Absolute vs. incremental encoders

[sinha_nsit]

The problem with incremental encoder is that the axes have to be referenced in every new machining session. But the advantage is that it does not need a battery. So, why is incremental encoder not preferred? Referencing would take just one minute.

[fomaz]

Originally Posted by sinha_nsit So, why is incremental encoder not preferred? Referencing would take just one minute.

I can recall some disavantages:
You would need home and limit switches. Those can fail. More wires.
In large machines (3000 mm courses), or heavy machines (lower speed), that would take more than a minute.
Fanuc would not sell baterys

but there should be others issues also.

[fomaz]

I remember 2 another issues that are important:

Many axis machines (+10)

Or if you are on a job that homing is simply impossible, since your tool is inside of the work, and there is no path to home (will colide with the work). In that situation, if you have a power fail.... no homing, no position, then you are on a big problem!

Filipe

[sinha_nsit]

Right.
In case of power failure, you know where you are, when the power is restored.
Other points are also important.
Thanks for your input.

[mactec54]

Hi sinha nsit

Encoders the only difference between the two is the battery all machines should be homed after a shut down or power lose anyway, if some one does not change the battery on a regular time every 8-10 months, the battery gets weak & does not work very well, most modern machines Home on start up so there is no need to have a absolute encoders If the power go off when you restart the machine it will home anyway

With some of the manufacturer of the machines they make a custom battery so you have to by the battery from them this is the gotch thing so you have to buy the battery from them

Fomaz
Limit switches have to be there, they have nothing to do with encoders, a home switch is the only switch used in the encoder circuit & this is how they work

The machine travels to the switch, when the switch gets triggered the Motor/Encoder then backs off the switch & finds the Z pulse, this is then your machine reference 0.0 for that axes

If your control has soft limts set then this activates them as well from that Z pulse reference

[Al_The_Man]

There are also two types of absolute encoders, differential output with battery retention, and 10 or 12 bit binary.
For some reason, Robots have traditionally used the binary output absolute encoders.
Al.

[sinha_nsit]

I do not have enough knowledge of machine hardware. So, some questions:

Is "Home switch" a different type of limit switch?
Do we have both a home switch and a limit switch (some distance after the home switch) in a machine with incremental encoder?
Do the terms "dogless referencing" and "referencing with dogs" apply to incremental encoders only?

[fomaz]

Originally Posted by sinha_nsit I do not have enough knowledge of machine hardware. So, some questions: Is "Home switch" a different type of limit switch? Do we have both a home switch and a limit switch (some distance after the home switch) in a machine with incremental encoder? Do the terms "dogless referencing" and "referencing with dogs" apply to incremental encoders only?

Normally a machine with incremental encoder would have both limit and home switch (all mine machines (home made) have). Also they have software limits.

My machines with absolute encoders (and those are not made by me) do not have physical limit switchs.

About the dog or dogless, I cannot help.

[Al_The_Man]

If you need a home switch and dog, there is only one, usually there is an over travel switch just after the home switch.
The home switch is nothing special as it is just used to tell the controller to look for the encoder index.
Dogless means no home limit is used because of either the machine uses absolute and retains the position at power off, or use a seldom used method that runs the machine to a physical stop with the current limit in place, when the machine sees no encoder motion for a certain time it assumes the stop is home and registers so.
Al.

[stevo1]

So just for my own knowledge is it safe to say that if you have absolute encoders and the position is stored at power off then the only thing needed would be a switch for hard overtravel? If the machine needs no reference then it does not need a limit switch to find the position? And the softlimits are set via parameter setting not physical switch.

We have 1 machine that has this. The MTB manual specifies that the axis reference is controled by the PMC. There are no physical switches for referencing the machine...only for hard overtravel.

Stevo

[sinha_nsit]

You are right. It is exactly like this on my lathe. No home switch. Only a hard overtravel switch. The software overtravel limits are set through parameters (1320 etc.) in MCS.

Absolute encoder is definitely better, but there are two things which I do not like about it:

1. The 6V Battery is custom battery which has to be purchased from Fanuc, obviously at a high price. (I was wondering if we could use a cheap commercial battery, by soldering the connector of the original battery to the new battery box)
2. If we fail to replace the weak battery in time (these must be replaced within about 2 weeks after CNC starts displaying the weak battery warning), the position data would be lost. If we re-reference the machine (through parameter 1815), it would not be same as the previous reference. As a result, work offset would have to be repeated.

Finally, a basic question:
The computer on which we work retains all data, including operating system, on its hard disk for ever, even if it does not get power. (A button cell is used only for real-time clock) Why do CNC machines not have hard disks for retaining data?

[Al_The_Man]

Originally Posted by sinha_nsit 1. The 6V Battery is custom battery which has to be purchased from Fanuc, obviously at a high price. (I was wondering if we could use a cheap commercial battery, by soldering the connector of the original battery to the new battery box)

Generally you can sub the battery if the new one is the same technology and Ma-Hr size as the original.
There are quite a few PC based systems now that use HD storage.
Al.