Servo Savings
- Published: December 31, 2002, By Edward Boyle, Contributing Editor
Ward/Kraft, a private, nationally known manufacturer of business forms, labels, and documents, also manufactures all its own machinery in-house. Five years ago, the company conducted a cost/benefit analysis comparing line-shafted and servo driven machines.
Ward/Kraft, a private, nationally known manufacturer of business forms, labels, and documents, also manufactures all its own machinery in-house.
Five years ago, the company conducted a cost/benefit analysis comparing line-shafted and servo driven machines. The converter found that over time, the investment in servo technology would pay off in faster production times, less waste, and fewer mechanical repairs.
Ward/Kraft also found a partner in Siemens Energy and Automation, which supplied what Siemens refers to as a “totally integrated automation” approach to its business. The plan called for designing and building all the machinery in-house and installing servo systems on all the machines created.
Strategic Investments
The plan is working. Established in 1972 in Fort Scott, KS, Ward/Kraft has doubled in size over the past ten years. According to Steve Adamson, Ward/Kraft electrical engineer, the key to this success is a top management willingness to invest in the future, especially in its machinery division.
Adamson says most companies the size of Ward/Kraft will not make the upfront $1 million or $2 million investment in an in-house machinery division. Ward/Kraft is unique within the independent printing industry, he says, and that gives the company a competitive edge by lowering overall costs up to 30%.
As part of the strategy, Ward/Kraft evaluated numerous potential vendors looking for one to supply the automation technology and service needed to build the machines. After working with Siemens Energy and Automation on previous PLC and software retrofits, the converter tapped Siemens to help build Ward/Kraft's first servo-driven finishing machine, the Versatile Pressure Machine [VPM04]. “We gave them the opportunity because of the commitment they made to our program,” says Adamson.
Line-Shaft Problems
“The older versions of the VPM were all line-shaft driven,” Adamson explains. “One main 20-hp motor ran the entire machine via a line shaft. Registration throughout the machine was not consistent, and mechanical breakdowns were much too common. The reinsert system, also driven by the line shaft, was a mass of belts and pulleys and also contained a correction gearbox, correction servo, encoder, and other sensors and components.”
Adamson says the registration adjustments on previous VPM machines were made using hand cranks and a correction gearbox. The company also experienced difficulties with accuracy when making small adjustments via the correction gearbox, he adds.
“During setup, the quicker the operator can register the machine, the less waste is involved,” says Adamson. “Minimizing waste has always been top priority when redesigning machinery.”
The VPM04 is a freestanding, four-axis finishing machine built at the Fort Scott facility and shipped in segments to the company's Fredericktown, OH, plant. According to Adamson, it took about six months to build the machine. Ward/Kraft worked closely with Siemens to find the right servo solution to the line-shaft problem.
“We've been able to achieve better registration at higher speeds,” Adamson reports. “In older line-shaft versions of the VPM, speed changes would cause our register to move out. But now everything is so tight we can speed up and down without having registration problems.”
Ward/Kraft receives rolls of paper specific to the type of job it's going to print. Most of the time its products are 24 in. or less, and the majority of them fall in the range of 14-16-in. wide.
The finishing process starts when preprinted paper rolls are loaded onto a splicer. The paper feeds into the VPM04 machine's reinsert station axis at a top speed of 500 fpm. Once the paper is reinserted, it goes through two coating stations representing the second and third axes. A fourth axis on the finishing station perforates and cuts the paper into 11×8-in. sheets. The product is then folded and stacked. If needed, the paper can be run back through another machine for additional printing.
Good Communication
Four Simovert Masterdrives MC drive units communicate with one Siemens S7-315 programmable logic controller (PLC) over a high-speed Profibus-DP communications system. The drives' technology includes a virtual master communicating speed and position data to all drives over Profibus at 12 Mbaud. The PLC and drives are housed in a cabinet adjacent to the VPM04.
