DRO/Electrical Question

After my original Z axis readout died on my mill/drill I fitted a new 6" vertical scale and a remote display as the scale display was rather hard to see where it's sited.

I found that when switched on the remote display powered the read out so I dispensed with the battery however the display now 'drifts'. You can zero it, turn your back and it's changed, re-zero it turn your back changes again!

I initially thought it was binding somehow but that's not the case. If I replace the battery in the readout the drifting stops. Now that's all well and good but with everything switched off any accidental movement of the quill switches on the readout for X number of minutes and eventually the battery drains and starts to drift again without you realising.

I was wondering if anyone had come across this or had any ideas of a fix/workaround.

Regards,

Paul

Hello Paul,

This may not be relevant, but earth bonding both the machine and the case of the display is a prerequisite for satisfactory operation of my mains powered DROs on both lathe and mill.

For a completely separate battery powered system it is hard to see why you are experiencing what you describe, but there could be stray potentials involved that over-ride the OFF command. It involves little to try bonding.

Regards Brian

If I understand your problem correctly, the DRO has an auto on functions so if the Z axis is moved it automatically switches on and runs down the battery. I notice mine performs this erratic display when the battery is low.Mine does not have auto turn on though so it's not a problem.

The answer would be to just wire in a switch on the remote display so you can turn it off when not needed.

If your remote display is the same has mine the battery pack is centre tapped so a double pole switch would be needed to completely isolate the supply.

I get the occasional glitch with my homebrew system. I'm increasingly of the view this is, at least partly, voltage spikes on the data cable.

I have seen it advised to leave the battery in the DRO - this seems foolhardy to me as you will be charging a non-rechargeable battery and it could burst. A more invasive but preferable alternative (that I'm going to adopt) is a to fit a small electrolytic capacitor in the battery compartment. 1-10uF seems about right for such a small current drain.

Neil

This is a common issue when using digital scales. The battery acts as a reservoir to 'smooth' the incoming supply and reduce interference.

The same smoothing effect can be obtained by removing the battery and soldering a suitable value capacitor across the battery terminals.

If you search any of the forums/websites relating to Shumatech DRO's, they will contain info on adding capacitors to the scales to correct the 'jitter' in the readings.

** EDIT ** - Here is a link that should explain everything in detail - http://rick.sparber.org/Articles/JO/JO.pdf

Edited By LADmachining on 09/05/2014 21:12:28

While investigating the problem of jitter on Chinese scales connected to the Shumatech DRO350 I did an experiment to see the effect of small changes in supply voltage had on the reading. I have put details of this test on the Yahoo Shumatech forum. it is message number 10417 (May 6th 2007) This should take you directly to this message but you may have to join the group. This showed that the reading does change with supply voltage. There is another problem that has been reported by a number of people where the display changes by exactly 0.2" (I have not seen this problem.) This seems to be the result of noise spikes being coupled into the scales via the cable connecting it to the DRO. It is suggested putting an 100 nF ceramic or mylar capacitor in the scale as well as the electrolytic capacitor.

Les.

Hi John,
That does not look like an electrolytic capacitor. Are you sure it's 200 uf not 200 nF ?

Les.

I'm not sure if it is the same issue you are having but I fitted set of dro's to my X2 ( I think ARC sell the same type ) and they ran well on battery power but connect the power supply that came wth them and the readout would flicker and was impossible to zero.

I deduced that it was noise from the switch mode power pack and a quick check with an oscilloscope confirmed this.

I decided to try and smooth this out using some large value electrolytic capacitors ( standard and low ESR types in parallel ) and I got pretty close but there was still some flicker.

I decided that the frequency of this noise may be also causing a problem for the display so I changed over to the good old transformer rectifier type power pack and hooked up the 2 low ESR capacitors across the terminals and was rewarded with a nice stable readout.

I would guess that some where inside the display there is a clock or oscillator that was being affected by the noise from that cheap switch mode supply.

Occasionally the readout on the Z axis will not zero but will read 0.02 mm and I found that when this happens if I press and hold down the zero buttons on the display and the scale ( mine has a small display on the scale also ) the readout will settle on 0.00.

Neil,

I wonder if there is a diode in that DRO that would stop the power supply from charging the battery?

Ian.

