Flow Dynamics and Automation, Inc. Tips

Tips - The Use of Proportional/Servo Valves

These valves modulate hydraulic parameters according to electronic reference signals. They can be used in open or closed loop applications to modify speed, direction, force and accurate motion of a hydraulic cylinder. Open loop applications typically involve the acceleration or deceleration of a load and are not required to stop accurately. For this reason valving used to accomplish this are not coupled with Temposincs position transducers and usually have positive overlapping in the center position of the spool. This overlapping (deadband) is not detrimental to the ability to accelerate or decelerate. In these applications the use of feedback devices internal to the valve is not necessary.

As the requirements for accuracy are introduced, the type of valve used to accomplish high repeatability, changes. To close in on a specific pressure or position you must have a valve that has a high response time. I.E the time for the valve to react to a change in reference signal. This is expressed in the engineering data of each valve as frequency response. This response number must be measured at the same degree of phase lag ( typically 90 degrees) in order to have a meaningful comparison of valves. If for example, you require the cylinder to return to the same place within +- .002" of an inch, you would require the system to be capable of several things. First, to know where the cylinder is at all times, secondly likewise to know the position of the spool in the valve at all times, and third to have no deadband in the spool of the valve. The positive overlap or “deadband” is detrimental to accuracy because it allows spool movement around the center position without a corresponding change in flow. The requirements for this type of accuracy would dictate the following: A. Temposonics position feedback in cylinder itself. B. Zero overlap (no deadband) in the proportional valve and C. Position feedback device on the main spool of the proportional valve. All of these parameter must then tie back to a Delta motion controller to control the entire operation. The valves must also be “tuned” to the program either in the amplifier card driving the valve or in the Delta motion controller. The parameters that must be tuned and their definitions are as follows:

Bias: Driving current required for bringing the valve to a null point
Dither: The pulse frequency of the driving current.
Scale: The relationship between the driving current values and the values of the reference signal
Ramps: The time required to change driving current to the valve following a step change in reference signal
Gain: The factor which modifies the loop error, to correct the values of the driving current I closed loop controls.

While all this may seem very complicated, rest assured that the staff here at Flow Dynamics and Automation are well versed in setting up a variety of applications using this technology. Once installed the “program” is burned into a plug in chip that may be replaced like any other part. Therefore the technical maintenance requirements are minimal and simple and simply do not require much attention.

If you have an application that requires the above described technology, give us a call or e-mail us a sales@flowdynamics.net and we will be happy to assist you.

Tips - Repair Facilities

I couldn’t help but chuckle a little upon reading Check list for choosing a repair facility in the October I & A department. Irving Weiner makes good sense with his six important points, and I agree with him. However, customers should go a little beyond a few bells and whistles in their search. Let’s face it, there are many quality independents out there – but some shoemakers as well.

A case in point: a local customer with about 100 Moog servovalves in the shop – who formerly dealt with one of the large independents – would send out three or four of these servovalves for repair each and every week on average. Our company, the Moog distributor in this area, finally convinced this customer to send the Moog servos back to the manufacturer for evaluation and repair. During a 6-month period, almost 100 previously “repaired” servovalves were sent to Moog, evaluated, serviced, and returned to the customer. During the ensuring six months, only two servovalves had to be sent out for repair by this customer. The savings during that period: $72,800.

In addition, Moog initiated a tracking system which provides the customer with a complete record indicating when each and every valve was repaired previously, how long it was in service, when it next needed repair, and what the possible cause of failure was.

This is not to be construed as an indictment of all independent service shops, because there surely are some very good ones; but among the problems many component manufactures find is the use of non-factory parts, substandard testing equipment, and some times, poorly trained employees. My tip to Mr. Customer: take a good, hard look at who will be doing your service work and ask the difficult questions.

Do we have an axe to grind? Nope! We are an independent repair facility – and have been for the past 40 years.

James D. Palmer Sr.
General Manager Tri-State Hydraulics, Inc.
Charlerio, Pa

Reprinted from HYDRAULICS & PNEUMATICS November 1997

Moog Industrial Controls Division
Moog Inc., East Aurora, NY 14052-0018