The Top 5 Mechanical Considerations For Electrical Engineers
June 24, 2010 by admin
Filed under Commentary, Design, Featured Mechatronic Articles
Problems can arise in a mechatronics project because mechanical and electrical engineers often do not have sufficient experience or understanding of their counterpart’s discipline. Here is one application engineer’s advice on how to avoid five of these common problems when specifying parts for an electromechanical system.
Getting the right motor is critical, which means matching it up to the overall system’s mechanical components. In other words, electrical system design can only occur once the mechanical design has been fine-tuned to match the performance specs of the application.
Mechatronic systems require a complex inter-play of electrical and mechanical systems to accomplish increasingly demanding tasks. But, because mechanical engineers and electrical engineers are often educated within their single area of expertise, oversights occur that can lead to higher costs and less reliable performance. As a mechanical engineer involved in the sizing, selection, and start-up of mechatronic systems, I’ve seen these problems both during the design phase and in the field. To help bridge the gap, I’d like to offer my electrical colleagues five important things to consider in the design of mechatronic systems.
Complex mechatronic systems require careful advance planning to ensure optimum performance and the best total cost of ownership.
Consideration #1: Total Cost of Ownership is everything
Of course mechatronic systems should be designed to achieve the best possible performance for the lowest cost over the long term. However, the need to document cost-savings on an annual basis (rather than over the lifetime of the system) can be a powerful motivation to look for less expensive components to keep the up-front cost of the mechatronic system low. Indeed, the choice of seemingly simple components can cause painful headaches later. For example, the use of less expensive bellows-type couplings between the motor or gearbox and the actuator input shaft are a perfectly appropriate choice in many stepmotor driven applications. In fact, a slightly spongier coupling can provide a certain amount of damping in pick-and-place applications moving heavier loads. The sacrifice, of course, is precision, and in many mechatronic systems, which are typically servomotor driven, it is best to use stiffer elastomer-type couplings. Using less expensive couplings may save money in the short term, but if the required stiffness is not achieved, redesigning and retrofitting the motor mounting system can easily cost three or more times the money you initially saved with the “economy” coupling. Plus, you’ll have the added cost of downtime and lost production, which is not a good trade-off. Total Cost of Ownership (TCO) is really the single most important consideration in any mechatronic system design, and the four remaining points will also contribute, ultimately, to lower TCO.
Consideration #2: Always look at the mechanics first
It’s very important for electrical engineers to get involved in, or at least understand, the mechanical design of a mechatronic system before forging ahead with the electrical design and controls. Trying to size and spec the electrical components before the mechanics are defined can lead to wasted time and rework, since parameters such as inertia and torque are heavily influenced by the choice of mechanical components. Bosch Rexroth engineers use a system called LOSTPED to help size and select the mechanical components that are best matched to the performance requirements of the application. LOSTPED is simply an acronym that stands for Load, Orientation, Speed, Travel, Precision, Environment and Duty Cycle. It is a systematic review of all the performance and design attributes that need to be considered, with the end goal being the optimum system design for each application. Not following this process can result in larger or more expensive mechanical systems than are needed. For example, if an OEM or end user forces the design to accommodate a specific motor without considering the LOSTPED criteria, larger mechanical components may be required to handle the motor torque or inertia than those actually needed for the application. The same goes for control systems. If a ball-screw-driven actuator can achieve 0.01 mm repeatability, you need to make sure that the encoder can meet or exceed this spec; otherwise you won’t be able to take advantage of the ball screw’s precision. It is human nature to want to use components that are in stock or that the user is familiar with, but each system deserves its own review to ensure that the components and the overall system are optimized for cost and performance. Otherwise, you may leave money on the table or fail to get the system performance you need.
Clean and careful cable management is often overlooked, but can help you make sure that even tight spaces don’t cause problems. Failure to consider cable management can result in shortened system life, physical incompatibility with the ultimate operating environment, or even fires.
Consideration #3: Don’t try to fit a square peg into a round hole
The last example is common enough that it’s worth emphasizing as an important consideration of its own: Don’t try to fit a square peg into a round hole. Many electrical engineers are familiar with particular motors and drives, or are pressured to save money by using components they already have. Using a motor that’s physically too large, however, can cause mounting issues. It could supply too much torque for the linear module to handle (causing mechanical failure or breakage); or it could cause inertia and settling issues. Unpredictable settling can be a particular problem in precision applications, such as dispensing, pin insertion, or precision assembly in semiconductor and medical applications. If the motor is oversized and has too much inertia, the actuator may have a difficult time achieving the desired position, resulting in longer overall cycle times than required. Particularly in precision applications, you should try to size the mechanical and electrical components to achieve as close to a 1-to-1 inertia match as possible. Keeping power consumption to the minimum required for the application is also important for customers who are looking to reduce their environmental impact and make their manufacturing operations more green.
