Redeploying robots
January 1, 2007
PLC robot controls can be used to rebuild or retrofit a variety of electric and hybrid robots, and allow DIY maintenance that avoids the expense and delays of calling an outside service engineer. |
If you don’t have the budget to buy a new, high-performance robot, consider beefing up your existing automation.
As molders search for new automation paths, they are finding a few alternatives to buying new robots. One option is to buy used robots that sell for less than new robots. When buying used, equipment age, maintenance, and runtime are important considerations. Late-model robots are usually in good condition, robots five to nine years old require refurbishment, and robots 10-plus years old usually need to be remanufactured.
A second option involves buying fully rebuilt robots from a qualified remanufacturer. Rebuilt robots reduce the risk of buying robots “as-is†that might need service and are usually half the cost of new robots with a full-service warranty.
Rebuilding allows molders to improve motion controls with servo technology for better speed, torque, and repeatability. Controls based on established vendors have an inherent advantage of easy DIY (do-it-yourself) service, plus PLC parts and training availability.
PLC controls provide 100% debug transparency, so a trained electrician can see all operational details, including program code, database, and processor details. This is a major advantage over embedded and custom robot controls based on proprietary designs and specialized parts. Such familiar PLC software allows in-house engineers and technicians to troubleshoot problems without outside service expense and time delays.
As a third option, molders can refurbish their existing robots, sometimes in-house. Many PLC- and CNC-based robots can be retrofitted with off-the-shelf controls. Those looking to save money by improving their current robots should consider these types of controls, with inherently easy software that requires minimal training for troubleshooting and repair.
From refurbish to remanufacture
The variety of gantry robots in the plastics industry ranges from simple pneumatic actuators to induction motors driven by inverter (frequency drive) to fully articulated servomotor systems. Although most electric motors use encoders for position feedback, some inherently simple electric robots move an axis to end stops for position control. These basic robots require more setup time and have less flexibility, but are very easy to repair. If running the same part 24/7, you can save money with such low-tech systems.
Another important point of interest involves speed and torque curves. Induction motors (460V AC, 60-Hz, 4-pole) with frequency drives normally run up to 1800 rpm. One option with induction motors is “overspeed†wiring. Wiring a 1-hp motor for 230V and running it up to 460V at 120 Hz can double the horsepower to 2 hp and the speed to 3600 rpm while maintaining adequate torque. This double-speed option provides low-end servo speeds at a much lower cost, especially with single-drive, sequential-motion robots.
Don’t think that you can only afford pneumatics, or that you must buy expensive servo robots. Each molding machine’s applications, job changeovers, tooling, and pre/postmold operations define what type of robot system provides the best ROI.
Your level of technical expertise and manpower availability will help determine what type of robots to purchase. If you run the same job six days a week, or inmold time is not important, a simple robot system can yield the best ROI. If you need high uptime but don’t want to pay high prices, a rectilinear robot with simple PLC controls can optimize your ROI. If you need high-speed motion or have complex applications, an advanced servo robot will provide your best payback.
Take notice in buying all-pneumatic traverse pickers: The cost savings quickly dissipate as you learn the importance of speed control, jogging, teaching points, and smooth motion control. A hybrid robot that has one pneumatic axis, preferably the strip stroke, or a hybrid that uses an induction motor with limit switches for the traverse axis is a better alternative. But two or more pneumatic actuators will significantly reduce your financial payback. Better alternatives: Wait until you can afford all-electric robots; buy a good used or rebuilt robot; or consider leasing contracts.
When to rebuild? When you have several of the same robot, and the robot’s mechanics are reliable and capable of faster speeds. Obviously, robots that don’t have strong axis design are not a good candidate for rebuilding. Since most rebuilders are in the United States and Canada, it’s easy to find replacement parts for robots made in North America, which makes rebuilding feasible. Attention should be given to spare parts, technical support, and programming with foreign-designed or -assembled robots.
As shown in the table, “Product and warranty definitions,†there are many different options when rebuilding robots. You can select the level of new performance you want. Be aware that retrofitted and refurbished robots usually come with a limited warranty covering new parts and programs, and fully rebuilt and remanufactured robots come with full warranties lasting one to two years. With all electromechanical wear-and-tear components replaced, such robots are like new and may even offer better performance than new robots, especially hybrid systems.
