Evaluating Servo-Controlled Presses
Since the first servo-controlled press system was introduced in 1991, the success of this technology in intelligent automotive assembly and test systems has spawned a number of competitive products in the market. When there was only one product to consider, the choice was easy. Today, it is more complicated.
There are three aspects to consider when evaluating the true value of a servo-controlled press system in any auto manufacturing application: mechanics, controls/software, and support. Here are things to look for in each category.
The system needs to be robust enough to survive in the production environment and there are some specific component characteristics that are critical to ensure this happens.
Press Heads—The weight of commonly available servo presses varies by as much as 400%. Heavier is definitely better. Light weight presses may be less expensive to produce, however they tend to cost much more down the road with downtime and maintenance concerns.
Ballscrews —A servo press is not a “standard” application for a standard ball screw and commercial ballscrews are not engineered for these operating conditions either. The most cost-effective, long-term solution is a custom-engineered ballscrew.
Sizing is also an important consideration. The absolute minimum size for a servo press ballscrew should be 2.5 × dynamic load to rated capacity.
Motors—To achieve maximum service life, motors should be sized to run in the continuous current range over the full range of the press rather than requiring peak current at some points.
Gearing—Lower gear ratios reduce motor fatigue by lowering the ratio of servo rpm to spindle rpm. Lower rpm is better. It is also quieter.
System Sensors—Force sensors, typically strain gage load cells, should be physically integrated into the press head assembly to give maximum protection.
Position sensing is typically accomplished via the servomotor encoder. It is important that the encoder provide sufficient accuracy and repeatability to meet application requirements.
Other Important Features—The ability to perform in compression and tension and in any orientation; the ability to run at rated capacity for any duty cycle; and the ability to move to and hold a specific force indefinitely without a brake and without damaging the motor, gearing, sensors or ballscrew.
Motion control and monitoring functions should be integrated as much as possible to simplify system configuration, support upgrades and changes, and provide the maximum amount of flexibility and reconfigurability. Some of the more common requirements are listed below.
1. Motion control capabilities:
- Press to an external dimension, which should include the ability to interface with external digital
- Move to a rate of change function
- Applied velocity
- Applied torque
- Closed-loop torque control
- A command language to create custom command routines
2. Software data analysis functions:
- Curve math
- X & Y crossing
- Global macros and variables
- Global variables
- Monitoring over steps
- Dy/Dx function
- Local macros
- Live expressions
- Monitoring over macros
- Gaging minimum, maximum, average over steps and macros
- A full math engine with an expression language (to create evaluation commands)
In general, the more of these capabilities available in a standard unit, the more capable and flexible it will be.
Service & Support
Service and support are essential, and all of the standard evaluation criteria apply. Talk to users with similar applications if at all possible rather than relying entirely on the supplier’s statements.
Application engineering experience and technical support resources are important. A good track record in real-world applications is an essential starting point, and a bad one should at least be a red flag and probably a deal-killer.
Finally, a focus on the long-term Total Cost of Ownership rather than initial capital cost will pay large dividends. Servo presses are not “throw-away” items. The ability to reconfigure them for future applications should be an important consideration in the selection process.
This article was first published in the 2013 edition of the Motorized Vehicle Manufacturing Yearbook.
Published Date : 11/13/2013