Consideration #4: Remember “Jerk”
“Jerk” is the rate of change of acceleration, or the “build-up” of the acceleration of the axis. It is the limitation of the jerk parameter (how fast you’re accelerating) that lets you experience the acceleration of the roller coaster without developing whiplash. Acceleration is important in mechatronic applications to achieve the desired move in the required time, but if the acceleration speed is too high, vibrations could result and cause a loss of positioning or premature wear of components. On the other hand, if electrical system designers don’t consider the magnitude of jerk required, the motor may be undersized and the system won’t perform as required.
Always look at the mechanics first. The LOSTPED acronym can help you consider every possible detail needed for sizing and selection of electromechanical components: Load, Orientation, Speed, Travel, Precision, Environment, and Duty Cycle.
Consideration #5: Cables must be managed
Cable management is one of the most frequently overlooked attributes in mechatronic systems; and it can be costly. Cables and cable tracks require physical space, and fast, multi-axis motion often requires cables that can handle tight bends and high duty cycles. Particularly when designing the cable management system, theoretical operation and space requirements can be very different from that required in the real world. Once installed in the machine or factory, the system’s active environment may include adjacent machinery, factory walls, posts or beams, and other parts or tooling that cause interference with the cable management system. Proper strain relief for cables is also critical, because cables that are bent and twisted beyond their specifications pose a safety hazard in the form of fire or short-circuits.
It all adds up to TCO
The ultimate goal in the design of any electromechanical system should be to achieve the optimum performance, mechanically and electrically, to get the job done with the lowest total cost. In the real world, many such systems are designed by teams of engineers from both disciplines working in tandem. This is the ideal model, but with the discipline of mechatronics being relatively young, the problems I’ve described here happen often because of insufficient experience or understanding of their counterpart’s discipline. These five tips aim to help bridge this gap and prevent costly, unwelcome and time-consuming surprises.
Custom Transfer System Adds Value by the Millisecond
February 14, 2010 by admin
Filed under Automation, Design, Motion Control, Technology
Services and products from hydraulics, pneumatics, electrics, and linear technology were linked by Rexroth engineers to produce a custom engineering concept for Swiss company Mikron Machining Technology. “The fact that Rexroth offers coordinated components from pneumatic, hydraulic and electric drive technology right through to high speed control enabled us to select the most suitable characteristics for specific functions,” said Rolf Held, design manager, Mikron. The result was a machine tool that makes real added value out of milliseconds.
The Mikron Multistep™ XT-200 has up to 54 NC axes and can be extended as required.
In a production environment, fractions of a second count and can accumulate to the extent that they affect cycle times. Automated transfer systems play a key role in many industries, particularly when metal parts must be processed using a number of different machining sequences. Suppliers to the automotive industry, for example, machine a number of items considerably more economically using intelligent transfer units. The machines pick up workpieces in clamping devices and transfer them automatically to the individual machining stations where they are drilled, milled, turned, chamfered or de-burred. Threads are cut and knurled profiles applied. Even peripheral processes such as installation operations or checks can be integrated into these transfer operations. With the transfer concept, all parts can be machined simultaneously.
The Multistep™ XT-200 is setting new standards for transfer systems – especially for the control speeds and the drives used for the various functions. The system makes precision manufacturing possible in non-stop operation. At the same time, the individual stations work practically hand in hand.
Extremely short chip-to-chip times ensure nearly continuous machining, and the system can even be used for high speed cutting. A key advantage is that it combines the productivity of a linear transfer machine with the flexible re-tooling capability of a machining center.
The concept is based on individual interlinked dual spindle modules, which can be used on a stand alone basis, or spread over up to four modules. Five interpolating CNC axes and up to 144 tools machine complex small and medium series parts on five and a half sides without remounting. If the parts are automatically re-mounted in-process, it is possible to machine six sides. The Multistep™ can be adapted to the production volume at any time. In addition, a loading and unloading station can assume the component feed function.
Without a break
While the main advantage of this machine is precision manufacturing almost without a break, further advantages come from the short chip-to-chip time of less than a second and the unusual dynamics. Accelerating the Rexroth CKK linear systems up to 1.4 g to 52 m per minute and spindles with speeds up to 40,000 rpm make for short machining cycles. This is where drive technology from Rexroth comes in: rodless pneumatic cylinders from the BRP Rexmover Series with a diameter of 50 mm and a stroke of 400 mm, as well as a linear axis Type CKK20-145 for strokes of up to 1,100 mm. The maximum force on this axis is around 72 kN in the direction of movement.
“At the end of the day it’s the number of milliseconds that we gain from a number of different points that is the decisive factor,” said Held.
The chuck for the C-axis in the loading and unloading station is pneumatically activated.
