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Allan Waterfall
05-09-2009, 04:30 AM
I've just noticed on my Sino DRO fitted to my mill,that if I power down and then move the table,when I power back up the display reads the power down position but does not read the new table position...is this normal?

It's very frustrating to have to reset the absolute zero back to the corner of the vice if I've moved the table whilst the DRO was switched off.
I thought a DRO should remember the zero after being switched off regardless of where the table is when it's switched back on,not the position it was switched off at.
I don't want to leave the DRO permanently switched on.

Allan

Evan
05-09-2009, 04:41 AM
In order to know where the corner of the vice is it has to know where the table is when powered up. To know that it has to have absolute position encoding.

macona
05-09-2009, 05:47 AM
Some higher end DROs have battery backup and can keep track of where they are if the display is off.

Simplest solution, dont turn the DRO off. Power consumption is negligible and its not going to hurt it one way or another. newer DROs use LED light sources in the scales unlike the old DRO scales with little incandescent lamps.

Allan Waterfall
05-09-2009, 05:49 AM
I set the absolute position to the corner of the vise and zero the display,the DRO remembers this.If I power down and don't move the table at all the zero position is still there when I power back up.

However,if I happen to move the table whilst it's powered down,when I then power back up the absolute position still reads zero which it was before I powered down,even though the table has been moved to a new position and shows the new position as the zero absolute.

So basically what's happening is that the DRO forgets the absolute that I set when it's powered down if this makes sense.

Allan

Evan
05-09-2009, 05:57 AM
Absolute position encoding means that the system is always aware of where it is regardless of the cycling of power. It is a hardware feature and is more expensive than a system that must be "taught" where zero is. A dial caliper is an absolute device whereas a cheap digital caliper is not. Good quality calipers usually have absolute position encoding, meaning they don't have to be zeroed.

The same applies to DROs. I don't know anything about your specific DRO but is sounds as though it only remembers relative position and is not absolute position aware.

Allan Waterfall
05-09-2009, 06:07 AM
Absolute position encoding means that the system is always aware of where it is regardless of the cycling of power. It is a hardware feature and is more expensive than a system that must be "taught" where zero is. A dial caliper is an absolute device whereas a cheap digital caliper is not. Good quality calipers usually have absolute position encoding, meaning they don't have to be zeroed.

The same applies to DROs. I don't know anything about your specific DRO but is sounds as though it only remembers relative position and is not absolute position aware. http://www.digital-caliper.co.uk/3_axis.php?osCsid=kii8hp9cteot11crebonoqcvr5

I assumed power down memory would remember a setting the same as my vernier and mic do when they're switched off.I certainly expected it for the price.

Allan

Barrington
05-09-2009, 06:12 AM
Allan

In response to your initial question - yes, this is normal for many devices.

In simple terms:-

Your scale is made up of a long series of identical marks. so it can only determine relative position by counting them as they go past. Turning off the DRO stops the counting, so it loses track.

An absolute scale would have markings which will indicate positon at all times, i.e. each mark is effectively unique so the position is known immediatley the device is powered up.

Some relative devices only 'pretend' to turn off but keep the counter running...but then people complain that their batteries are running down even when they're off :)

Cheers.

Allan Waterfall
05-09-2009, 06:22 AM
It seems that my only solution is to wind the table out of the way to get out of my shed and then turn the DRO off and hope I don't accidentally move any of the handles before I switch back on.

Allan

japcas
05-09-2009, 07:49 AM
Like Macona said, the simplest solution is to just leave it on. It wouldn't cost more than .50 cents a month if that. You'd never notice it and it won't hurt the dro.

SGW
05-09-2009, 08:45 AM
My Acu-Rite does have absolute positioning. If I turn it off an on, then move each axis a couple of inches, it can figure out where it is. One of the reasons Acu-Rite and other high-end DROs cost more....

Carld
05-09-2009, 08:46 AM
Or you could, move the table to the home/absolute 0 position then move the table an even number of inches to clear the door and 0 it there and turn it off. Then when you come back to use it turn it on, move it the same number of inches to home and reset 0.

Why don't you just reposition the machines so the doorway is clear at all times. It seems to be a safety issue having it set up the way it is.

GadgetBuilder
05-09-2009, 10:55 AM
Depending on your DRO, it may be able to recover the zero with a little help from you. There's a video at DROPROs showing how to do it and my Meister Top10 has the same capability but implemented a little differently.

Scroll down to "Power Off Memory" for the video:
http://dropros56k.com/Videos.htm

Then have a look at the manual for your DRO to see if it has a similar feature.

