As part of the process of manufacturing power steering pumps for the automotive industry, a metal bushing is pressed with a measured force into the case.

Horizon Technology Group (Wyandotte, MI) recently incorporated electrohydraulic controls into the control system for a bushing press. This saved Horizon hundreds of thousands of dollars in production costs for its customer. It also streamlined the pump manufacturing process to eliminate unneeded fixtures and setup costs.

Horizon’s previous system used standard on and off solenoid valves to control the press cylinder. Limit switches attached to the fixture provided gross information to the controlling programmable logic controller (PLC) to tell it where the cylinder was. The motion was fine-tuned using timing loops run on the PLC without providing precise distance information. The cylinder encountered a hard stop at the maximum distance that the bushing could be inserted. The PLC set the press force to be initially low and then increased it as the bushing was inserted farther into the case. The reaction force was transferred to another cylinder via a sliding plate, where a load cell measured force. Because of static friction and hysteresis, it was difficult to maintain precise control. A data acquisition system recorded the press force and alarmed the operator if the distance was not met or if the required force to accomplish the task was below or above acceptable limits. The window of acceptable force was set to be relatively large to avoid false alarms.

Because the old system lacked precision controls and had relaxed alarm settings, it was not error-proof. If a bad bushing reached Horizon’s customer, the cost could be enormous. The steering gear manufacturer might assume that an entire batch of assemblies was defective, causing it to return the lot for 100 percent screening. The design of the old bushing press also required that the press cylinder and plate system be changed for each different steering assembly type. This resulted in duplicated fixture development costs. Horizon also had to deal with the cost of maintaining and exchanging multiple fixtures.

To improve the quality of the process and cut manufacturing costs, Horizon installed an electrohydraulic system to control position and monitor force of the bushing press cylinder. The old timing and limit switch-driven system was replaced with precision hydraulics from the Motion & Control Sales Div. of Parker Hannifin (Troy, MI), controlled by an RMC100 electrohydraulic motion controller from Delta Computer Systems (Vancouver, WA). The new system can be programmed to set the press cylinder to any position. One set of tooling can be used for any length bushing. With more precise control, productivity has also significantly improved.

The motion controller in the new system acquires its position information from a magnetostrictive displacement transducer mounted along the cylinder’s axis. Analog pressure transducers with high-resolution D/A converters and scalable outputs are mounted inside the cylinder. They provide continuous information on the force being applied by the press. To simplify system integration and reduce interfacing costs, the motion controller supports a direct interface to the transducers. The motion controller drives the cylinder by providing analog drive levels to a proportional hydraulic valve. The system incorporates a human machine interface (HMI) that interrogates the motion controller to figure out net force. This information is then used by the HMI to trigger alarms if the values are unacceptable.

Using the new control system, positioning the cylinder is 10 Arial more accurate than the old time-based control system (±0.003 inch vs. ±0.025 inch). Horizon’s customer estimates that the overall improvement in efficiency of its production process is around 10 percent, due mainly to the elimination of most of the setup Arial when it changes the pump type being manufactured.

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