Troubleshooting Techniques potx

A stick-slip condition on the machine slide can cause a null hunt discussed in the next section in the drive.. Poor surface finish can result where load forces are fed back to a drive tha

5 Troubleshooting Techniques

The time is gone when machines, controls, and drives were ‘‘add-ons,’’ as shown inFigure 1

Early machines used add-on drives These drives had one thing in common: There was no interaction among the control, drive, and machine

To a large extent these machines were open loop

Today’s numerically controlled machines are tightly integrated machine systems There is a great deal of interaction between the machine, drive, and control It is this interaction that results in problems with stability, surface finish on the work part, and accuracy The actual block diagram for today’s machine systems looks more like the block diagram shown inFigure 2

The primary difference between the machine drives of yesteryear and those

of today lies in the number and type of feedback signals that occur with modern control systems It is true that early drives had load forces fed back

to the drive, and these forces could cause a droop in the feed rate with load (called load regulation) Nonlinearities (stiction, lost motion, etc.) would not affect the stability because they were outside the servo loop, but they could cause inaccuracies By and large, drives were ‘‘hooked’’ onto a machine and

in spite of resonances in the mechanical drive, stick-slip, lost motion, etc.,

the machines performed with less than acceptable accuracy by today’s standards With the ever-increasing demand for more accurate machines, more and more of the machine was included in the control/drive feedback loops Today the machine, control, and drive are an integrated machining system The existence of resonances between the drive motor and machine slide can make the drive unstable A stick-slip condition on the machine slide can cause a null hunt (discussed in the next section) in the drive Poor surface finish can result where load forces are fed back to a drive that has poor servo stiffness A contouring drive with poor resolution also demonstrates poor low feed characteristics affecting the surface finish Electrical noise feedback into transducer cables, as the result of poor shielding and isolation, can cause stability and accuracy problems

With just about everything in the machine system being a variable that can affect machine accuracy and stability, where does the technician start to look for the cause of a machine that will not perform properly? It is virtually impossible to diagnose the trouble of a malfunctioning machine with all the control loops closed Therefore, the control and machine should be

Fig 1 Machine control system block diagram

Fig 2 Machine control system block diagram with nonlinearities

separated With a hydraulic drive the separation should be at the servo valve connector With a DC electric drive the separation should be at the input to the electric motor With brushless DC drives the battery box should be connected to the drive amplifier input The troubleshooting should now proceed in the following sequence

Closed Velocity Loop The next logical step in the ‘‘setting up’’ or troubleshooting of the servo drive is to close the tachometer or velocity loop and check the performance

Hydraulic Drive The velocity loop performance can be checked by closing the servo loop external to the control A battery box connected to the velocity drive input can be used to operate the servo drive independently of the control By operating the velocity drive back and forth and putting step changes in input voltage to the drive, the servo performance can be observed for stability Any unacceptable performance must be corrected before proceeding further

Electric Drive Setup instructions for the electric drive are provided by the drive supplier This type of drive can also be controlled by a separate variable DC voltage applied to the drive amplifier input terminals Likewise, by moving the machine slide back and forth and putting step changes in input voltage to the drive, the servo performance can be observed for stability

Unacceptable performance must be corrected before proceeding to close the position loop

In either the hydraulic or electric drive the performance of the velocity loop should be checked with an external DC voltage source The numerical control voltage sources such as the feed-forward voltage should not be used for trouble shooting The most obvious test to make with the velocity loop is

to make sure the drive can be controlled from standstill to the traverse rate Whether the feed rates are smooth or jerky, especially at the lower feed rates, should be observed With either electric or hydraulic drives the velocity loop gain should be set at its highest possible setting The velocity loop performance is directly related to the gain With too low a gain the drive will be sluggish and exhibit a speed droop with applied load One of the prime reasons for poor surface finish with numerically controlled machines is a velocity loop with too low a gain (thus also poor performance and bandwidth) There are engineering evaluation tests that can be used to

ascertain the servo loop performance However, these tests should only be made by a trained analyst For general use the gain setting can be made by raising the velocity loop gain until the drive goes unstable and oscillates The oscillation is usually observed as an audible rumble often called ‘‘tachometer growl.’’ When this condition occurs the gain should be reduced until the

‘‘growl’’ ceases The gain should then be reduced slightly further to allow some margin in reliability of the gain setting With electric drives the gain setting is often a component part of the setup instructions

