Created Date: June 10,2009

Created by: Bob S BN

I have seen some high precision microswitches(snap acting, mechanical) that may be that accurate.

I 've also seen some sensors for air cylinders with magnetic pistons that actually feed back an analog signal over a position range of 1/2 " or so.

I'm sure there are a few other options as well, keep looking.

What type of actuator are you homing (mechanical, pneumatic)??

Good Luck,

Created Date: June 10,2009

Created by: daveinarmstrong

I am positioning a metal fence on a panel saw. This fence is driven by a screw. When the panel saw is first started up the fence is driven to the zero position (Home) and then the PLC uses this zero point as a starting point to calculate measured cuts from.

I am going to try a inductive proximity switch and test the accuracy.

Created Date: June 12,2009

Created by: jonesjeremya

Dave,

The precision of the "switch " is relative to the positioning speed while the Axis is homing. I am a technical engineer of woodworking machinery and have many years experience on different brands of horizontal beam saws, as well as virtually all other types of woodworking equipment.

There is really little difference between mechanical and inductive in this application. In my opinion, I prefer mechanical, and not a snap action. As long as the switch is a "quality switch "

The switch just needs to be "repeatable ". Each time the pusher axis homes, the switch is opened or closed and the value in the controller set as the reference point is set. It does not matter that the switch is accurate, it matters that the switch is highly repeatable. There will always be some level of discrepancy when you are indexing a moving axis.

If you are having positioning issues, or mis-sized boards produced by the saw, the problem may not be the reference switch.

I may be able to help further.

Good Luck

Jeremy

Created Date: June 14,2009

Created by: daveinarmstrong

I used a dial gauge against the fence and drove the fence to home several times. I am getting 0 to 0.020 " variation during the test. The last 2 weeks the Absolute cut was off by 1/4 " and i am thinking that the bracket holding the home position switch must be moving somehow, but there doesn't appear to be anything hitting the bracket. this machine was built in 1988 and uses a brake motor to position the fence. With your experience does this 0.02 " variance seem normal?

I will be upgrading this machine with a spare Automation Direct PLC i already have and a new servo fence motor because the existing PLC is starting to fail.

Created Date: June 14,2009

Created by: jonesjeremya

Thanks for the info Dave.

I do not have much first hand experience with Casadei particularly. I took one apart to make room for a new Giben several years ago. Gibens have used telemechanique roller head limit switches in the past.

.020 " is excessive, no doubt. So, if I understand you correctly you are homing the axis repeatedly, and have measured discrepancies of 0.000-0.020 " during each homing cycle using a dial indicator which remains stationary during the tests?

The problem "could " be the switch that is used for homing. Have you tried replacement? It could be that the switch just isn't repeatable any longer. Could be mechanical wear or dusty contacts, etc. I would replace the switch with the same type as OEM to eliminate the possibility of the switch, and to make sure you are using a equal quality switch.

If you still have the discrepancy after the switch replacement, I would look closely at the motor/brake combo. Does the brake really hold/stop the motor shaft from turning? Does the brake stop the motor instantly? There are usually adjustment posts to adjust the brake plate and the friction plates to be plane with one another. Generally the rule of thumb is to use a feeler guage of about 0.5mm when the brake is disengaged and set the plates evenly. When the feeler guage is removed the motor should turn freely while the brake is disengaged. When the brake is unpowered and engaged, the motor shaft should be locked with no possibility of turning.

The other thing to consider is the age of the machine. Is there excessive wear to the drive components such as gear boxes, screw drives, rack/pinion units, etc. It's really more or less a process of elimination to discover the culprit. I have used AD products to retrofit a number of older woodworking machines and components. This looks like a good application for a AD PLC, maybe a sureservo, and a cmore touch screen.

Hope this helps,

Jeremy

Created Date: July 07,2009

Created by: allukes

Have you thought about adding an enocoder with home pulse to the screw?

Depending on the resolution required and how slow you can drive the screw, you would not need a high speed counter module to accomplish it. You can home to the prox then to the home pulse on the encoder to get the resolutoin you need. You just have to work through the numbers to find one that will fit your needs.