John

kf2qd
05-09-2009, 11:51 AM
Your DRO will rezero on the power up position unless it uses absolute scales. Most inexpensive DROs use an incremental encoder - has 2 signals - typically called A & B - pattern goes like this - A off B off, A on B off, A on B on, A off, B on and back to A off B off - when turning in one direction and - A off B off, A off B on, A on B on, A on B off, and back to A off B off - you can get the neumber of counts and direction from this pattern. (Called a GRAY CODE, there are also 3, 4, 8 and more bit GRAY CODES) A 2 bit GRAY CODE is very easy to decode and works on a huge number of machine encoders.

Absolute encoders require more bits (and some require batteries - like on a Mitsubshi robot). And because of the more bits they use a different interface and are more expensive.

You could make a hard stop at one end of your table travel and always run the table up against that stop before powering things down. Want to be careful how hard you tighten up against the stop. Not knowing what kind of encore system your DRO is using - some encoders have a Z signal - only changes state at one point in the encoders feed back -
A B Z
0 0 0
1 0 0
1 1 0
0 1 0
0 0 1
1 0 1
1 1 1
0 1 1
0 0 0
1 0 0
etc...

If you could see the different signals, move against your stop and with the Z signal ON you could then zero the DRO.

(Probably don't have any access to the individual signals, and Z probably isn't there any way...

Do you know what type of signal your scales send to the readout? Might be able to hook up a couple of LEDs and decode A & B and use them to set zero.

Evan
05-09-2009, 12:16 PM
(Called a GRAY CODE, there are also 3, 4, 8 and more bit GRAY CODES) A 2 bit GRAY CODE is very easy to decode and works on a huge number of machine encoders.


The 2 bit pattern is quadrature encoding. The encoder is arranged to turn each bit on and off 90 degrees out of phase from the last transition giving 4 different possible values than can be represented by 2 bits, hence the name "quadrature". That isn't Gray code although it shares the characteristic of only changing one bit at a time. The reason for using Gray code with larger word lengths is that it only ever changes one bit of value as it increments or decrements. This means that if the value produced by the encoder is jittering it jitters one count only in the least significant digit of the decoded value. An encoder that uses standard binary code has the potential for a 1 bit change to cause a jitter from min range to max range if using complementary binary codes.

Also, a binary encoder may change as many as wordlength-1 bits at once. If they don't all change at precisely the same instant intermediate error values will be produced. That can't happen with Gray code.

The main thing that separates quadrature encoding from Gray code is that it cannot provide absolute position information, only direction information.

Allan Waterfall
05-09-2009, 12:49 PM
Depending on your DRO, it may be able to recover the zero with a little help from you. There's a video at DROPROs showing how to do it and my Meister Top10 has the same capability but implemented a little differently.

Scroll down to "Power Off Memory" for the video:
http://dropros56k.com/Videos.htm

Then have a look at the manual for your DRO to see if it has a similar feature.

John
Thanks for that link,unfortunately it doesn't apply to my system and I can't find anything in the manual either.

Thanks kf2qd and Evan,but your replies are a bit like me asking you the time and you start to tell me how a watch works.

Allan

Barrington
05-09-2009, 12:51 PM
That isn't Gray code although it shares the characteristic of only changing one bit at a time.

A bit off the subject, but just to clear up any confusion for anyone looking at data sheets - quadrature encoding IS also widely known as a '2 bit repeating Gray code' - and is a terminology commonly used by any number of encoder manufacturers.

Cheers

Allan Waterfall
05-09-2009, 12:55 PM
My Acu-Rite does have absolute positioning. If I turn it off an on, then move each axis a couple of inches, it can figure out where it is. One of the reasons Acu-Rite and other high-end DROs cost more....

Thanks,my DRO does that,it's when I move the table whilst it's turned off that's the problem.
Anyway,now that I'm aware of it I'll just leave it permanently switched on.

Allan

danlb
05-09-2009, 01:40 PM
The Sino DRO has a button labled HA. The manual calls it sleep mode.

It shuts down everything except the encoders, so it does not lose any position.

According to the manual, you don't power it off, but simply press the HA key. At least that's what it says for the model SDS6-3V, and I've verified that that is what happens.

When sleeping, it pulls only 2 watts. When on, it pulls 4. That's less than a night lite. That's a difference of 1.5 kwh a month, or 25 cents in my neighborgood.

Dan

Evan
05-09-2009, 03:16 PM
A bit off the subject, but just to clear up any confusion for anyone looking at data sheets - quadrature encoding IS also widely known as a '2 bit repeating Gray code' - and is a terminology commonly used by any number of encoder manufacturers.