In the standard version, the Multistep™ is fitted with a high-speed CNC Rexroth IndraMotion MTX. Up to 64 axes can be operated in twelve CNC channels independently of one another. The maximum extended version features 54 axes that are required to work in parallel. “Using any other approach would mean that we would need at least two controls and we would have to combine these with each other,” said Held.
The PLC can process 1,000 instructions in 60 ms. At the same time the CNC offers, when controlling eight axes, an interpolation cycle time of 1 ms maximum. The Rexroth IndraDrive servo drives have integrated safety functions for secure hold and safe movement. “Also of interest is the so-called feedback capability, with which the generator capacity of the motors is fed back into the network during the braking operation,” noted Held. Mikron uses the force of hydraulic components for clamping the direct drive B/C axes. The tool clamping mechanisms in the motor spindles that close by means of spring assemblies are opened hydraulically. Here the valve blocks are the same for all spindle variations.
Movement of the Z-axis for the loading and unloading station is activated by a Rexroth IndraDrive Servo drive. In addition, the pneumatics ensure rapid, safe workpiece handling. Control is through a field bus.
When it comes to workpiece handling in the loading and unloading station as well as workpiece transfer, it is pneumatics that takes care of speed and safety. With the HF03-LG “light generation,” Mikron uses a light and compact variant of the HF valve series. It has a narrow valve width, yet can flow up to 700 standard liters. By using plastic plates, the weight can be reduced even further. The pneumatic and electric controls are located towards the front and arranged in one direction, thus offering increased installation potential, compactness and the possibility of adapting to the space available. By way of an alternative to the traditional multi-pole connection, a field bus connection is used.
Problem-free commissioning of Rexroth IndraDrive in the Mikron Multistep™ XT-200 control cabinet.
From a single source
When it comes to compressed air treatment, Series AS2 maintenance units feature a modular structure. The individual air treatment processes are brought together in maintenance units made from high quality plastic. Filtering, closed-loop control, lubricating and draining – the configuration is geared to user requirements. With the patented oil-fill system, the oil is directly extracted from the storage tank by suction using a hose. This means that the maintenance unit is protected against fouling by oil.
The maintenance units for the pneumatics are located, like the hydraulic power unit and the master control, in a separate control cabinet. The cabinet also houses the central lubrication, power connection and the fire extinguishing system. This arrangement corresponds to the modular structure of the Multistep™ and, by ensuring simple and rapid access to central components, guarantees that the unit is maintenance friendly.
Bosch Rexroth Group
www.boschrexroth-us.com
Mechatronics: New answers call for new questions
February 10, 2009 by admin
Filed under Motion Control
By Richard Vaughn, Product Engineer
Bosch Rexroth Corporation–Project Management Dept.
In a short time, mechatronics has evolved into a universally accepted engineering concept. It integrates mechanics with electronics – and with engineering itself. The result is expanded technological capabilities and assembly-line successes like the Cartesian multi-axis robot. Because it enables more flexible automated production, users can precisely control parameters such as weight, speed, reach, and work envelope. That is why mechatronics can be the answer to a variety of design challenges.
Mechatronics enables the development of creative designs that extend the capabilities of existing robotics and controls to new levels.
But obtaining the right answers requires asking the right questions – from the very first stage of a design. A mechatronics approach is a 3-stage ongoing process:
—Design – configure the system to accomplish a specific task.
—Integration – determine how components work together.
—Implementation – achieve optimum value in daily operations, and prepare for future changes.
As an example, consider the parameters involved in an application such as pin insertion in automobile underbodies traveling on an assembly line. Proper design begins with the determination of mathematical factors such as payload, travel distance, desired speed, and axes size. Then there are questions of machine control, motor size to deliver proper speed and torque, and even the operator HMI. There is also the key question of how much it all will cost.
The design of a mechatronics system requires a multidisciplinary focus — to root out potential difficulties before they grow into time-consuming, costly and distracting problems.
Here are a few specific mechatronics challenges—and some tips for handling them.
Keep envelope restrictions in mind. Consider work envelope restrictions including walls, supports, and safety barriers to avoid physical interference. The difference between length of a module and length of stroke is also crucial, especially when selecting linear actuators. A rodless actuator’s “dead length” means the actuator’s stroke is shorter than the apparent length of the cylinder. The best approach is to use a 3D simulation, rather than reconfigure system elements later in the project.
Find a proper protocol. Approach the marriage of controls and drives from different sources cautiously – it can lead to problems, especially when using off-the-shelf protocols such as PROFIBUS, DeviceNet or Ethernet. Some off-the-shelf protocols, such as Bosch Rexroth’s IndraControl components, can communicate with many proprietary controllers, but this may not be true of all protocols. Problems may arise if a controller running DeviceNet is added to a platform running PROFIBUS. Similarly, if your plant runs Ethernet, you may not be able to “plug in” a component from any vendor. During the specification phase, you should ensure that compatible off-the-shelf control systems are available for expansion or reconfiguration.