The PLC provides a speed set point to the virtual master, performs parameter changes for various machine setups, and ensures proper sequencing on machine startup and shutdown. The processor also performs machine function interlocking.
Each drive controls a Siemens 1FT6 servomotor, one servomotor per axis. The reinsert drive unit is 5 hp and controls a 4.7-hp servomotor. Two coating stations also each have 5-hp drives and 4.3-hp servomotors. The final finishing drive is 15 hp and controls a 13-hp servomotor. The drives were programmed utilizing Drives ES software running under the Step 7 Simatic Manager engineering environment.
One communication cable connects the system together. With the ability for motion control with Profibus DP, the entire system was simplified, notes Adamson. (Traditionally, a cable for a field bus and a cable for a motion bus were required to perform electronic line-shaft and sequence-control functions.)
The built-in capabilities of Profibus DP include a synchronous and broadcast modes, as well as peer-to-peer communication and clock-synchronized tasks. This allows the field bus and motion bus to be one and the same, resulting in hardware and engineering savings. The motion control with Profibus DP was configured with Step 7 hardware configuator and Drives ES, enabling multi-drive, high-speed, peer-to-peer synchronization of position and speed set points.
Step 7's Simatic Manager provides an integrated development environment, out of which come the following:
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Drives ES Basic-programmed, design-time drive parameterization sets can be saved to disk or copied between projects. Drives ES Basic also can be used to monitor and modify drives parameters on-line.
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Function blocks from the ladder logic program run-time drive parameterization, including technological constants. The function blocks used by Ward/Kraft (PDAT_ASYNC) were supplied with the Drives ES Simatic software.
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Run-time drive process data R/W, including control words and set points, is handled by function blocks called from the ladder logic. These function blocks (PCD_SEND, PCD_RECV) also are included with the Drives ES Simatic software.
A New Address
The Drives ES Simatic function blocks map the PLC ladder logic to the drives through a simple hardware configuration address. The complete range of PLC memory addresses, including data block structures, then are available to the drive communications function blocks within the ladder logic.
“Normally, we would use Simolink for drive-to-drive communication via fiber-optic cable and then use Profibus for PLC-to-drive communication,” says Adamson. “We decided to use Profibus to handle all drive-to-drive and PLC-to-drive communication because of hardware cost savings and programming simplicity when utilizing the Drive ES software package.”
The HMI is provided via a Siemens TP170 touch panel mounted on a central control counsel. During the reinsert process, a color mark sensor hardwired to the drive is used to read a preprinted ballot mark on the paper. This allows the drive to take a real-time sample of the paper position every repeat and compare it to an actual position on the reinsert servo.
Errors are corrected by the reinsert servo utilizing a position trim value within the drive. This maintains consistent paper position throughout the coating and finishing process and allows for excellent registration. According to Adamson, in the past operators had to rely on mechanical hand adjustments to correct registration problems. Now, because of servo technology, the operator can adjust the registration with the touch of a button.
The TP170 touch panel is a Windows CE-based platform providing monitoring and control functionality at various locations around the machine. The built-in MPI (multi-point interface) Profibus port on the TP170 allowed for seamless integration into the S7 network back to the PLC as well as the PC workstation. The HMI screens were developed with Siemens' HMI configuration package ProTool that can be integrated into the Step 7 environment. The TP170 supports a remote control mode that receives HMI configurations automatically via ProTool on the PC workstation and reboots itself.
Cost Savings Bottom Line
The end result, says Adamson, is higher speeds, reduced waste, and increased profitability.
Over time Adamson expects to see the waste figures diminish considerably compared to line-shaft-driven machines, making the company more competitive because these savings go right to the bottom line.
CONVERTER INFO
Ward/Kraft
2401 Cooper St., Fort Scott, KS 66701
620/223-5500; wardkraft.com
SUPPLIER INFO
Siemens Energy and Automation, Alpharetta, GA; 800/964-4114; sea.siemens.com
