Information on digital angle gauge.

From another thread discussion started about digital angle gauges. This information should allow them to be coupled to shafts to measure the angular position. It should also enable the building of a remote display or sending the data to a DRO. In my photo album "Digital Angle Gauge" there are a number of pictures of the internal construction of these gauges.

Here are some notes I have made as a result of investigating how they work. Particullaraly the protocol of the communication between the encoder board and display board.

Notes on Wixey digital angle gauge.

The principle of operation is an eddy current damped pendulum coupled to a rotary encoder that uses the same principle as digital calipers.

Battery - is connected to metalwork

6 pin connector between boards.
NOTE This is a 12 pin plug & socket between the two boards but the two rows are paralleled so there is only six connections.

With display up.
Staring from left hand side of connector. (Call this pin 1.)

Label
Pin 1 Battery + +3V
Pin 2 EA (Generated by display board)
Pin 3 DA Data (generated by read head board.)
Pin 4 CLK Clock signal (Generated by display board)
Pin 5 Battery - GND
Pin 6 VDD This is at 1.495 volts with respect to ground (Generated
by display board.)

-----------------------------------------------------------------------------------

All signals referenced to ground (Battery - ve)

Clock signal. (Originates on display board.0

Amplitude 3 volts

Static state +3V (Will call this logic "0" )

Clock burst repeats every 22 mS

-----------------

Clocks occur in 6 groups of 4

Group structure.

Goes to ground for (Logic 1) for 125 uS
Goes to +3V for 40 uS

Goes to ground for (Logic 1) for 100 uS
Goes to +3V for 40 uS

Goes to ground for (Logic 1) for 100 uS
Goes to +3V for 40 uS

Goes to ground for (Logic 1) for 100 uS
Goes to +3V for 70 uS This is the space between groups. After the last group the signal stays at +3v until the start of the next burst

Next group starts

-----------------

DA signal Data

I think least significant bits come first. (Have now confirmed this.)

Data bit starts on positive going clock (Going from logic "1" to logic "0" ) of clock signal.
So need to clock data on negative going edge (Going from logic "0" to logic "1" ) of clock signal.

-----------------
EA signal
static state +3V

At +3 for about 16 mS (Logic "0" )
At 0V for about 6.6 mS (Logic "1" )

Goes to ground about 2.8 mS before start of clock burst. (Logic "1" )
Goes high about 500 uS after end of clock burst. (Logic "0" )

-----------------

Reading data

The data seems to use the same protocol as BIN6 linear scales but with different timing values.

Power up count value.
The count value on power up seems to be on a 200 (Hex) boundary + a count that seems to be almost an absolute value if the position of the encoder is not moved. As there are 18 radial tracks on the encoder wheel 200 probably corresponds to the spacing of these tracks which will be 20 Deg. On initial tests I thought that 10 degrees corresponded to 250 counts but the above information would make it more likely to 256 counts for 10 degrees. So one count corresponds to 10/256 = 0.0390625 degrees. Also one full rotation would be 256 x 36 = 9216 counts. When I get time I will mount the gauge on my rotary table to do some more accurate measurements.

I hope this will help others find some more uses for these gauges.

Les

Edited By Les Jones 1 on 23/03/2014 17:34:11

Edited By Les Jones 1 on 23/03/2014 17:34:32

Edited By Les Jones 1 on 23/03/2014 17:35:58

Les,

Many thanks for posting this

A very useful reference ... Much appreciated.

Will you also be sacrificing one of the CPC/Duratool units for our benefit ?

MichaelG.

Hi Michael,
I decided to sacrifice the Wixey one as it did not have the spirit level on the top. Also I had already opened it up a short time after buying it which as you can see from the pictures involved pealing the overlay on the front to access the screws. I will keep one of the Duratool ones in the house and one in my shed.

Les.

Hi, all very interesting Les but a bit above my head -- not good on electronics ! I have one of these and find the battery runs flat even when switched off, the only way to stop this is remove or isolate the battery when not in use. I guess this is not a problem if the tool is in use every day but for me this is not the case.

Any thoughts on this? I did take the matter up with the suppliers and it seems this is the way they are made ??

John

Hi John,
Mine is the same as yours regarding the battery going flat so I also remove the battery as I only use mine rarely. Prompted by your comment I have just measured the current in the "off" state. It is about 30 uA I have just looked up the capacity of the Cr2032 battery. It is 225 mA H so it would discharge in 7500 hours which is 312 days.
I'm not convinced mine even lasted that long. I don't think there is enough room to fit a proper switch.

Les.

How about a tiny reed switch and then make a simple pouch/stand for it (sort of a rectangular cup) for it to fit into when not in use with inbuilt magnet to turn off the switch?

Is there room for such a switch? You might even be able to make a simple clip containing the magnet.

Great stuff Les. I have been looking at one of these devices to make an accurate model engine propeller pitch gauge to interface and store propeller data to a laptop.