Hello Ian and other readers, you say you connected two low ESR capacitors across the terminals, what type of capacitors and what were the values please. I'm thinking of fitting DRO to my mill and that information would save time and avoid frustration to know how others have over come problems.Ted

Gents,

Thanks for all the replies and advice. I put a meter across the battery terminals and get 1.2v so the remote is putting out a voltage across the battery so that's coming out, strange that there was no mention of this with the remote displays instructions.

As general consensus is to put a capacitor across the battery terminals that's what I'm going to try first (I think I have a bag of mixed ceramics hidden somewhere that I couldn't bring myself to throw away)

I'll let you know how it goes.

Paul

I have not used them in this application, but this type of "supercapacitor" might be worth a try.

... They are a useful half-way-house between a battery and a capacitor.

MichaelG.

Hi John Fletcher,

They were 4700 uf - 16volt low ESR ( low impedance ) but you won't get them into the battery compartment like Bogstandard has shown as they are much bigger than the monolithic capacitor he has used which is also a low impedance type as well .

The unit I built is nothing more than a 12v transformer type plug pack with 2 separate regulator units mounted inside the motor control box of my X2 Mill , there are 3 dc power sockets fitted also ( power in from the plug pack ,power out to the DRO display and power out to the digital tachometer ) .

I think the important part was that the transformer type power pack operates at 50- 60 hertz compared to a switch mode type that operates at many thousandths of hertz and may be close to or a harmonic of the frequency of an internal clock or oscillator in the DRO .

Ian.

Hi Paul,
I think the discussion has drifted from the original problem. Your problem seems to be related to varying supply voltage to the read head on the scale. You do not seem to have the jitter problem caused by noise superimposed on the 1.5 volts to the scale. (I am making the assumption that the scales are the original 2 x 24 bit protocol scales that are powered with a 1.5 volt silver oxide button cell and the remote display unit contains a battery of some kind.) More details of the scale type, the remote display and weather the remote display is powered by internal batteries or an external power supply would help identify the cause of the problem. With no battery in the read head on the scale could you measure the voltage at the read head both with the remote display switched on and switched off. (I suspect that that it is never truly switched off.) I have done quite a lot of work with various types of Chinese scales (2 x 24 bit, 7BCD, BIN6 and 21 bit) but I have no real knowledge of the remote display unit. I have always built my own display units.

Les.

Edited By Les Jones 1 on 11/05/2014 09:39:23

Hi Les,

The scale takes a single 3v 2032 cell. The voltage at the scale terminals is 1.7v regardless of the display being on or off. As the display can be powered by batteries or external power I've removed the external psu from the equation by fitting batteries, 2x AAA, to the display with the same result.

I did fit a 200nF tantalum cap across the scale terminals last night but that didn't make any difference. I have got a few electrolytics but at much higher values but I don't know if that's worth a try?

Hopefully there should be a couple of pictures below, if not they'll be in my album.

Thank you all very much for your help,

Regards,

Paul

Hi Paul,
Can you confirm that I have all the facts correct.

1 You are not using the battery (CR2023) which would be fitted in the read head if the scale was used without a remote display

2 The remote display is powered by two internal AA batteries

3 These AA batteries are alkaline batteries NOT rechargeable Nicd or Nimh (Rechargeable batteries only supply about 1.2 volts compared with 1.5 volts from alkaline or zinc carbon.)

4 It is these AA batteries you are replacing to fix the problem.

5 By "replacing the batteries" you mean that you put new ones in. Not just remove and refit the same pair of batteries.

This suggests that the batteries are becoming drained (Which you probably realise.) When you say the batteries eventually drains how long are you talking about - hours or months ? I would expect the information on the remote display would give some idea of the expected battery life. If so how does this compare with the battery life you are getting ? The switching back on of the scales when they see a change of position is a characteristic of the scales so there is nothing you can do to get round this. Fitting capacitors will not change this behaviour nor will they make the scale give reliable readings with flat batteries.

Les.

Hi Les,

Sorry I think I may have confused matters with my one line comments (was my birthday yesterday so still running on wine fumes).

1. Correct

2. The remote display is powered by an external switched mode supply (6v I think, can't get to it at the moment to check as it's stuck behind my mig welder). Changing to internal AAA batteries, alkaline (it can take either batteries or external power) makes no difference

3. See 2

4. No, fitting the 2032 back into the scale fixes the problem but I don't want to leave it there if the remote display is supplying a voltage to it.

