Distributors find that teamwork makes the difference

Two fluid power distributors join forces to give a bathtub manufacturer the single-source solution it craved

By Dave Cameron, Contributing Editor -- Industrial Distribution, 3/1/2006

How do you move a red-hot, 300-pound cast-iron bathtub from one oven to another? For one major kitchen and bath products manufacturer, the answer is, "as smoothly and efficiently as possible."

However, that strategy was becoming increasingly difficult to perform with the company's 15-year-old ActionBot industrial robots. The robots, which are 15- to 18-feet tall with a 20-foot reach, have three primary motions: rotation, lift and extend.

Here's how they work: On the manufacturing line, four ActionBots place bathtubs into and out of ovens for enamel finishing. The first robot places the bathtubs into the north and south lines for the enamel-glass application process, in which the enamel melts onto the tub. The remaining robots remove the tubs from the furnaces and place them into secondary furnaces for an additional coat. The tubs are then moved to an inspection station and exit line.

Using a combination of 1970s vintage Moog hydraulic actuators, closed-loop hydraulic valves and electric drives with outdated controls, the ActionBots were plagued by "bounce" on the rotation and extend operations—problems that were causing lost time and damaged product, according to the company.

To combat the problem, the firm decided to retrofit the ActionBots with updated controls, and sought a single supplier to provide both the drive and control, and the hydraulic solutions. After discussing the project with several vendors, they chose a newly formed partnership between two fluid power distributors: Brookfield, Wis.—based Controls for Automation, Inc., which provided the drive and control elements, and Green Bay, Wis.—based Fluid Systems Components, Inc., which provided the hydraulics.

Though the distributors had never worked together before, their decision to join forces was aimed at offering the customer one seamless resource to the manufacturer (Bosch Rexroth) and simplifying the entire process.

Greg Catalano, CMA sales engineer, worked with Andy Baumann, FSC engineering manager, to work out the details of the partnership—a partnership that not only spelled success for the customer, but also created an enduring relationship between the two distributors.

According to Catalano and Baumann, other vendors had offered the customer solutions that included multiple product lines to meet the application requirements. But for the bathtub manufacturer, the CMA/FSC team offered one product source, and that meant one company to call with issues, one purchase order to place, and one service group to deal with.

Baumann described the partnership as a unique opportunity to combine FSC's hydraulics expertise with CMA's drive and control expertise, exclusively using one manufacturer's products.

"As we began to examine the project, it was immediately evident that we were a good match," says Baumann. "The synergy between us was remarkably intuitive, and the solution came together almost effortlessly."

To increase throughput and decrease product damage, the manufacturer wanted to change the rotation axis to electric servos for better response and positional accuracy. On the lift and extend motion, electric servos would have been too expensive, and the position is not as critical for those axes.

Working together, CMA and FSC determined that the best solution was to upgrade each ActionBot by adding a Rexroth DKC02.3 electric servo drive with an MHD112D servo motor for the rotation axis; a Rexroth HNC100 hydraulic servo drive for the linear axes, including lift and extend functions; and SERCOS communication to the individual axis controller.

Servo motors provide precise movement such as turning, while servo drives monitor the motion commands to the motor, feedback from the motor, and synchronize operation with other sections of the robot. The DKC02.3 electric servo drive with MHD112D servo motor controls the rotation in which the robot spins from the bathtub pickup point to the oven. The Rexroth model HNC100 hydraulic servo drive is simply a special drive for hydraulic components.

SERCOS is a high-speed communications protocol developed specifically for digital control of multi-axis movements. It connects the individual axis controllers (via a cable) so that the movements of all the axes are coordinated and synchronized. An axis of movement is simply in/out, up/down, back/forth action.

According to Catalano, the previous up/down and in/out linear axes were Moog hydraulic actuators; the rotation axis was a Moog hydraulic closed-loop motor; and the control was an analog product made by Giddings & Lewis. These components were replaced with a new control package, which included a Rexroth PPC controller with VisualMotion programming language, and SERCOS interface to the electric and hydraulic servo drives. The high-speed, fiber optic communication protocol allows for multi-axis control with perfect synchronization between axes.

The MHD112D servo motor has absolute feedback, so no homing is required after power-up, and the DKC02.3 electric servo drive has two encoder inputs: one from the servo motor and another mounted to the machine. This allows for redundant position checking of the rotational position of the robot. Since there are four robots, their proximity to each other could allow contact, so this redundant position check was critical to the application from a safety standpoint.

"With the old system, the manufacturer only had proportional control," explains Catalano. "The new solution now provides proportional, integral and derivative control, which yields faster moves with greater control."

The PPC controller provides Ethernet and ControlNet interfaces on-board, which allows communication to the robots from the master PLC and from the company network at the same time. The controller can "see" production rates via the Ethernet connection at the same time the PLC is issuing the next job batch on the PPC. This seamless connectivity was a requirement that the distributors were able to meet with the built-in technology of the PPC motion controller.

Using state control, or active damping, the axis previously oscillated until it found the correct position. Now, the state control senses pressure on the cylinders through cylinder-mounted transducers, so there is no wait for stability before executing the next motion, which improves throughput by one to two seconds. Re-machining to eliminate a spacer at the gearbox allowed the robot's full torque to be used, and brought an overall improvement of four to five seconds.

The resulting performance gains allowed the customer to exceed the previous production rates of four robots while using only three robots. The customer then chose to rebuild the fourth robot to boost production rates even further.

In addition, the robots now move smoother due to the PID control algorithm, which makes for less vibration at the end of the robot. The diagnostics supplied by the SERCOS link between all the drives offers significantly simplified troubleshooting.

In the past, the only diagnostics were a green light for "OK" and a red light for "Fail." Now, there are thousands of diagnostics to lead the operator directly to the problem, saving time, and allowing the operator to get the machine back up and running in a fraction of the time.

Catalano and Baumann agreed that the timing was the greatest challenge to retrofitting the ActionBots.

"The manufacturer insisted on completing the retrofit of the first ActionBot over a New Year's holiday shutdown, giving us less than two weeks to complete the retrofit," says Baumann, who also noted that an expensive penalty clause was in place to ensure production would not be lost.

To survive the time crunch, the team created a two-step test prior to the retrofit. The first was to simulate the machine at FSC's facility to ensure that the control and drives would communicate, and that the HNC could "talk" to the existing hydraulic cylinders. The second step was to try out the hardware on an ActionBot over a Thanksgiving holiday shutdown prior to the manufacturer's official New Year's shutdown. During these tests, the partners discovered many hidden issues that would have made the retrofit a disaster.

"Since we did the tests ahead of the scheduled retrofit, we de-bugged the problems and had a proven solution ready," says Catalano.

The first retrofit was completed in the week-and-a-half window of time allotted.

According to the customer, installation and start-up went smoother than anticipated. One area that improved drastically was the servo following error, which is the difference between the actual position of an axis (i.e., the robot arm) and the position it was commanded to be in. An application typically has an error tolerance that determines the precision of movement. Speed, the complexity of multi-axis movement in a robot, and torque can affect the position tolerance.

In order to achieve previous running speeds, the customer accepted tremendous following error, the distributors said. This caused poor control of the actual path on coordinated motion, and resulted in scraping of tooling or damage to the product. Now, the ActionBots provide precise placement of the bathtubs and less chance for a serious crash.

"Not only did we learn a great deal about the product's capabilities, we also learned a great deal about our capabilities and FSC's capabilities as a team," Catalano says. "This project created a great partnership between two distributors that has carried over to other projects. It was a collaboration that showed our customer that Bosch Rexroth distributors carrying complementary product lines could work together to solve a demanding customer application."