose and maintain…

IO-Link is cost effective. Before going on the field with the material and physical solution, it helps for reducing the number of engineering hours, shortening the design studies, reducing the preparation of the commissioning. When arriving on the field, the commissioning itself is faster. As mentioned above, IO-Link uses standard cables. It also helps reducing spare-part inventories notably thanks to intelligent multi-purpose devices.

IO-Link also simplifies diagnosing the sensors/actuators network, and organizing maintenance campaigns. Expanded diagnostics functionalities allow for remote diagnostics down to the field-device level, for cable break detection, and for device-specific diagnostics using IO-Link. And because it is simple and robust, easy and fast to maintain and repair, IO-Link contributes to increasing the runtime of the machinery systems. Typically – and not going too deep into details here – each field-device is described via an “IO Device Description” file (IODD). This file contains information such as the manufacturer of the device, its model number, serial number, the device type. It also contains parameters related to the application. These parameter values can be changed remotely (via the master), and so the machine can be adapted remotely and on-line, in order to fit the next coming production batch. At the same time, while parameters of a link are residing in the master (hardware), these parameters can also be reset, adjusted, modified dynamically during the production process. And if a master unit needs to be replaced for whatever reason, it simply requests pre-configuring (in the office or in the lab) and replacement (on the field). In such cases, it is just a “mechanical change” to operate in the cabinet of the machine. Powerful, isn’t it? And Easy.

In summary, IO-Link is factory automation and machinery oriented. Innovative IO-Link machine-concepts ensure simplified installation. It standardizes interfaces and cabling systems (parallel, analog, digital) to a unique type, and modular-machine concepts are inherently supported. Function modules and tool-assistance allow for a highly automated parameter setting. This leads to faster and easier tasks from design to commissioning, and maintenance.

And, of course, all these simplifications dramatically lead to the reduction of documentation and of training costs.

Che cos'è l'IO Link?

Motion Control

Quite a lot of information is available for typical IO applications. Much less easy is to find something to read on applications for motion control.

Well, from technical literature producers point-of-view, motion control is of great interest when it comes to advanced motion control applications dealing with high-speed and/or cycling positioning… and/or applications typically requiring a lot of interpolation calculations… and/or applications involving electrical gears and/or cams…

But on the other side, while motion control manufacturers and vendors can serve those complex applications, many users simply look for designing simple applications. Here, as IO-Link supports only quite slow automation cycles, it can perfectly serve such applications.

In those applications, sensors are measuring and controlling process values and parameters such as: angles, distances, frequencies and pulses, levels, positions, pressures, rotation or slide counts, rotational or linear speeds, temperatures, etc.

Measurement ranges and thresholds have to be set variably according to various production batches. While those changes used to be made manually (with the risk of human error to be handled and corrections to be managed), IO-Link allows for them to be set remotely. An IO-Link master is capable to recognize the connected sensors, actuators, displays. Before running a production, it will check for the configuration parameters of all the devices. While not complex in terms of speed or interpolations, many motion control applications get a lot of parameters involved. And there, the dynamic setting of the application shows a real benefit.

The global machine market is hotly challenged. Competition is high! In such a contest, the jury is composed of… the users! And users look here for reliable machines. And they expect that these machines can quickly pay for themselves.




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Dual loop: Advanced Control Techniques for Real World Drivetrains

Venerdì, 26 Gennaio 2018

www.galilmc.com

Dual loop: Advanced Control Techniques for Real World Drivetrains

The goal of a mechanical system built for motion control is to accurately move or position a load. In an ideal system, the load is rigidly coupled to, and directly driven by, a linear or rotary motor. There are many systems for which this drive method is not an option and a drivetrain is required to drive the load.

Some common drivetrains include:
  • Ball or lead screws
  • Belt and pulley
  • Chain and sprocket
  • Gear trains or rack & pinion
  • Hydraulics

Each of these drivetrains introduce errors in the system that influence positional accuracy and repeatability of the load. These errors can be introduced for a variety of reasons: compliance, drive slip, pitch variance, backlash, friction, misalignment, and wear are among the most common. Most errors are a result of tolerances and compliance, both of which are unavoidable in real world systems. Some position errors can be assumed constant with time, such as constant backlash. Others vary with position, time, or loading such as pitch variance, wear, and compliance. In the case of hydraulics, the position of the load is subject to dynamic factors such as variation in pressure, temperature, deadband or hysteresis, and others.

For closed loop systems, the most straightforward configuration for the feedback is measuring the position of the drive motor. Systems built with components that have tight tolerances, are structurally rigid, and comprised of low wear components, can achieve a high degree of accuracy and repeatability with the feedback located on the drive motor. However, the components required have higher cost comparatively, which adds to the overall cost of a system.

