Has anyone used Chinese MSR5000 magnetic read heads? Having issues.

[lens42]

I replaced a dead scale on my mill with this magnetic scale from Ali Express:

https://www.aliexpress.us/item/3256..._main.11.61f91802lC21Gk&gatewayAdapt=glo2usan

It uses a read head labeled "MSR5000" which is a common item on Chinese commerce sites. I thought the read head might be bad because as I slide it along the scale (being careful with alignment) the output signals disappear (while watching with an o'scope) every 4mm or so in a very periodic way with position. Also, when the signals disappear, the fault light on the head comes on. I've attached a little video of the issue. Needless to say the TouchDRO output numbers are all wrong when this happens. I tried a different read head (M-MDRO brand borrowed from another machine) and it does not drop out like that. I also tried the MSR5000 on a different mag scale and it did the same thing. I thought my read head must be bad, so I ordered another MSR5000 from a different vendor and it exhibits exactly the same problem, so I no longer think the fault is there.

Can anyone shed any light on this issue. I've done a few TouchDRO installs with mag scales in the past with no issues, but those were with more expensive parts. Are all these MSR5000 heads no good? It seems doubtful since they seem widely sold. Thanks.

 

[ycroosh]

These are "hot garbage", to put it politely. There is a sticky on this forum that cover recommended scales and a detailed page on touchdro.com. All ofnthe info is there.
Regards
Yuriy

 

[lens42]

These are "hot garbage", to put it politely. There is a sticky on this forum that cover recommended scales and a detailed page on touchdro.com. All ofnthe info is there.
Regards
Yuriy

I understand they may have issues at that price, but what I'm seeing makes them completely non-functional for any purpose. The quadrature outputs appear and disappear at different positions along the scale. Can all of them really not work at all?

 

[ycroosh]

I understand they may have issues at that price, but what I'm seeing makes them completely non-functional for any purpose. The quadrature outputs appear and disappear at different positions along the scale. Can all of them really not work at all?

OK, let me try again:
ALL Chinese magnetic scales are bad. Even 1um brand name scales that use 2mm pole-to-pole tape, (i.e. Ditron and Aikron) have very pronounced periodic error, and are at best +/-10um scales, not 1um or 5um as advertised. The no-name stuff from AliExpress that is sold under MSR500 or MSR5000 is hot garbage. The issues range from +/-100um error to unstable signal, etc.

500mm of magnetic tape from EMS or RLS will cost you $100 (EMS is abut $5 per inch), or about $50 from a reputable Chinese source (plus shipping, of course). The rock-bottom cheapest sensor on the market is around $36 (and still needs the line conditioning circuit, cable, enclosure, and DB-9 connector).

The scale you linked sells for $63 with free shipping for 500mm version. it has one review "Recommend store; not tested yet" and the specs look like this:

I might be too cynical, but this whole thing screams "run the other way, this is garbage".

I particularly enjoyed +/-30-50um/meter. In other words, these are 100um scales, practically speaking, since that error probably happens on the usual 5mm cycle.

 

[lens42]

OK, thanks. I was able to return everything on Ali Express. I understand bad scale accuracy, but I still don't understand why two MSR5000 read heads from different vendors simply did not function. I was able to get the crappy scales to at least generate quadrature output with other heads (Renishaw and M-DRO), but the MSR5000 heads would go in and out of fault mode as I moved them over the scale so 30% of the pulses would never get sent. At this point it's moot since I ordered a Ditron optical scale from eBay. The whole magnetic scale saga was just so I could cut the scale to match existing mounting holes on the mill, but the Ditron eBay seller will send me a scale cut to exactly my spec'd length.

 

[ycroosh]

The "complicated part" in magnetic scales is the sine/cosine interpolation. Sine/cosine is the important keyword - it's VERY hard to produce clean sinusoidal signal from the sensors. In the real world the signal from the HALL effect sensors is closer to a rounded triangle. Brand name tape gets closer to sinusoidal, but not perfect. The rest is taken care internally (usually digitally, these days), but for that, the tape has to be matched to the reader and overall be very consistent batch-to-batch. The tape itself is expensive too.
Even the "good" Chinese scales have horrendous cyclic error because they don't compensate for the signal shape very well. I suspect they burn some default calibration data based they came up on day 1, regardless or where the tape came from. I would also not be surprised if the sellers on AliExpress found some factory that makes something that looks like encoder tape (it's not that different functionally from refrigerator magnets, so the "bar" can be pretty low). If the calibration assumes values ranging form 30 to 200 (let's say they use an 8-bit ADC), and your tape has values between 5 and 230, you will have spots where the sine/cosine wave forms will be clipped.

So, this is how it can happen technically. As far as "I still don't understand why two MSR5000 read heads from different vendors simply did not function":
1. We don't know that it was 2 sellers. It could be (and very likely was) a single drop shipper with two different store fronts.
2. This is AliExpress you are talking about here. This is the same place that sells plastic "stones" inset into gold plated plastic rings and call them "100% solid gold ring with Amethyst", genuine "Rolex" watches for $13, and iPhone's running Android...
3. Even for China/AliExpress the listing price is "too good to be true". Wholesale price of the sensors used in these heads, directly from China is $36 in 1000 Pcs. increments, without the tape and protective extrusion.These ultra-cheap listings smell like "3rd shift" overruns, or quality rejects from somewhere. I.e. they had to be sold to the retailer well below the cost of the parts. In my experience, "your thing doesn't work and you will get 100% of them returned" almost never stops the sellers from trying to offload the stock.

Regards
Yuriy