Thanks * Danny M *

Hi Jason,
No the data is not in BCD format it is in straight binary. I do not know why is is grouped in 4 bit nibbles. It is the same format as BIN6 calipers. (Sometimes called HF or Harbor Freight in the US.) but the timing is different. Last week I bought some of the Aldi calipers which also use the same data format but again the timing is different. There are some digital calipers that use BCD format (This is called 7BCD) but the only one I have seen myself I bought from Neto many years ago. The zero is set in the display unit by storing the value from the encoder when the zero button is presses. The value displayed is then the difference between this stored value and the current value from the encoder.

Hi OMG,
I like your read switch idea but to switch it off the way you describe would require a normally closed reed switch. They do exist but I think they are difficult to obtain. You could turn the idea around and put the magnet on when it is in use. Another alternative would be to use a mosfet with the gate pulled up with a very high value resister and a normal reed switch between source and gate.

Hi Danny,
Your idea should be feasible. To read the data from the encoder I used a slightly modified version of the interface I designed to interface scales with "Yuriy's Android DRO" This basically converts the raw data from the scale to a formatted ASCII string that starts with a letter for that axis (X, Y, Z) then a minus sign if needed then the value in decimal with leading zeros suppressed. Details of this interface are on my website. If you are interested I can send you the source code to get you started. It would mostly need code removing that you would not require for your application.

Les.

Les,

Looking at the photos in your album, I was struck by the thought that the performance of these devices might be limited by the engineering build-quality.

Obviously, [as supplied] the resolution is prescribed by the display ... 'though there might be better available if read to computer. ... But; I suspect that the accuracy and precision could be improved if it was rebuilt with decent bearings to avoid "stiction".

Any opinion ?

MichaelG.

Hi OMG,
I've just tried your idea with a normal type reed switch. I used an 2N7000 FET . The source is connected to battery negative, The drain to the negative power connection on the gauge. A 10 Meg ohm resistor from the gate to battery positive and the reed switch between source and gate. It works fine. There would be room at the top of the unit and between the 2 PCBs to fit the reed switch and components.

Les.

Hi Michael,
There is little if any stiction I can detect. The resolution is limited by the encoder. 1 bit from the encoder is 0.0390625 degrees. (Call it 0.04) So the real readings will go like this. 0.00, 0.04, 0.08, 0.12, 0.16, 0.2, 0.24 etc. This is different to the 2 x 24 bit protocol Chinese scales where one bit is 1/20480 of an inch. ( 0.000048828125" ) This is about one twentieth of a thou. The scales display to half a thou. You might think here that it would be worth using the least significant few bits but they are not very stable. I think the low order bits are worked out by an analogue method from the relative strength of signals on the track pattern of the read head.

Les.

Thanks, Les

That's good to know.

MichaelG.

I have just tried connecting the display part of the gauge to the encoder part via a 1 metre length of screened cable. (About 5.5mm dia. ) The unit still worked and there was no noticeable degradation of the signals when looked at on an oscilloscope. I think I have found out all I need to know about the unit so it will be re assembled until I decide what to use it for.

I think Danny has a good use for one of these gauges. Danny, The interface I used for testing communicates via a bluetooth serial link so you would not even need to physically to the laptop.

Les.

Hi Les,

Fitting a positive on/off switch into the battery positive line would certainly end the faff of having to remove the battery after each use. To save me stripping my gauge to find out, do you think it would be possible to fit one of these **LINK** to the casing?

Hi Otley,
There would not be enough room for that type of switch. (Or any other type of switch I have seen.) I think OMG's suggestion is the best way either having a magnet in the case to switch it off or sticking a magnet on the outside to switch it on. The only other way I could see would be to have a nut soldered to the battery negative terminal in the top right hand corner of the back and having a screw through this nut which made contact with the internal metalwork when screwed in.

Les.

I don't think it's that bad, Les. On this side of the pond, a quick look showed a fair number of small, glass NC reed switches available (and in stock) from DigiKey and Mouser........ Newark/Farnell/Element-14 (whatever they go by this week) also purport to sell them but their link to that page is broken.

I'm sure the situation must be similar in Europe.

(I just took a quick look at Farnell UK. As you say, NC reed switches don't seem to be a big thing. However they do have several styles of CO glass reed switches which would do)

Edited By OMG on 24/03/2014 21:39:11

Hi OMG,
I found some changeover ones on Ebay. I'm not sure if they would close again if the magnetic field became very strong. It occurred to me that you could put a reed switch on each side and put a magnet on one (Or both) sides and on on the bottom of the case. That way it would shut off no matter which way you put it in the case. Disk magnets like the ones in the base of the gauge would be ideal. The case for my Nexus 7 has a magnet in the front cover which puts it in standby mode when it is closed. When I first got it it sometimes also shut off when the cover was folded against the back. I solved this with a small piece of steel sheet in the back of the case to shield the sensor from magnetic fields from the rear.

Les.

Hi Les,

Would there be enough room for one of the tiny audio coaxial jacks? You could then wire the (external) plug to act as a shorting (or non-shorting) bar.

Hi OMG,
I don't think there is enough room.

Les.