The Absolute Encoder; benifits and function

Absolute encoders as opposed to Incremental encoders do not require a homing procedure. The measuring standard and priciple are vastly different from that of an incremental Encoder.

Absolute positions typically uses a "bit" principle. Esentally instead of having 4 channels offset 90 degrees from each other. the encoder has X number of sensors that can be on or off. The "X" is the number of bits the encoder outputs.

The easiest way to display this is via a table:

You can see to the Left that based on the Measuring Step a very unique position can be determined by how many and which specific bits are on or off. For instance in the chart to the left, If the poistion (accross the top has a measuring step of 1um Than at a position of 20 um you can see that it would have bits 1, 2, 3, and 17 triggered or activated and bits 4-16 deactivated or not triggered


This allows for greater reliabiablity, and stability on machine tools, and other measuring devices.

The one major benifit to Absolute positioning on a Machine tool is that upon startup the machiner knows its position and if you were in the middle of a project you will not have to back off and re-reference you machine. It will read the machines absolute position upon startup.

Some Basic and General Information on Troubleshooting

As a general rule I follow these basic guidlines for troubleshooting linear scale problems:

First of all pay close attention to the specific error message received. Many controls have different designations for the causes of different faults. You can like find a detailed description of the fault in a manual which can narrow your search.

Relating to feedback specifically

Problem: You get a fault immediately upon moving an axis, regardless to position or to speed, or you cannot reference an axis.

Things to investigate.

1) I recommend always starting with the cable in a situation like this. It will likely be your least expensive fix. This can simply be done by running a cable on the outside of the machine and seeing if you are able to move/referece an axis. If you can than you can either reroute the cable through the existing path or create a new cable path that would achieve better resistance to electrical noise.

2) If the cable does not fix your issue you likely have 1 of 2 issues.

a) a faulty readerhead: replace the readerhead or consult a service company that has the proper test equipment.

b) paticularly in the case of a scale not referencing. Many incremental linear scales have a reference mark selector via a shadow plate, magnet, or other method. Check both the location of the selector and the Dog or switch, and in the particular case of a magnet, make sure it has not corroded and lost its magnetism.

3) If you have verified that the above conditions are met and satifactory then there is likely something mechanical and would check for binding or slipping of a coupling.

Problem: You get period faults from the encoders that you can not trace to a particular location or procedure.

Repeat the above procedure:

Things to investigate.

1) I recommend always starting with the cable in a situation like this. It will likely be your least expensive fix. This can simply be done by running a cable on the outside of the machine and seeing if your errors are corrected . If they are than you can either reroute the cable through the existing path or create a new cable path that would achieve better resistance to electrical noise.

2) If the cable does not fix your issue you likely have 1 of 2 issues.

a) a dirty linear scale: carefully follow manufacturers rules for cleaning or send to your preferred repair company.

b)a faulty readerhead: replace the readerhead or consult a service company that has the proper test equipment.

3) If you have verified that the above conditions are met and satifactory then there is likely something mechanical and would check for binding or slipping of a coupling.

Problem: If you get very repeatable faults in the same location regardless to speed or load.

Things to investigate.

1) Cable first... Inspect encoder to see if linear encoder is scratched or dirty in the problem spots. If there is visible damage to the measuring standard than you will likely need to replace the scale. If you can clean the standard then try that prior to replacing.

2) If the Linear encoder is functioning properly than you very likely have a mechanical problem. Linear encoders are pretty "dumb" they really either work or they don't. If the measuring standard remains uncomprimised the electronics of an encoder are note likely to manufacture condition to throw a fault repeatedly for no reason what so ever.