The increased cost for precision components is not always feasable or justifiable and more cost effective components must be chosen for the drivetrain. The trade-off is increased errors introduced by the drivetrain from looser tolerances or the addition of components with compliance. If the errors introduced by the drive mechanics are greater than the system requirements, a different feedback configuration is needed. Without feedback on the load, little can be done to compensate for the position errors introduced because there is no way to measure the load's actual position.

An alternative encoder configuration puts a single encoder directly on the load. The resolution of the load's position is a direct function of the encoder resolution. However, this configuration tends to be unstable because the non-linearities and resonances of the system are now part of the control loop.

The solution to maintaining system stability while compensating for errors in a drivetrain is to use a dual feedback configuration or dual loop. This combines the stability that is inherent with feedback directly coupled to the drive motor with the added information from the load encoder. This results in the ability to position a load accurately, despite an imperfect drivetrain.

There are two basic requirements for a system to use dual loop. The first is a load encoder with resolution at least 2 times the position accuracy required for the system. This is a general rule of thumb when deciding the appropriate encoder for a system. The second requirement is that the resolution of the motor encoder should be at least 2 times the resolution of the load encoder, taking any gear reduction into consideration. This is required because 1 count of motor encoder movement must account for less than 1 count of movement of the load. If this condition is not met, 1 motor count can result in more than 1 count of movement of the load, causing dither about the desired position.

Standard Dual Loop

The primary components of a dual encoder control loop can be seen in Figure 1 where the PID control filter is broken into two parts. The inner loop is comprised of the derivative gain (D term) that receives velocity feedback from the motor encoder and adds pure viscous damping to the control loop. This term is responsible for adding stability to the system. The velocity feed forward (FV) is utilized here to compensate for the phase lag between the inner and outer loop.

block diagram of the standard dual loop pid filter
Figure 1 - Block diagram of the standard dual loop PID filter.

The outer loop is comprised of a proportional gain (P term) and integral gain (I term) that closes the position loop on a load encoder. This is also known as a PI filter. The P term is responsible for the responsiveness of the system while the I term is used to compensate for steady state position error of the load. The trade-off for a dual loop control filter is that tuning is more complex than a standard PID filter, and a different tuning methodology must be employed to tune this type of control loop.

Advanced Dual Loop

A properly tuned standard dual loop is able to compensate for mechanical imperfections and accurately position a load but it is not always able to achieve the dynamic system requirements. For systems that require higher bandwidth a more advanced control loop is required.

The primary difference in the advanced dual loop is that an additional P term has been added to the inner loop. This has the benefit of adding responsiveness and stiffness to the inner loop. From this modification comes an increase in system response.

advanced dual loop block diagram of pi outer and pd inner loop filters
Figure 2 - Advanced dual loop block diagram of PI outer and PD inner loop filters.

The trade-off for increased bandwidth is additional control loop complexity. The advanced dual loop can be accurately described as having two distinct control filters, one for each encoder in the system. The difficulty lies in optimizing both filters to work in conjunction to achieve the system performance requirements.

Dual Loop Case Study

For the purposes of this paper, a system was constructed using a non-ideal drivetrain. It was constructed with a brushless rotary motor coupled through a belt and pulley to a ball screw to achieve linear motion. The carriage slides along two polished rods with linear bearings. The load being positioned was coupled to the carriage through a plastic mount. A schematic of the test apparatus can be seen in Figure 3.

There are various mechanical imperfections in this drivetrain that contribute to load position errors. The belt is subject to backlash and stretch which can result in resonances. The ball screw will have backlash and pitch variances. Finally, load position error can be introduced by flex in the plastic component that mates the ball screw carriage to the load, or from misalignment of the components that make up the drive assembly.

Three control configurations will be compared:
  • Encoder on motor only
  • Standard Dual Loop
  • Advanced Dual Loop
For each control configuration, the motor will be tuned using a profiled move with the following parameters:
  • Move: 25.4 mm (1 inch).
  • Acceleration: 500 mm / s2
  • Deceleration: 500 mm / s2
  • Speed: 178 mm / s
The tuning will attempt to achieve the following goals:
  • The amount of error during the move must be within +/- 30 μm.
  • At the end of the move, settle with an error of less than +/- 15 μm as fast as possible.
schematic of test system constructed with a non ideal drivetrain
Figure 3 - Schematic of test system constructed with a non-ideal drivetrain.

First, the theoretical resolution of the load needs to be calculated. This is done by taking the motor's rotary resolution and scaling based on the pulley gear ratio and the pitch of the ball screw. The theoretical linear resolution with a motor encoder having 4,000 counts per revolution is:

theoretical resolution of load
Equation 1 - Theoretical resolution of load.

This results in a resolution of 1.25 μm. The system requires that the load is placed to within +/- 15 μm or +/- 12 motor encoder counts. In order to measure this, the minimum resolution for an encoder attached to the load should be 7 μm or 143 counts per mm. For the purposes of this study, an encoder with 250 counts per mm was used for tuning and positioning the load while an encoder with 1,000 count per mm was used for characterizing the system.

