10 Design Tips for Machine Control

How many times have you used something and wondered if the engineers who designed the product ever used it? To make life a little easier for anyone working on a machine, here are 10 design tips in integrating the latest automation, controls, instrumentation, sensors, logic, and communications to save time and effort, and avoid getting (or giving) grief later on.

10/01/2004


At A Glance

 

  • Use standards, networks

  • Consider sensors, logic early

  • Advanced controls help actuation

  • HMIs go beyond eliminating buttons

How many times have you used something and wondered if the engineers who designed the product ever used it? To make life a little easier for anyone working on a machine, here are 10 design tips in integrating the latest automation, controls, instrumentation, sensors, logic, and communications to save time and effort, and avoid getting (or giving) grief later on.

1. Research, study, quantify

Get information, make a plan, and stick to it to achieve anticipated benefits. There's a fine line between being flexible and allowing feature-creep to eat into your time and make the product overly complex.

When priorities are set, start with areas where the best 'bang for the buck' can be obtained, according to Michael Gurney and Ed Diehl, co-owners, Concept Systems Inc., an industrial system integrator. Statistical process control and quality control analyses can provide useful information, they add.

A modular machine design and manufacturing approach can meet the conflicting needs for increasingly customized machines and reduced machine development times (ease engineering effort), while lowering OEM manufacturing costs, says Larry Komarek, automation product manager, Phoenix Contact Inc.

'Break the machine or family of machines into functional subassemblies where each subassembly contains the mechanics, automation/control elements and software routines. Customer customized configurations or 'small/medium/large' product families are created using combinations of these,' Komarek advises. 'Pre-designed, pre-documented and pre-tested subassemblies are quickly manufactured,' lowering costs.

2. Standards, safety

Standard interfaces enable use of 'best-in-class' system components and reduce obsolescence woes related to proprietary control systems, according to Gurney and Diehl.

Intellectual knowledge gathered in design software can be transferred to machine code, avoiding reworking the design parameters. Some design software provides data translators for STEP and IGES file formats. STEP, the Standard for the Exchange of Product Model Data (ISO 10303), describes how to represent and exchange digital product information.

Designing distributed controls takes advantage of group motor installation provisions in the NEC code [article 430.53 (c)] to reduce the number of branch circuits. Doing so reduces costs, helps eliminate EMC noise, and provides flexibility to easily move modules without rewiring and re-commissioning, says Rich Mintz, electronic products manager for SEW-Eurodrive.

In the interest of safety, anyone working on globally used machine designs since the NFPA-79 2002 revision should meet IEC 60204-1 Category 4, the highest threshold of safety (requiring safety relays and safety interlocks), according to Larry Sunday, Schneider Electric product manager for machine safeguarding products. The 2002 revision aligns NFPA-79 with IEC 60204-1, an international standard. The U.S. practice of using standard relays in a basic control circuit, known as 'control reliability,' may not be enough for Category 4, Sunday warns.

3. Sensors, machine vision

When designing, 'don't leave sensors as the last item,' warns Joseph Dolinsky, technical marketing manager, Banner Engineering. Smart and very small sensors offer certain benefits. Often, 'a less-expensive sensor could solve the problem, if mounting space could have been allocated.'

Deciding how best to handle inspection and process feedback depends on a number of factors, says Mark Sippel, principle product marketing manager for In-Sight Vision Sensors at Cognex Corp.

Does feedback need to be shared between multiple discrete inspection, rejection, and other stations, or will one controller coordinate inspection results with ancillary tasks? How will inspection results be used and communicated? If I/O devices are used for communication, what types and locations are required to function with other controls? Lighting, power, and data transfer should also be considered.

4. Logic platforms

Start the 1,000-mile journey of machine design with the right logic platform, says Rahul Kulkarni, product manager, industrial data acquisition and control, National Instruments. 'Controllers today can be in the form of a PLC, PC-104, PCI, PXI, or custom circuit board,' he says; account for all functions the controller needs to perform (such as logic control, process control, motion control, automated inspection, and machine condition monitoring), before deciding on a controller platform.

