The machines at my workshop have meter counters with hall effect sensor switches, which convert shaft rotation to electronic pulses. The switching signal is send to their PLCs' high speed counter input. The PLCs are located at the other side of the workshop, around 300m away.
Recently, discussions arose within my team about the measuring accuracy of the system. Surely there are many factors that could affect the accuracy. One of the them being discussed concerns whether that long signal wire, traveling through the cable bridge, would introduce interference which affect counting accuracy.
Also some believe that higher frequency gives better accuracy. (By increasing frequency I mean adding more sensing points to the encoding disk on shaft, currently 1 pulse/rotation)
One thing worth noting is that the "accuracy" here is not about resolution, but the final accumulated sum - the total amount of material through the machine. Every machine run takes typically
3 >2 hours resulting in about 2 billion 184,000 pulses at current frequency. (i.e. ~1,200RPM)
I am not really an electrical guy, but my logic tells me increasing the frequency give better resolution, but not necessarily accuracy to the total sum. Maybe on the contrary it loses more pulses (but each pulse represents less rotation too). And about the noise/interference thing, I really have no idea. However I do believe moving the disk to the driven shaft, instead of the motor shaft, will increase accuracy (but which will decreases counting frequency notably)
Provided that we don't re-run the cables, will increasing / decreasing the frequency make the result better / worse ?
As pointed out by @jonk and @Russell McMahon, the numbers are unlikely, I did some serious calculations and updated the figures.
The PLC is Mistubishi PLC, the counter's max response frequency is 100kHz.
The question actually is not about how to improve, but what has been done that actually improved things. Because many changes were done in one project, we are debating about what contributed least/most to determine how to move forwards. I find the information confusing, so I'd rather rely on physics.
Basically what has been done is:
- moving the encoding disk to the driven shaft, to eliminate belt slips.
- meanwhile increasing sensing points from 1 to 4. (pulley reduction = 2:1)
- use an additional PLC + HMI 10m away from the hall effect sensor.
The accuracy we're discussing is reported as a 1%-5% difference variation of a direct variable (wt).
Unfortunate limitation :
- there is no oscilloscope to compare the signal from 10m to from 300m away.
- the accuracy is based on indirect monitoring of the mach output weight, the pulses/meter is just a factor in the weight formula.
Again, I find the information unreliable, so I'd rather look for clues in physics... Will electrical science support a 1~5% stability improvement through the above changes ?
Some more details to add. The proximity sensor is an Omron
E2E 24VDC E2B-M12KS04-WP-C1 sensor. The signal is dumb on/off pulses, not digitally encoded.
The Mistubishi PLC's X000 high speed (100kHz) counter input has a default 5us
input filter delay (I think that's the hysteresis time, though the FX doc never mentioned it as hysteresis).
The encoding disk is not really a disk, but a pole or 4 poles in the revised version, close to the size of the cylindrical sensor head. So the ON cycle is very short compared to the OFF cycle at each rotation.
The PLC is
FX3U-64MR AC powered model. The proximity switch is tied to X0 input point, not extension module. The code directly read/reset
C235 which is counter for X000, with no self-baked counting logic in PLC code.
According to hardware manual jy997d16801k.pdf
For input point X000 to X005: Input form = Sink/source Input signal voltage = 24V DC +10%, -10% Input impedance = 3.9kohm Input signal current = 6mA/24V DC ON input sensitivity current = 3.5 mA or more OFF input sensitivity current = 1.5 mA or less
There is more info in the User's Manual [Hardware Edition]
sect. 10.2 JY997D16501. I can't find a public download link, so I uploaded it to a site I found. However the site removes files every couple of hours. Just in case, here's some screenshots: