| Home & Axis Limit Switches |
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sparkscompass |
| Posted: Monday, 15th Mar 2004 10:58:31 AM EST (#77) |
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My desktop mill has no home or axis limit switches. I would like to add them. In the CNC Zeus manual, zero is reputed to be accurate within .0005 using "good quality axis limit switches." Can you recommend some that would allow this desirable accuracy? |
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support |
| Posted: Wednesday, 17th Mar 2004 06:49:15 PM EST (#78) |
The limit switches I use are Square-D lever type switches #AW12BA series C and have a watertight enclosure. The accuracy stated is derived from actual testing and applies only to repeated machine homing to verify the accuracy of the homing cycle. In actual use if a machine sets overnight the results can vary depending on the integrity of the machine or a multitude of conditions such as temperature changes or oil settling out of way surfaces.
The switches are likely too large for most desktop machines and I believe most any good quality switch or even those infrared led break switches will yield very good or possibly better results.
Setting the homing parameters correctly is also important in achieving an accurate home cycle. The feed rate to the limits should be maintained reasonably slow for safety, but the return rate from the limit should be slow. The program checks the switch between every step on the return move looking for the switch to close then adds the switch clear distance after detection.
Jeff
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sparkscompass |
| Posted: Monday, 22nd Mar 2004 07:52:49 PM EST (#79) |
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I've looked at a number of solutions for the axis & home limit switch and settled on an Optek OPB804 slotted optical switch available from Digi-Key. This U shaped housing has an LED and phototransistor mounted so that a thin vane will interrupt the beam and provide positive switching information in an easily fabricated fashion. The CNC Zeus manual states that the (shorted to ground)limit switches open when a limit is hit. I assume that means that the program is looking for a "1" logic level (5 volts) at the limit. I should then hold the designated ports low until the switch activates, driving them high. I also assume that same X,Y,Z ports are used both for the (positive) limit and for the (negative) zero point. Are my assumptions correct? Thanks for your information. |
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support |
| Posted: Tuesday, 23rd Mar 2004 12:13:16 AM EST (#80) |
You are correct! A logic “1” on the port limit switch inputs would indicate a limit has been reached.
A logic “0” is required for normal machine operation.
These logic levels work well with optical switches like the OPB804 or the CNZ1414A from Digi-Key because the output transistor holds the line low (logic 0) state as long as the led light reaches the detector side. If the beam is interrupted then the transistor turns off and the line returns high (logic 1).
Only one switch per axis is required and should break the beam near the end of travel in each direction accounting for deceleration. If two switches are required on one axis then they can be wired in series so it takes both switches to maintain the low (logic 0) level and will go high (logic 1) in either switches beam is broken.
The limit switch wiring should be shielded if running in a conduit with motor drive wiring since even very short pulses on the limit switch inputs can be detected and stop program operation. The primary symptom of this is if the machine stops program operation for no apparent reason and resumes by pressing ENTER.
Jeff
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