Programming tools should be easily understood and the resulting code should be modular, scalable, and reusable. Choose a control architecture solution to help keep the design simple, says Dan Seger, global OEM technical consultant, Rockwell Automation. From controllers and networks to peripheral devices and operator interfaces, making decisions only on lowest procurement costs may cause troubles later. Also, interconnectivity increases efficiency and lowers costs, when pieces of equipment 'plug together and talk to each other natively rather than working through how to get devices to communicate,' says Seger. The inexpensive ways to make devices communicate are 'rarely easy to use and tend to eat up a lot of time during start up and maintenance.'

He suggests separating programming for control and information sharing for best results. Structured data forms, pull-down menus, and libraries of code-based objects arranged by functions also help.

5. Actuation devices

Choose from a myriad of pneumatics, hydraulics, and electric actuation devices, depending on application needs; controllers for motion also bring choices. Should a motor-control unit (MCU) or a digital signal processor (DSP) be used for motor control?

Kedar Godbole, motor control strategist, Texas Instruments, recommends looking for performance for innovation and system cost reduction; balance of performance and price (optimized price/performance ratio); efficient C/C++ core for fast program development; high level of analog integration (numerous control peripherals); and flexible, easy-to-use tools.

Controls almost always can be more useful and efficient than mechanical friction. A variable frequency drive (especially closed loop) can double and sometimes triple the cycles with less maintenance and reduced wear and tear on equipment, compared to clutch brakes or similar solutions, says Jan Lindholm of SEW-Eurodrive. Lindholm explains, 'VFDs come in sensorless vector, flux vector, and ac blushless servo and can solve a multitude of drive challenges.'

6. Screens vs. pushbuttons

Evaluate benefits of screen-based human-machine interfaces (HMIs) versus pushbuttons. Replacing buttons with touchscreens has benefits; HMI toolkits have templates for easy adaptation and reuse.

HMIs offer other benefits, such as data interconnection and diagnostics. Joe Rubino, software product marketing manager, Omron Electronics LLC, says diagnostics offer uptime advantages and hardware and software, easy connectivity, and reduced programming, a value worth the cost.

7. Networks, communications

Networks save on wiring and communication translations, inside and beyond the machine. David Crump, marketing communications, Opto 22 says, machine-to-machine (M2M) communications increase machine connectivity, leverage wireless technology, and connect equipment to the enterprise. These enable remote equipment monitoring and activation, real-time reporting on manufacturing, and other operations activities. M2M also helps machine designers track the machine's lifecycle, identify trouble spots, improve future designs, and unleash opportunities for 'new revenue opportunities by providing value-added services for customers, such as proactive equipment maintenance, and repair dispatch,' Crump suggests.

NI's Kulkarni says wireless technologies vary; for M2M communication between a remote computer and a factory HMI/SCADA system hundreds of miles away, satellite or cellular wireless communication is ideal. For wireless communications between a controller/PLC and a HMI/SCADA system, IEEE 802.11 is best.

However, for controller to sensor communication, 10-100 m, technologies like Bluetooth and Zigbee are more effective, Kulkarni says.

Rockwell Automation's Seger says to 'avoid devices using RS-232 or standard Ethernet that require writing proprietary communication drivers to support data transfer,' which consumes time instead of adding value.

Inside, at the I/O level, using a secure termination technology can save assembly and troubleshooting time. Dean Norton, Wago marketing manager, says Wago screwless Cage Clamp termination technology reduces wiring time up to 60%, eliminates need for re-tightening required every 6-8 months in some applications, and allows greater flexibility. 'The machine builder can prewire and install all I/O requirements on the machine and drop in the controller with the right fieldbus protocol later,' explains Norton.

8. Power

Consider purchasing power-related systems, rather than designing them, to save time and effort.

Power supplies, buses, management systems, and intelligent components all can report into the HMI for greater integration and efficiencies. Isolation, monitoring, and shielding can ensure minimal power-line and EMF disturbances.

9. Integration

Think about a machine in context of surrounding production flow to improve efficiencies. Fredrik Jönsson, CEO of FlexLink Systems AB, says '30% capacity utilization in a machine line is still a common figure;' even more can be gained by taking a broader view of the entire production process.'