5. By replacing the battery I just meant putting it back into the scale.

Regardless of the type of power source to the remote display the reading 'drifts' unless I place the 2032 cell back into the scale which, from what I've learned from the above posts, appears to be a bad thing if a voltage is being supplied to it by the display.

Thanks very much for your help.

Regards,

Paul

Confession, that may be relevant. My homebrew display boards use a forward biased red LED to provide 2.2V for the readouts. LEDs work really well as voltage regulators (look inside a solar calculator, you will usually find an LED inside the case wired across the OPV panel. I worry this is a bit high, so i plan to fit an ordinary silicon diode in each supply line as well as the smoothing caps, which will drop it to 1.6V - less than the voltage of a new cell.

Neil

Hi Paul.
Thanks for answering all the questions clearly. I now have a clearer understanding of the problem. I hve two 6" calipers using the 3 volt lithium battery. One was bought from Aldi in 2008 and the other in January this year from Lidl The set bought in 2008 was the first I had seen that used the 3 volt battery. I did some investigation at the time and posted the results in the file section of the Yahoo Shumatech forum. Here is a copy of that information.

NOTES ON CALIPERS WITH 3 VOLT LITHIUM BATTERIES

The scale of this type that I bought was bought at Aldi supermarket in the UK

Battery + ve Connected to metalwork and right hand track on socket
Battery - ve Not connected directly to anything.

Interface connections (With caliper the right way up. ie the connector at the top.Counting left hand track as pin 1))

1 -ve (Note on this version ov the scales it is not connected to the battery -ve)
2 Data
3 Clock
4 +ve (Connected to battery +ve)

Note about pin 1

Pin 1 is about - 1.6 volts with respect to battery + ve with the scales switched on. With the scales switched off
it is about -1.8 volts with respect to battery + ve

Time period from start of one data frame to the start of the next about 180 ms
length of data frame (Full sequence of 2 x 24 bits) about 500 us
Clock period about 7.5 us (133 Khz)

Tests with DRO350
Does not work with oversampling turned on. Works with oversampling turned off
They also work with the battery removed. so it looks like the main scale elecronics still only require about 1.5 volts to work.
There must be a regulator in the -ve rail to drop the 3 volts to 1.6 volts.
The DRO350 also puts the scale into fast sample mode OK
The only problem is that the scale switches itself off after a time. It may be possible to stop this from happening by shorting the on/off button - I have not investigated this.

Tests with YADRO dro.
Reads the data from the scales but seems to get confused when the sign of the display changes to -
(Possibly the way this scale represents negative numbers is different. I have not looked in detail at the problem. issuing the rx
commands (x is the port number the scale is connected to starting at 0) from a terminal program to the YADRO int4 (The YADRO box)
returns the hexidecimal value from the scales. This should make it easy to analyse the problem. I think the YADRO uses the relative data ( 2nd data packet) but the DRO350 uses the abrolute value (1st data packet)

All I have done with the set that I bought from Lidl this year is to look at the output on the oscilloscope to see if they used one of the known protocols. (They did but I can't remember which one.) When I get time I will do some tests powering them from 1.5 volts via the data connector.

Hi Neil,
I have found that changes in the value of the 1.5 volt supply changes the readings. I found that changing the voltage from 1.55 to 1.50 changed the reading by 16 (Decimal) counts (1 Count is 1/20480" (About 0.02mm) I would recommend using an LM317 type regulator.

Les.

I have seen this 0.2" problem recently and it baffles me.

I actually see it as about 5mm because I rarely have the calipers set to imperial. I have several pairs of the common Chinese calipers of various ages (one pair about 15 years). Mostly they are the type that remember the last reading which means they retain 'calibrated', I just pick them up to use, take a measurement and then realise it has about a 5mm anomoly. I have never actually checked to see if it is exactly 0.2" as I just re-zero and start again.

I have used this type of calipers for as long as they have been around, but have only seen this problem in the last couple of years so maybe its due to the aging of some component.

Ian P

Edited By Ian Phillips on 12/05/2014 10:05:57

Hi Les,

Can you clarify that change in reading with voltage. is it an absolute change (i.e. every reading moves along a bit) or incremental?

When replacing a read head recently, I noticed the head had two beryllium copper 'brushes' to keep it in good contact with the body of the scales. I will prod around in the defunct head to see what is connected to what.

thanks

Neil