Hydraulic Drives Machine piping should be checked to assure that all return lines run directly

to the ‘‘tank.’’ The drain lines should have a loop at the motor to be higher than the motor Any restriction in the drain line can blow out a motor seal

A blown seal is obvious with a puddle of oil on the floor Hydraulic pressure and return line pressure can be checked at the servo valve as a rapid way to determine if a piping problem exists Return line pressure at the motor should be less than 100 psi when the motor is rotating

Electric Drive Wiring for electric drives does not pose the same problems as hydraulic drives However, motor armature cables carry currents with very fast rise times from pulse-width modulation (PWM) DC drives and brushless DC drives These cables should be isolated as much as possible from tachometer feedback cables and position transducer (resolver or Inductosyn scales) cables

Open Loop Battery boxes can be used for both hydraulic and electric drives

Hydraulic Drive The battery box connected to the servo valve can be used to move the machine slide back and forth A normal machine slide will move (break away) with about þ0.004 A or less This current may not necessarily be symmetrical around zero The servo valve mechanical null adjustment may cause a slight servo shift if it is not centered

Electric Drive For DC electric motors, the breakaway current should be approximately less than one-half the rated current Excessive breakaway currents in either drive indicate a sticking or binding machine slide Vanes or pistons in a hydraulic motor and armature winding slots in electric motors will cause a cogging effect when these motors are operated as an open loop The cogging

is noticeable at low speeds It is normal and should not be a cause for alarm

A properly operating tachometer loop will smooth the open-loop cogging action Machine slide stick-slip can be observed with a battery box in cases

of friction slides With antifriction slides (hydrostatic or roller bearing) stick-slip should not be a problem

In the case of friction slides a breakaway current two or more times the run current can cause a position loop null hunt if the stick-slip is inside the position loop

Hydraulic drives usually have some amount of internal leakage This leakage is a form of damping for the hydraulic resonance in the drive It will

be of some benefit in damping and will permit higher gains to be used When the leakage becomes excessive and variable, the low feed rates may become jerky This is the same resultant effect obtained from varying loads on a low-gain velocity servo loop The cure is the same for both situations The velocity loop gain (and thus the bandwidth) should be set to its highest feasible setting With some hydraulic motors the leakage has been found to

be excessive and the motor should be replaced Some control suppliers make

an unnecessary practice of adding cross-port leakage to all hydraulic motors Cross-port leakage in the form of a drilled set-screw plug in the servo valve manifold should only be used if needed Cross-port leakage reduces the sensitivity of a drive and if required should be kept as small as possible The diameter of the drilled plug should be about 0.001 in With the larger hydraulic motors, experience has shown that a 0.0025- to 0.003-in diameter orifice will suffice

In general the setup and troubleshooting of electric drives has been found to be simple and reliable The numerous variables associated with hydraulic drives have proven to be challenging at times

Closed Position Loop Each control manufacturer will probably have a different technique for closing the position loop The one thing in common to all controls is that the position loop gain must be set at a predetermined value The techniques vary for setting this gain It is quite common to set the gain by measuring the position lag (command position7actual position, called the following error)

at some given speed In other cases the output position responses to a step input position may be measured and recorded on a chart recorder The position-loop gain is then adjusted until the desired response is observed on the chart recorder Whatever the techniques used to set the position-loop gain, the control manufacturer will have a step-by-step procedure for these adjustments

Once the position-loop gain is set to the prescribed value, the servo-driven axis may not perform properly for a variety of reasons These are some symptoms of faulty position loops:

1 The servo drive may be unstable or it may null hunt at a low frequency

2 The feed rates may not be smooth

3 The drive may not be stiff enough

4 The drive may not position accurately

To relate all the possible faults to the symptoms would result in endless discussion The next section attempts to relate different kinds of problems to their possible causes

Table 1illustrates the causes of problems

Poor Surface Finish Causes:Other than tooling problems, a poor surface finish can result from a poor low-feed contouring control If the low-feed-rate contouring control does not operate smoothly it can be defined as a drive with poor drive resolution (the difference between breakaway error and run error is too large) The smaller the control signal required to cause the drive to move, the smoother is the drive and the better the surface finish The surface finish

is also related to the bandwidth of the velocity loop and the position loop Poor surface finish is very often the end result of a low gain (and thus low bandwidth) tachometer loop