Integrating information from machine tools yields benefits beyond the machine. Several major engineering design and production organizations have shown that STEP databases, for instance, can reduce the cost of production planning and manufacturing control by between 35% and 75%, according to Step Tools Inc.

10. Groundwork

And, just when you think you're done, set aside time to assess, evaluate, gather input, and lay some groundwork now for the next design cycle, while the project's fresh in your head, because with two dozen other things to do, it won't stay fresh long!


For related products, visit www.controleng.com/buyersguide; for system integrator help, go to www.controleng.com/ integrators; also see these Web sites:

www.appliedproduction.com

www.bannerengineering.com

www.cognex.com

www.conceptsystemsinc.com

www.flexlink.com

www.ni.com ( National Instruments )

www.opto22.com

www.omron.com/oei

www.phoenixcon.com ( Phoenix Contact )

www.rockwellautomation.com

www.schneiderelectric.com

www.seweurodrive.com

www.steptools.com

www.ti.com ( Texas Instruments )

www.wago.com


Online Extra for 10 Design Tips for Machine Control

Save time, effort, grief in designing machine controls: Decentralized control technology (DCT) promotes modular design. For instance, when adding another filler or capper to a bottling line, wires can be disconnected, moved, and plugged in again, instead of rewiring and recommissioning, which saves labor and downtime. New installations also benefit. For more on the topic from Control Engineering, along with a summary of the 10 tips, click here .

Motion control comparisons: Compare motor-control unit (MCU), digital signal processor (DSP) for motion control, with the following information provided by Texas Instruments.

MCU/DSP comparisons for motion control

MCU

DSP

Methodology

Proven, intuitive, empirical

Deterministic, math-modeled

Evolution

Higher integration, attempt to do some DSP functions

High MIPS, lower power consumption

Technology

- “Old” process nodes optimized for integration
- Squeeze the max out of each nodes

- Leading edge digital processes
- Move quickly to a finer process geometry

Major drawback (traditional)

Unable to offer enough MIPs as new system requires more deterministic control & faster time-to-market

Pure logic DSPs requiring a plethora of analog companion chips

Strategy moving forward

Strive to offer more MIPS while keeping integration advantage

No compromise: One-chip that cost-effectively combines DSP(digital) and analog


Texas Instruments says its DSP-based TMS320F28x controllers offer a control-savvy core, flash memory, flexible, robust peripherals, a, low device cost, computational and math efficiency, and “C” programmability.

Memory Architecture Comparison

DSP Memory Architecture

MCU Memory Architecture

Harvard architecture (multi bus)

Von Neumann architecture (single bus)

2-4 memory accesses per cycle

Typically 1 access per cycle

Usually No caches–on-chip

SRAM May use caches

Small to medium address reach

Medium to large address reach

Separate external address and data buses

External buses may be multiplexed

 

Integration Comparison

DSP Integration

MCU Integration

Traditionally have been MPU vs. MCU (this is changing…)

Single chip solution

Traditionally limited selection of on-chip peripherals (this is also changing…)

Wide set of general purpose peripherals (SPI, SCI, WDOG, Timers, CAN, LCD…)

DSP peripherals optimized for end application

General-purpose peripherals

Bode plots helps resolve motion-design problems
When designing closed-loop control systems, measure the system frequency response to solve problems that are hard to find in other ways, recommends Erik Brewster, applications engineering manager, Motion Engineering Inc., part of Danaher Motion. Tools such as frequency response plots (“Bode plots”) help illustrate problems.


The example, explains Brewster, plots data from a simple brushed motor with an inertia wheel attached to the motor shaft. Standard PID tuning results in audible “growling” before getting satisfactory performance. A quick frequency response test shows a resonant peak at 685 Hz. Filtering this peak out allowed a six-times increase in control loop gain with the same amount of audible noise, control stability, and increased disturbance rejection. Brewster says to keep in mind that there is no motor coupling or similar normal sources of mechanical resonance. Mechanical resonance comes from the mass resonating with the motor shaft deflection, he says.

www.motioneng.com


Related reading

Click here for more Control Engineering design tips for machine control from sources at Banner Engineering, Phoenix Contact, and Rockwell Automation.





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