To further clear up any confusion 2 bit quadrature encoding as it is usually implemented does not meet the definition of reflected binary coding, which is what Gray code is in this context. Most quadrature encoders aren't used to produce all four states reliably in quadrature when encoding step and direction information. Decoding commonly only notes which bit changes first to determine that movement has occured and in which direction and only responds to either a falling or rising edge but not both.

There are quadrature encoders available that do produce accurate quadrature signals. These may be considered repeating Gray code encoders although they still do not provide position information. The decoder will be level sensitive rather than edge sensitive.

Why true quadrature encoding isn't often used:


x1, x2 & x4 Decoding
The quadrature signal produced by incremental encoders has 4 state changes per quadrature cycle. A 500 CPR encoder has 500 cycles (2000 quadrature states) per revolution. x1 decoding means that the external electronics counts only state per quadrature cycle, so there will be 500 counts per revolution. x2 counts two states per quadrature cycle (1000 counts per revolution). x4 counts every quadrature state (2000 counts per revolution). A properly implemented quadrature decoder can reduce noise by examining the current quadrature state and ignoring the signal if the next detected quadrature state is invalid. If this is not done, shaft vibrations (often caused by stepper motor ringing) may accumulate counts in one direction when the counter should be oscillating up and down. Quadrature signals are easily converted to step/direction or up/down clock signals using the LSI Chips.

http://www.quadratureencoders.com/support/glossary#glossary_quadrature

Most often the decoders operate in X2 decoding configuration.

Interestingly, the above page, which is a glossary of quadrature encoder terms never mentions Gray code.

Allan Waterfall
05-09-2009, 03:46 PM
The Sino DRO has a button labled HA. The manual calls it sleep mode.

It shuts down everything except the encoders, so it does not lose any position.

According to the manual, you don't power it off, but simply press the HA key. At least that's what it says for the model SDS6-3V, and I've verified that that is what happens.

When sleeping, it pulls only 2 watts. When on, it pulls 4. That's less than a night lite. That's a difference of 1.5 kwh a month, or 25 cents in my neighborgood.

Dan Thanks Dan,
I'll check the manual again,but I thought I read that the HA button doesn't work on the mill 3 axis read out.

Allan

EDIT.....just had a look and it doesn't work as is stated in the manual.

Now for Evan to explain the manual printing process and the way ink is made and all the electronics in the printing press etc.etc.

nheng
05-09-2009, 04:06 PM
Absolute encoding, seen in some (Mitutoyo, others) digital calipers, requires power be kept on via the battery.

On Acu-rite's with "Position-Trac" and other DROs , reference marks are either coded into the glass or in some cases, like some Heidenhain scales, the reference mark can be physically moved by moving a segment of the scale "cap". In the Heidenhain case, I think it is either magnetic or optical.

On my late model Acu-rite, you either need to move both axis to pick up the reference, or key on "no reference" at the power-up screen.

Den

Carld
05-09-2009, 04:32 PM
Allan, your making a mountain out of a mole hill :rolleyes: I told you an easy way to move the table and still find home but you seem to have ignored it. Your looking for a complicated way to solve a simple problem.

clutch
05-09-2009, 04:41 PM
I've just noticed on my Sino DRO fitted to my mill,that if I power down and then move the table,when I power back up the display reads the power down position but does not read the new table position...is this normal? Allan

I have some ancient sony magnescope dro's. That is normal for them.

Clutch

clutch
05-09-2009, 04:47 PM
At work we have a newer Sony DRO on the bridgeport. Two power switches. A rocker on the back that powers it down, a soft key on front that turns off display. The former looses stored reference positions, the later maintains them.

Clutch

danlb
05-09-2009, 05:03 PM
Thanks Dan,
I'll check the manual again,but I thought I read that the HA button doesn't work on the mill 3 axis read out.

Allan

EDIT.....just had a look and it doesn't work as is stated in the manual.



Very strange. The SDS6-3V is a 3 axis mill model. MY manual says it does work. :) My use of it says it does work. I never turn off the power and always use the HA button.

You have an older or cheaper model perchance?

Dan

Barrington
05-09-2009, 06:04 PM
Dear reader, Please skip this post if you have a life, or anything vaguely interesting to do... ;)


To further clear up any confusion 2 bit quadrature encoding as it is usually implemented does not meet the definition of reflected binary coding, which is what Gray code is in this context.
But 2 bit quadrature encoding is exactly reflected binary ! It is generated from the (not very useful...) 1 bit Gray code and provides the basis for the following 3 bit Gray code !


Decoding commonly only notes which bit changes first to determine that movement has occured and in which direction and only responds to either a falling or rising edge but not both.
I cannot comment as to whether this is common or not - it isn't in my world, but I would regard this as unacceptable for use with almost anything more critical than a front panel control rotary encoder.