Stiction and high-friction machine ways can also contribute to poor surface finish A machine way is the surface area where a machine slide makes contact with the structure of the machine Nonlinearities such as nonuniform leakage, as found in hydraulic motors, can definitely cause surface finish problems

Cures: It is recommended that all feed drives use the highest tachometer loop bandwidth (highest gain) possible A wide bandwidth

Table 1 Causes of Problems

Problem

Possible causes

Poor surface finish

Unstable position loop

Null hunt

Tach loop growl

Feed rate not smooth

Low stiffness

Insufficient accuracy

Note: X, Definite cause; p, possible cause.

tachometer loop will minimize the nonlinear effects of hydraulic motor leakage The machine slide should have no lost motion (backlash) in the mechanical drive train, and the machine ways should be antifriction (roller bearings, hydrostatic, Rulon way liners, etc.)

Unstable Position Loop Causes:There are several causes for an unstable position loop, with the most obvious being too high a position-loop gain for a given system When resolver feedback is used at the drive motor, the nonlinear mechanical problems such as lost motion, stiction, and low spring rates will not affect the stability of the drive However, when direct slide feedback is used (using Inductosyn scales) these mechanical nonlinearities are inside the position loop

Cures:For contouring servo drives the position-loop gain should be in the ‘‘soft servo’’ range (0–2 ipm/mil) The machine dynamic problems can be minimized by using a low position-loop gain when direct machine slide position feedback is used Some control manufacturers use 0.6 ipm/mil position-loop gain on all axes In general, most large machines with direct feedback should have position-loop gains of about 1 ipm/mil It is very important that lost motion (backlash) be absent from drives with machine slide feedback If a wound-up gear train should lose its windup it will probably result in an unstable position loop

Null Hunt Causes: A null hunt is a form of instability A null hunt is a very-low-frequency oscillation sometimes called a limit cyle oscillation Lost motion (backlash) can cause a null-hunt condition when combined with large friction forces Usually a backlash condition inside a position loop results in

an unstable drive The fact that the period of oscillation may be fast or slow

is not significant Stick-slip on a machine slide can definitely cause a null hunt In general, a null hunt is possible when the breakaway friction is at least twice the running friction The null hunt associated with stick-slip can also be aggravated by a spring (unstiff drive screw) inside the position loop With hydraulic drives, excess leakage will cause an unstable position loop, which will oscillate at a low frequency and appear as a null hunt

Cures:For most cases of null hunt caused by backlash, antibacklash gearing (wound-up gearboxes) should be used When the windup is lost in a wound-up gearbox the position loop is almost guaranteed to be unstable Friction-type slides should be avoided For machine ways using roller

bearings, hydrostatic ways, or Rulon way liners, null hunt from stick-slip should not be a problem Excessive cross-port leakage on hydraulic servo drives will cause a null hunt, and care should be taken that if an orifice is required for hydraulic damping the orifice is in place There have been cases where the orifice was omitted The orifice is located in the servo valve manifold Hartman motors have enough internal leakage and do not require additional cross-port leakage

Tachometer Loop Growl Cause: The instability of a velocity loop will cause an oscillation at a frequency high enough to be audible, thus the term ‘‘tach loop growl.’’ The most significant cause is trying to set the loop gain too high for the existing parameters

Cures:The most obvious cure for an unstable tachometer loop is to lower the loop gain However, if the loop gain must be lowered the total contouring drive will have reduced performance

Feed Rate Not Smooth Causes: There are numerous causes for an unsmooth feed rate Excessive and varying hydraulic leakage can also cause a varying low feed rate in hydraulic drives Other things, such as the drive nonlinearities of stick-slip, lost motion, low mechanical resonances can affect the drive smoothness Cures:A prime consideration is to maintain as high a tachometer loop gain (thus bandwidth) as possible assuming other restrictions, such as underdamped hydraulic resonances and mechanical resonances, have been dealt with

Low Stiffness Cause:Stiffness as referred to here is the servo drive loop stiffness It is a measure of how many lb–in of torque is required to rotate the motor shaft a given number of degrees Too much leakage will reduce the stiffness of a hydraulic drive

On a machine, the stiffness measured at the motor increases by the square of the drive ratio The stiffness can also be measured at the machine slide where the stiffness increases by the square of the ratio/lead Drive stiffness is discussed in depth in Part II

Cures: To maintain adequate stiffness the design of the drive must include the proper sizing After the drive is assembled the only variables left

to adjust are the loop gains