Troubleshooting tachometer input on DIY DRO Adapter Kit

[svideo]

I've been using the DIY DRO Adapter Kit for Glass Scales for a few years and it works great. Now I'm looking to add tachometer support to the system and am running into some trouble getting the input to read reliably.

This is on a PM728-VT mill which came from PM with an Ancer RPM320M tach mounted with sensors. I'd like to retain the existing tach display on the machine. The existing tach runs at 5v logic levels so I can't directly feed that signal into the TouchDRO. Instead, I've wired up a level shifter so I can output the signal at 3.3v levels.

Checking scope traces, the resulting signal performs like I'd expect. I've added a 0.1uF cap across the power rails to smooth out some noise but it didn't make an impact. Blue trace (mostly covered) is +3.3 from the DRO PCB. Yellow trace is the signal coming back. Voltage p2p is 3.2V like I'd expect, the signal is reasonably clean without a lot of ringing on transitions, and the timing seems correct.

Zooming into a single pulse, there doesn't appear to be any ringing or overshoot, transition time is around 100us-ish.

When I set the speed such that the pulses are showing a frequency of 60Hz, the mounted tach shows 900 RPM which suggests 4 pulses per revolution. 30Hz shows 450, 90Hz shows 1350, so everything on the machine tach is ruler flat and works like I'd expect. When I set the speed controller, the RPM display doesn't bounce around and just shows the value.

In Touch DRO I've set the tach at 4 pulses per revolution and the device is showing an RPM but the value bounces around ±10% and the values are way off. Here's an example (sorry for the mess, guts are spilling out while I figure out what's happening here):

Here we see the machine tach (correct) at 905 RPM, while TouchDRO is 1815. Hey no problem, just set it to 8 pulses! Sadly, that relationship isn't quite as simple as 2x. Here are some example values:

Pulse frequency (Hz)	Machine Tach (RPM)	TouchDRO (RPM, set to 4 pulses)
29.8	445	1110-1380
60.2	910	1695-1950
90.2	1355	2220-2505
119.9	1801	2580-3075

Any suggestions as to what might be going wrong here?

 

[ycroosh]

I've been using the DIY DRO Adapter Kit for Glass Scales for a few years and it works great. Now I'm looking to add tachometer support to the system and am running into some trouble getting the input to read reliably.

This is on a PM728-VT mill which came from PM with an Ancer RPM320M tach mounted with sensors. I'd like to retain the existing tach display on the machine. The existing tach runs at 5v logic levels so I can't directly feed that signal into the TouchDRO. Instead, I've wired up a level shifter so I can output the signal at 3.3v levels.

Checking scope traces, the resulting signal performs like I'd expect. I've added a 0.1uF cap across the power rails to smooth out some noise but it didn't make an impact. Blue trace (mostly covered) is +3.3 from the DRO PCB. Yellow trace is the signal coming back. Voltage p2p is 3.2V like I'd expect, the signal is reasonably clean without a lot of ringing on transitions, and the timing seems correct.
View attachment 489506

Zooming into a single pulse, there doesn't appear to be any ringing or overshoot, transition time is around 100us-ish.
View attachment 489507

When I set the speed such that the pulses are showing a frequency of 60Hz, the mounted tach shows 900 RPM which suggests 4 pulses per revolution. 30Hz shows 450, 90Hz shows 1350, so everything on the machine tach is ruler flat and works like I'd expect. When I set the speed controller, the RPM display doesn't bounce around and just shows the value.

In Touch DRO I've set the tach at 4 pulses per revolution and the device is showing an RPM but the value bounces around ±10% and the values are way off. Here's an example (sorry for the mess, guts are spilling out while I figure out what's happening here):
View attachment 489508

Here we see the machine tach (correct) at 905 RPM, while TouchDRO is 1815. Hey no problem, just set it to 8 pulses! Sadly, that relationship isn't quite as simple as 2x. Here are some example values:

Pulse frequency (Hz)	Machine Tach (RPM)	TouchDRO (RPM, set to 4 pulses)
29.8	445	1110-1380
60.2	910	1695-1950
90.2	1355	2220-2505
119.9	1801	2580-3075

Any suggestions as to what might be going wrong here?

Which version of the kit is this? Was this a DIY kit, or a "you put the box together" kit?

 

[svideo]

Which version of the kit is this? Was this a DIY kit, or a "you put the box together" kit?

This guy: https://www.touchdro.com/store/diy-dro-kit-for-glass-scales.html

I'm not much of a machinist but am pretty handy with board-level electronics so if there's any utility in grabbing captures at test points, data busses, JTAG, serial etc I'm setup for that sort of work and can provide whatever detail there.

Here's what that looks like:

 

[netmagi]

Could be signal. I just embarked on adding a tach to the spindle on my lathe for touchdro, and I also broke out the scope. There's some traces visible in the vid:

I think you're much more knowledgeable in electronics than I am, but the signal I was seeing from the hall-effect sensor definitely looks a lot cleaner.

Have you tried just popping a magnet on the spindle and feeding touchdro the signal from a hall-effect sensor just to confirm the input?

 

[netmagi]

Here's a screen grab from the video:

I was using 6 magnets when testing, so note that the frequency in Hz on the scope lines up x10

 

[ycroosh]

This guy: https://www.touchdro.com/store/diy-dro-kit-for-glass-scales.html

I'm not much of a machinist but am pretty handy with board-level electronics so if there's any utility in grabbing captures at test points, data busses, JTAG, serial etc I'm setup for that sort of work and can provide whatever detail there.

Here's what that looks like:

View attachment 489559


View attachment 489560

Sorry for the later reply - got bogged down with an unrelated "quick fix" for more than a week...
Your kit is running the latest version of MSP430 firmware (I checked the dates, and this was the only version that shipped with it). The firmware is pretty simple - above certain RPM it counts pulses per second and converts them to RPM; below the the threshold it uses time between pulses.
A common cause of minor fluctuations is inconsistent period between pulses. This can happen with some HALL effect sensor - based tachometers. The error you are seeing is pretty big, though, so it's probably not this. Plus, the scope screen capture looks reasonably good.


I don't have any of these kits left to test right now, but will be getting some MSP430 later this week (from Mouser). I'll try to reproduce this signal with my wave form generator and see how the firmware behaves.

Regards
Yuriy

 

[ycroosh]

Could be signal. I just embarked on adding a tach to the spindle on my lathe for touchdro, and I also broke out the scope. There's some traces visible in the vid:

I think you're much more knowledgeable in electronics than I am, but the signal I was seeing from the hall-effect sensor definitely looks a lot cleaner.

Have you tried just popping a magnet on the spindle and feeding touchdro the signal from a hall-effect sensor just to confirm the input?

Rich,
I watched your video (a few times). Thank you for the detailed review of the user manual. It was pretty eye opening I will clarify the pinout stuff (it sounded obvious to me when I wrote "input pinout", but I can read my own mind pretty good...). When I get a bit of time, I'm planning to order a bunch of HALL effect sensors and test how they work. For now, I'll at least add a short section on what "should" work.

Thank you
Yuriy