There are quadrature encoders available that do produce accurate quadrature signals. These may be considered repeating Gray code encoders although they still do not provide position information. The decoder will be level sensitive rather than edge sensitive.
Your point that quadrature encoders rarely produce outputs in exact quadrature is even more reason to describe tham as repeating 2 bit Gray coded, as that terminology in no way implies an exact relationship between the periods of each state of the cycle. If you agree that 'accurate' quadrature decoders "..may be considered a repeating Gray code encoders..", then I do not follow how less accurate ones cannot ?


A properly implemented quadrature decoder can reduce noise by examining the current quadrature state and ignoring the signal if the next detected quadrature state is invalid. If this is not done, shaft vibrations (often caused by stepper motor ringing) may accumulate counts in one direction when the counter should be oscillating up and down. Quadrature signals are easily converted to step/direction or up/down clock signals using the LSI Chips.
I don't understand the relevance of this to the discussion on terminology - and why would anyone use an improperly implemented decoder :confused: :D


Most often the decoders operate in X2 decoding configuration.
Agreed - because each phase may well be close to a square wave even if the two phases are far from 90 degrees apart.

I'm done.

Cheers

ckelloug
05-09-2009, 09:16 PM
Some posts here on the topic of Gray Code appear more accurate than others. Where oh where is oldtiffie when you need him?

Rich Carlstedt
05-09-2009, 10:01 PM
Simple Solution, leave power on
Complex solution, Buy an expensive DRO
(More $ to save $)

Electronics don't like power changes.
Longevity is a function of fewer on/off cycles.
My 1985 Spaulding DRO has been on, almost it's entire lifetime.
Was off in 2006 to change the battery which only lasted 21 years
( I knew the battery was bad, when we had a outage, and it lost position !)

Rich

Fasttrack
05-09-2009, 10:27 PM
Hmm ... Call me crazy, but I zero the DRO based on a chosen datum of the part and work from there. Sometimes it's the back corner of the vice, sometimes it's the center of a flange, sometimes it's the center center of a rotary table, sometimes it's the edge of a part - just depends. I've always felt more comfortable indicating off of the work than the vice, but then many times I can't use a vice.

I know it is common practice to work from the back left hand corner of the vice, but it doesn't seem like it would take very long to re-zero the scales - especially if you're just touching off with a wiggler or paper/endmill trick.

Also keep in mind that the DRO's I use are old and don't have nifty features like bolt hole patterns and whatnot.

macona
05-09-2009, 11:09 PM
Absolute encoding, seen in some (Mitutoyo, others) digital calipers, requires power be kept on via the battery.

On Acu-rite's with "Position-Trac" and other DROs , reference marks are either coded into the glass or in some cases, like some Heidenhain scales, the reference mark can be physically moved by moving a segment of the scale "cap". In the Heidenhain case, I think it is either magnetic or optical.


Den

Heidenhains use a magnetic sensor.

Allan Waterfall
05-10-2009, 10:15 AM
Very strange. The SDS6-3V is a 3 axis mill model. MY manual says it does work. :) My use of it says it does work. I never turn off the power and always use the HA button.

You have an older or cheaper model perchance?

Dan Hi Dan,

Mine's the same model as yours.

I emailed my supplier about the "HA" button this morning and have received a reply,the manual on page 14 says it does not work and he's contacting the manufacturer about this anomaly.

It does work in incremental mode but not on absolute...strange.

Excellent after service from Jade products,I didn't expect a reply until tomorrow at the earliest.

Allan

A.K. Boomer
05-10-2009, 10:49 AM
Im going to have to check mine in incremental now --- I have the same issue's but haven't tried anything fancy, Like Fasttrack I pretty much start from scratch each time anyways and like to double check anyhow,
Here is a thought --- My cheap ass digital calipers remember where they are even if "turned off" now I know that's because they have an internal battery but I would think my DRO does also.

I don't like leaving my DRO on as I do think the individual numeric lights have a life span -- If I know im going to be doing the same job the next day I just set my mills table back to zero and lock it down - or sometimes just lock it down where its at, (lock it down and then shut it off - then turn it on and unlock for the next session as the act of locking it down can change the position slightly - you want that to be recorded) it would be a little handier to have it remember but its also no real big deal - (to me anyhow) as its probably going to be double checked anyways.

danlb
05-10-2009, 01:04 PM
Hi Dan,


It does work in incremental mode but not on absolute...strange.

Allan


Since I installed the DRO, I almost never use my mill in abs mode, I did not notice that. HA works on all 200 memory settings too; as in [ZERO]1

Daniel