Incremental the basics

 The principle behind a Incremental position encoder whether linear or aotary is a sign wave or a digital square wave created from a standard with a gradient equally spaced on a standard.   For instance Heidenhain a leading manufacturer of measuring, control and display technologies has a patented Diadur process of Bonding metal to glass in very fine and equally spaced dashed on a piece of glass.   These dashes are spaced at gradients finer than 1 um.  In this instance there is a scanning unit or readerhead for the Unit which has a lightsource that shines through this glass.   where the glass is bonded with metal there is a shadow created.   On the opposing side of the glass there is a series of phocells that finely detect the light to dark transitions.   There are commonly 4 photocells that create the phase shifts of the output signal 0, 90, 180, and 270 degrees.   http://www.ssiquality.com/evalfeedback.htm this link displays the A and B or 0 and 90 degree sine and square wave patterns.     It does not display the A- and B- or 180 and 270 degree wave forms.    These are commonly used as a check so to speak and some processers only actually use the A and B Channels.  This is enough to determine position and Direction.   

Position:  

Position is determined by bringing the encoder to a known postion.  Once it has reached its known position or coordinate then it can monitor the number of wave forms produced based on a fixed length or angle it can determine its position.  This known position is usually a REF channel in or above instance it is a separate photocell on a seperate track than the Incremental track on the glass that triggers a sine or square wave at only at the exact same position every time. 

 

Direction: 

Direction is determined by monitoring the transition of the A and B signal channels. 

This is what the wave form would trigger if i change directions. 

ABABABABABABBABABABABABABA 

As you can see during a change in direction you see a clear transition in the order that your A and B channewls trigger. If the encoder changes direction it will see either the A or the B channel trigger twice in a row. 

A note on Spare Parts

When buying Spare Parts f your machines, particularly for highly critical machines in your shop.   Have an extra Encoder on the shelf. Delivery on some linear and rotary encoders can be significant up to 12 weeks.  

Stocking Linear Encoder Spares:   Don't just stock a readerhead.  Keep a full scale in your inventory.  If you make sure that you stock a full scale you can likely remove the readerhead from the spare and continue to use the original scale body... Then you are just purchasing replacement heads for the Spare Scale body... and you have a complete Scale at virtually all times.  Many Scales SPare parts are still available, you can pull any of the parts you need from your spare linear encder and get back up and running.  Then if you have to wait a couple weeks for additional spare parts you have the time and you can just continually rebuild the Spare encoder keeping your critical machine up and running as long as possible.    

Rotary Encoders are more commonly stocked items.  I would recommend just calling the manufacturer ot preferred distributor, and ask if the encoder is commonly stocked.  

As usual any questions or comments are welcome. 

The Tech 

Cable Routing and Protection

After a recent visit to a cutomer and finding another very strange issue that was the result of cabling issue; I thought it would not be a waste to discuss the importance of feedback cabling.

Some key things to remember about the routing and protection of feedback cables; following these can help prevent a long period of confusion and downtime on your CNC by properly protecting and routing your cables.

1 Your cables should be routed in the appropriate conduit or track to prevent pinching or binding the cable at too samll of a radius for the cable specifications.

2 Your cables should be routed away from and protected against any high voltage lines. I would receommend although it is more expensive to have two seperate tracks or conduits that you have a track for your low voltage lines and your high voltage lines.

3 Your cables should be protected from chips and coolants that would deteriorate or cut the cable shielding. ( In a situation that there is not sufficient protection, the machine should be cleaned and freed from any chips especially in any cable tracks or runways.

Cables can cause a variety of CNC problems. I would receommend having someone with the proper test equipment evaluate any scale problem prior to replacing equipment. Linear Scales and Encoders can be very expensive components of a CNC machine. If the issue can be located in a cable the cost to repair could be significantly less if it is found to be a cable from the begining than a cable after replacing other more expensive components.

Starting Out Simple

I had been thinking about a title now for about ten minutes now and realized that maybe I should just start out simple.  As you can see to the right I am Bret Golab. I have been cleaning and repairing linear scales in the lab for almost 10 years now. I have just under 5 years of field service under my belt.   

My intent in this page is to provide help to those that are the do it yourself kind of people.  Don't hesitate to comment with questions if you have any. 

Whether you have a single length guage and a DRO to a five axis CNC, the linear and rotary position devices (linear scales and roatry encoders) all pretty much operate under a few unique principles. I will cover them all to an extent throughout my posts so come back soon.