Axis Soft Overtravel (Soft OT) Alarms: Stroke Limit Function for Fanuc Controls
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A basic principle of CNC repair and troubleshooting that is suprisingly unfamiliar to alot of people is the soft overtravel limit (software overtravel). It does seem to come up alot when dealing with CNC machines. What is it and why does it generate CNC alarms? Here are a few basics:
Alarm Codes:
Fanuc 16,18,21 Series Controls: Alarm 500 (n Axis stroke limit +) and 501 (n Axis stroke limit -)
Fanuc 10,11,15 Series Control: Alarm OT 001 (n Axis stroke limit +) and OT 002 (n Axis stroke limit -)
The software overtravel limit, or stroke limit is a point (or a value) that is set to a CNC parameter that will stop the movement of an axis when the value of the machine position of an axis reaches the value set in the parameter. This value set to the parameter is a distance from the zero return point of the axis (machine position 0.000mm or 0.0000 in.). Typically, there will be two parameter settings involved; one for the positive direction and one for the negative. The unit used for the parameter settings will be the detection units used to calculate the machine position. It may be a metric value in millimeters, or 1/1000ths of a millimeter, or another value. To see how to calculate the detect units for Fanuc 16,18,21, and other newer controls, click here.
Again, the value set for the positive and negative stroke limit parameters are a distance from the zero return point of the axis involved. If the zero return point is near one end of the axis stroke (i.e. the Z axis of a vertical milling machine zero point is typically near the upper end of the Z axis stroke) you would find the parameter for the Z axis + direction stroke limit set to a relatively small value compared to the value for the – direction stroke limit. This is because the machine has very little stroke available in the + direction when the axis is at the zero return point, near the upper end of the stroke. However, in the – direction, almost the full stroke of the axis is available when the axis is near the top of the stroke at the zero return point.
In the case of a horizontal machine with the X axis zero return point located at the center of the axis travel or the center of the table, you might expect to see the + and the – stroke limits set to the same value, since the available stroke would be the same in eithe direction from the zero return point.
Keep this in mind: Don’t mess with the stroke limit parameters if you don’t need to or don’t understand what they are. The parameters for stroke limits are typically factory set by the builder of the machine and are not to be altered in most cases. Why? Because it’s a pretty good bet that the machine builder has already set these values to the absolute safe limit of axis travel. The more available travel that a machine has, the better it is when trying to sell the machine to potential customers. In other words, there most likely will be no “left over” axis travel available to extend the travel without causing some sort of crash. Either the ballnut will come into contact with the ballscrew thrust bearing housing at one end of the travel or the other, or the ballnut will travel beyond its moveable range and damage the ballnut, or a way cover may open or collapse further than it was designed to, or some other mechanical catastrophe will occur. However, there are cases when it is expedient to alter these parameters when performing service on a machine.
One tip before we go further: if you find that you are going to have to disconnect the ballscrew from the motor, but the axis still moves with the motor (as might be the case if you were going to replace a ballscrew, a motor, or axis thrust bearings), it would be worth your while to get some sort of reference position established before you disconnect anything. Why? Because you will alter the relationship between the servo motor and the ballscrew if you uncouple the motor from the ballscrew. Establish a reference point so you can make sure you are back to the original axis position when the repairs are complete. You could carefully move the axis in the hand-wheel mode and touch off a tool to some unchanging point on a fixture, (or the chuck on a lathe) using a guage block or a piece of paper). You would then record the machine position for all axes at the touch off point. We are only concerned with the machine position at this point. When the repairs are complete, this touch off point should be exactly where it was prior to the repairs. If it isn’t, you have changed the zero return point of the axis. This means that all of your positions for the part program will be incorrect and you will most likely crash the machine if you don’t do something about the position error. You will have to make the machine position match the reference point you had established prior to the repairs. You could:
- perform a grid shift adjustment
- move the zero return switch or the dog for the switch
- move the axis to the touch off point, uncouple the motor from the ballscrew, move the motor until the machine position is correct for the reference point, then recouple the ballscrew to the motor. This should at least put you in the ballpark. Use the gridshift to fine tune the position. Caution: Do not uncouple an axis motor from the ballscrew of a gravity axis without blocking the axis from falling. This would typically be the Z axis on a vertical milling machine, the Y axis of a horizontal mill, the X axis of a lathe. However, it could be any axis depending on the machine. If you don’t know for sure, don’t release the coupling. The axis may fall and injure you or damage the machine.
Ok, so here we go. The most common cause of an overtravel alarm is an actual overtravel condition. You have indeed reached the end of stroke and the control stops the axis movement with the alarm before you break something. Or maybe you have programmed the machine to go to an axis position that is beyond the stroke limit. The machine may stop you with the overtravel alarm before the erroneous line of the program with the bad position data is even executed (before the axis even moves). This can be confusing, since you have an overtravel alarm, but the axis is nowhere near the end of stroke. Answer: Correct the program.
You may have an overtravel alarm when the machine zero return point has changed for some reason:
- You might have overtravel alarm problems when a zero return switch (or “decel” switch) is used for zero return and there is a problem with the switch. The switch may be malfunctioning or being activated by chips or a broken or loosened mechanical part at the wrong place in the axis travel. The machine, of course, has no way to know the difference between the actual zero return dog for the switch or an erroneous switch activation. It will see the switch activate, then deactivate, then the zero point will be established pretty much right there (no matter where the axis actally is at the time). It may be at the center of the axis travel when it should be at the positive end. You are unable to reach the zero return switch dog due to the soft overtravel alarm. Note: this example would not apply to a machine with a dogless zero return system (without a decel switch). Although stroke limits are used, the zero return process works differently. For more information on this subject, click here.
- Another example could be when mechanical connection between the motor and the ballscrew has been released or changed for some reason. You might find that the zero return point has been altered during repairs and you may get soft overtravel alarms when you attempt to zero return the axis. Attempting to move the axis in the + direction towards the actual zero point (most machines I have dealt with that have Fanuc controls will move in the + direction when performing manual reference return) results in a soft overtravel alarm after only a few millimeters of movement in the + direction. Again, you can’t reach the actual zero return point because of the soft travel alarm.
- A third example: a crash has occurred and slipped the motor/ ballscrew coupling relationship. The result is the same: the zero return point is now going to be incorrect and you may be unable to reach the correct zero return point before getting the soft over travel alarm.
The cause of the mispositioning must be corrected before proceeding further.
After correcting the mispositioning problems discussed above, you may find that the axis in question continues to alarm out with overtravel errors. The zero return point may have been established in the wrong place and you cannot get past the stroke limit to the zero return dog. You may find it expedient to “open up” the stroke limit so that the axis is able to reach the zero return switch dog and complete the zero return function. When you have successfully executed the zero return process, you must then restore the stoke limit parameter to its original value.
By altering the stroke limit parameters, you can essentially eliminate the stroke limit completely by setting a value well beyond the moveable range of the machine. Record the original factory setting first. You can enter 8 digits: “99999999″ or “-99999999″ as the setting, depending on which way you need to move. This will set the stroke limit down the block or up through the roof. In other words, there will basically be no stroke limit at all. Proceed with caution.
Soft overtravel and other commonly altered Fanuc parameters for various controls can be found here.
Again, altering the stroke limits is a last resort and does present the opportunity for personal injury and/ or machine damage. Use extreme caustion. Do not forget to restore th original parameter settings when you are done servicing the machine.
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