Consider the implications of specifications. Specifications can have powerful, difficult-to-foresee implications for mechatronics. For example, a 480 V 3-phase motor may be ideal for a servo application, but not if your drive amplifier is only capable of 220 V – which may require retrofitting a transformer. A change from Class 1000 to Class 100 semiconductor production clean room conditions may require third party specification.
Build in cable management. Often, this is the last challenge addressed, leading to last-minute scrambles to avoid interference with motion and parts pickup. Rather, cable management for a gantry pick-and-place application should be one of the first factors considered. A program such as Bosch Rexroth’s camoLINE can offer predefined cable management and 3D modeling, letting you “drop in” components to ensure all components work cleanly together.
Cable management is an important and often overlooked consideration during mechatronic system design.
Find the right tool for the job. An application that requires complex interpolative motion, such as cutting or gluing circular seals on catalytic converters, requires an interpolative motion controller and device. Attempts to adapt point-to-point controllers for these applications can be time-intensive and deliver inadequate precision. The best approach is to determine the circular interpolation path and identify the needed controller performance; that in turn will guide the selection of drives, power requirements, I/O and other elements to achieve that performance.
Following some basic tips like these can help avoid common – and costly – problems like either over engineering and over sizing machines (resulting in heavy-duty capabilities that are rarely if ever used) or under sizing machines (not accounting for occasional increases in payload or run speed).
Either situation can unnecessarily increase automation costs, which might discourage implementation of mechatronics – another reason why asking detailed questions is essential.
Integration
Mechatronics is clearly a cross-disciplinary science, requiring expertise in mechanical and electrical engineering as well as electronics and computers. But few have a background in all these disciplines. Those with expertise in one particular area, such as electrical engineering, may end up doing on-the-job training in other aspects of mechatronics, or trying to learn how to incorporate components from an unfamiliar manufacturer.
One effective approach is to use the services of an integrator who specializes in mechatronics and is experienced in blending mechanics and electronics. Cross-disciplinary integrators are becoming more common as mechatronics applications expand, and the trend toward cross-disciplinary integration skill is consistent with the current industry focus on accomplishing more with fewer people.
Integration can act as a “force multiplier,” extending the capabilities of existing technology to create quantum leaps in production efficiency, reduced downtime, and cost savings. For example, an automotive production line can be made many times more productive by substituting different control commands for retooling, and an outboard support axis added to a 3-axis Cartesian robot creates a gantry device. Many similar solutions are possible for designers who adopt a multidisciplinary, full-system approach.
Use of 3D simulation during the design phase of the project can prevent the need for reconfiguring system elements later in the project.
After integration, the final step is implementation. But the final step should be well planned early in the process, or the result can be significant delays and added machine or production line costs. Avoid potential problems by clearly defining the roles and responsibilities of integrator and customer. This task can be a challenge in a process that blends a number of different engineering disciplines to create an integrated solution. The key is communication, right from the beginning — including detailed questions. For example, regarding system adjustments or changes, what is the responsibility of the integrator and what can be done by on-site personnel? The answers should be carefully documented to head off potential problems before they start. Of course, no one can foresee the future. But good implementation envisions the context in which a mechatronics system will operate.
Cross-disciplinary integrators are becoming more common as mechatronics applications expand.
As a cross-disciplinary process, mechatronics demands integrated thinking to go with integrated engineering. Part of this thinking involves the ability to envision the day-to-day operation of assembly line functions, including the working environment and the blending of electronic protocols, to anticipate and head off potential disciplines. Be prepared for the reality of cross-disciplinary requirements that may call for an integration specialist to get all the components working together. Finally, to implement the system, clearly communicate with everyone involved about their roles and responsibilities. For mechatronics to be truly successful, the development process must involve not only mechanical and electronic elements, but process elements as well: the key phases of design, integration, and implementation.
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Software tools speed integration
Proper design begins with determining mathematical factors such as payload, travel distance, desired speed, and sizing of axes. Bosch Rexroth developed “LOSTPED”—a multi-step analysis process to help designers gather information for specifications.
To help answer mechatronic questions, Bosch Rexroth developed “LOSTPED”—a multi-step analysis process for gathering information for specifications. LOSTPED stands for Load (the weight or force applied), Orientation (direction each axis is mounted), Speed (and acceleration), Travel (distance and range of motion), Precision (repeatability or positioning accuracy), Environment (operating conditions), and Duty cycle (duration the machine will run; example: 24 hr/day, 5 days per week). In an automotive assembly application, for example, duty cycle and assembly line speed are crucial to determine insertion arm size, motor size and logic control, along with many other key factors.
Bosch Rexroth
www.boschrexroth-us.com