To see if the system requirements are met by the current configuration, the position error will need to be characterized. Backlash was measured by driving the motor back and forth until motion is detected by the linear encoder affixed to the load. This value includes backlash from the belt under low load conditions and in the ball screw. The total distance that the motor must move to take up the backlash is 5 motor encoder counts and is within the system requirements for positional accuracy.

Pitch variance or misalignment can also affect how accurately the load can be positioned. Load position error is the difference from where the load should be if the drivetrain were perfect and the position read by the load encoder. The position of the load was read using an encoder with 1,000 counts per mm while the motor was indexed by 1 count at a time. Data was gathered by running the full length of the ball screw in both directions and averaged. Sampling was done with no load and at a rate of 125 counts per second to reduce the contribution from compliance.

The results of this test is shown in Figure 4. It is evident that driving the motor along this ball screw can result in an error that constantly changes. At its maximum, the load position will deviate by 82 μm, which is beyond the requirements set in place for this system. The source of these errors is due to manufacturing defects, resulting in a slight variation in pitch as well as slight deviation in collinearity. This error profile will vary from system to system as it is entirely dependent on the tolerances within each component.

load position error vs displacement along lead screw
Figura 4 - Load position error vs displacement along lead screw.

Finally, there are errors introduced by the compliance of the belt. This was measured by clamping the load and driving the motor open loop in the positive and negative direction to measure the amount of stretch from the belt. This was measured to be approximately +/- 550 motor encoder counts or 688 μm. It is clear that the biggest contributor to load error in the drivetrain is the belt and that the current system does not meet the system requirements for positional accuracy. In addition to position error, the belt's compliance also adds resonance at a frequency of approximately 34 Hz. With the drivetrain characterized, the three encoder configuration options will be explored in order to determine which gives the best performance.

The first option is to use the motor encoder to position the load. In this configuration, the position of the load will have errors introduced from the drivetrain without a means of correction. After tuning the system, the load was positioned in the middle of travel and the motion profile was plotted (shown in Figure 5). The end of the move is marked with a vertical line at 472 ms for reference.

The theoretical load position error, given the motor encoder alone is shown in red on the bottom plot and is always within the acceptable error band and converges quickly to zero error when the move is finished. The actual load error measured by the load encoder (shown in green on the bottom plot) has a steady state error of -137 μm at the start of the move. The load oscillates during the move and finally comes to rest with a steady state error of -110 μm, outside the acceptable error band. Based on these results, the motor is stable but the system is unable to position the load to the accuracy that is required.

motion profile and error of the system with a single motor encoder
Figura 5 - Motion profile and error of the system with a single motor encoder.

Next, standard dual loop was employed in an attempt to increase the accuracy and repeatably of the system. The error during the move does not exceed 55 μm, shown in Figure 6. The dual loop encoder configuration commands motion using the load encoder. Because of this, the motor encoder error is omitted from the plot and analysis is focused on the load position error. The error settles to be within the error band 112 ms after the move ends. Adding a load encoder and using standard dual loop, the system was able to always converge on a steady state error of 0 counts and has a much smaller error band. While this is an improvement over the configuration with only the motor encoder, the error during the move was not within the acceptable +/- 30 μm and further improvement is needed.

motion profile and error of the system with the standard dual loop
Figura 6 - Motion profile and error of the system with the standard dual loop.

Finally the advanced dual loop was employed in order to meet system requirements. After tuning, the characterizing move was performed and the motion was plotted. The advanced dual loop implementation was able to bring the peak error during the move down to, or below, 30 μm, shown in Figure 7. One count error (1.25 μm) was attained 108ms after the move had ended, showing improvement from either of the two previous configurations. The effects of the resonances have also been reduced to an acceptable level.

results from turning the system with advanced dual loop
Figure 7 - Results from tuning the system with advanced dual loop.

The test system was constructed with 6 μm of backlash, a maximum deviation of 82 μm due to pitch variance and misalignment, and 688 μm in compliance. The system required that the end position accuracy be less than 15 μm and that the position error be within +/- 30 μm for the duration of the move. The advanced dual loop was used and able to successfully move the load with less than 30 μm of error for the duration of the move and settle to 1.25 μm of error within 108ms. With this configuration, system requirements for accuracy and repeatability can be met.

Summary

Using a drivetrain can result in position errors of the load and can sometimes add instability or resonances to a system. Systems with a drivetrain that do not currently meet the system requirements for accuracy can benefit from the addition of feedback to the load and closing a position loop using standard dual loop. This is a standard feature on all current generation Galil motion controllers and is available using quadrature, SSI, or BiSS encoder feedback. Systems that require a further increase in bandwidth, above and beyond the standard dual loop, will benefit from the advanced dual loop. The advanced dual loop compensation is a separate firmware that is also available.

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Soluzioni per il Motion Control

Lunedì, 29 Magio 2017

Soluzioni per il Motion Control
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