Controls
Try looking at robotics servicing from in-house-DIY and vendor-call-in viewpoints. Are your molding jobs straightforward, allowing simple automation, or more complex applications where a robot vendor’s technical expertise is needed?
A control system based on an established PLC vendor will allow you to get spare parts, training, and specifications anywhere in North America. A PC-based or proprietary control may prevent you from buying spare parts, or getting technical information and training from anyone except the original manufacturer.
One technology that has been around for years, but lost popularity with the rush to full servo controls, is combining induction motors with inverters (frequency drives). These Indro (INverter DRive Operation) controls use servo modules with encoder feedback for position control, have the smooth motion, axis jogging, point teaching, and variable speeds of a servo robot, but perform one movement at a time to reduce overall cost.
These sequential-motion robots have less low-speed torque, and therefore less position repeatability and more current loading when jogging. Usually turning motors at 1800 rpm, Indro-based robots can hit speeds up to 1.5 m/sec with a 50-lb payload. Using a double-speed wiring option, motor speeds can be doubled to 3600 rpm, providing 3-m/sec (low-end servo) speeds without affecting motor quality or lifetime. (Vendors that support overspeed wiring include SEW, Yaskawa, and Baldor.)
Software
Simple PLC-based robot controls do not require end users to program the robot with a sequence of tasks. Instead, they provide logical groups of setup screens that turn various features on and off and select motion paths and inmold actions. These simple menu-based programs also provide timers and speeds to be set but do not allow for unlimited or complex applications, although a qualified electrician can add features via ladder-logic modifications. Such an approach is great for beginners with easy molding jobs and small molders that cannot afford in-house automation experts.
Rebuilding robots or retrofitting with servo motors and controls can increase speeds by 100% or more while improving repeatability from 2-3 mm to .25 mm. Servo controls can also increase payload capacity, and improve low-speed jogging and motion with smoother, more repeatable stepping. New servo controls offer many special features that allow for better inmold and postmold operations, increasing the number of jobs that can run on a given press. New controls also improve overall reliability, and can reduce mold changeover times via faster jog and teaching, more user-friendly screens, and fewer motion errors when running automatic mode.
Some gantry robots use a CNC programming environment. Here you can obtain ladder-logic or G-code training from the control manufacturer and develop your own application programs. Such a control requires a trained electrician to ensure proper programming. PC controls allow end-user scripting, whereby a predefined instruction set is used, usually in a Basic or Pascal programming language. Embedded controls require the user to get all documentation and training from the robot vendor since these controls are custom designs.
Finally, the latest and most advanced controls use a graphical user interface (GUI) for operator control. These designs combine the best of simple selection screens with user-friendly programming. GUI-based robot controls provide an excellent setup and jog/teach environment; but remember, like any custom control, you are dependent on one vendor for all parts, service, and support. Be careful of displays that overload you with information, display excessive messaging, and tend to lock up like MS-Windows.
Cost comparison
Gantry robot pricing varies widely, thanks to their wide size range (from 300- to 4000-mm vertical lengths), various options, and features. (See “Showing you the $,†p. 37.) But if you’re buying new robots, be careful how excited you get about low prices. Few vendors can sell 20-30% below list price without cutting back on product quality and aftersale support.
Remember, you are not buying a passive machine that sits in a corner performing a simple offline function. You’re buying a highly engineered “operator†that interfaces with your injection machine, mold, downstream equipment, production workers, and setup and maintenance technicians, for up to 24 hours a day. Do you really want a cheap robot that may be down 20% of potential running time? Will quality parts and new software releases be readily available in the future? Will your robot supplier have qualified technical support to help redeploy your robots a few years later?
Rather than buying cheap, consider rebuilding your older robots with simple, user-friendly controls. Easy-to-use robots allow a cost-effective way to automate without the complexity and programming of PC or embedded controls. If you’re a molder looking to improve automation on a set budget, consider the cost savings and improved reliability of rebuilt robots. Money saved redeploying existing robots can be used for other important projects, or to improve your profitability.
Bill Egert ([email protected]) is engineering VP for Logic One Robots (Addison, IL; www.logicone-robots.com), a provider of used, rebuilt, and new robots, and robot controls.
You May Also Like