Thread: Mars Lathe DC motor conversion.

Hi Rob,

I understand what you are saying, but both the sensor and tacho are powered by the same supply. So, where should the sensor GND come from if not from the supply?

Here's a quick drawing of the circuit. It's the same as was posted previously except the load resistor.

Cheers + Thanks.

Simon2012-12-08_17-36-51_190.jpg

Hi Simon,

Look at the pin diagram again on Ebay.

0.56" DIGITAL Red LED Frequency and Tachometer Rotate Speed Meter Two Function | eBay

Pin 2 is signal ground.

Pin 4 is powers supply negative.

They are not connected together. They are kept separate for a reason.

You seem to be mixing voltage and signal together.

Is the sensor ground a connector or earthing through the mount ?

My bet is it's a connector and should go to pin 2 on the tacho - which should also not be bridged to pin 4 as in your diagram..

Rob

I'm fairly certain, that's how mine is wired, the signal ground in most cases is common to negative as the tacho is looking for a change to the signal ground as a trigger. I had simmiler problems, I'm still betting its the settings on the tacho, what are they Simon?

I would sugest that you isolate your display & sensor power to test.
You seem to have extra pulses.
Get a 9v battery and connect to your sensor and then connect the output to pins 1 & 2, using a battery will eliminate any ripple you may have in your benchtop power supply and confirm a working circuit and correct settings.
Once you confirm a correctly working display you can then play around with power supplies.
Andy

Here's my circuit. I've used 3 core shielded cable.

ImageUploadedByTapatalk1354954375.808216.jpg

Mine is the same as Chris's except pin 2 and 4 are bridged........
My S1 setting is 1. and my S2 as also 1...

I don't think a little ripple in the power would be the problem, i am using 240v-15v tranny, bridge rectifier, 100?uf cap, 7812 reg, 10uf? cap. Not exactly the cleanest PSU out there.

Correction to my last post:
I am using 2 core shielded cable. The 2 inner conductors are + Vcc and Sensor output. The braided shield is Signal GND. I bought the cable at Jaycar.

Hi Stuart,

Thanks for the offer, unfortunately I had a big night last night (one too many ciders) and I'm recovering quietly at home. I have a couple of ideas that I am going to try. I'll remove the notched signal generating ring i made and tape a small single piece of scrap steel to the spindle and see what I get having one pulse per rev.

Also for others on here asking about the wiring for the sensor, it's identical to the PNP NO sensor here. It clearly shows that the signal output is between the output and GND of the sensor.

3_4_wdc.gif

Cheers,

I still think I must be doing something wrong. With this stuff it usually is operator error!

Simon

Hi all,

I've just been out in the shed and tried a few things. I isolated the sensor gnd from the tacho power in ground, effectively the same circuit as Chris's.

Still no good. I also had a look on the cro and the signal seems OK. I'll keep trying...........

Simon

Here are a couple of snaps from my sensor. The first is just a broad range one, the next 2 show the on and off times. The ramping on the off was causing me problems, this was before the pull up resister was used.

Hi all,

Just been doing some more trouble shooting. It appears that the problem is noise and/or interference. The speed sensor and tacho work beautifully when the VFD and motor are not running, hardly helpful! In fact if I take the sensor off the mill and hold it about 1M away and use some other form of signalling (eg a cordless drill with a nut taped to the chuck) it works great! but as soon as I bring it near the mill with the motor running it goes haywire!

Even if the sensor is not picking up a signal (just sitting on the table) and the motor is running, I get the same rubbish display on the tacho as I have been getting all along.

I have tried varying the carrier frequency of the VFD but no good. I have looked closer at the signal through the cro and it shows a square wave but on closer inspection you can also seen some very fine noise superimposed. This noise goes away when you turn of the motor (VFD still powered).

So, I'm not sure if I have a noisy VFD or if the sensor is playing up to the point that the signal/noise ratio has become an issue for the tacho.

I'm sure it's not meant to be this difficult!
Any ideas anyone?

Simon

Interesting.......i wonder if mine will play up once the VFD is wired. Can the VFD's be used on single phase motors, using just 2 of the phases? i might get some time later to try it. Just to remind me Simon, what sort of sensor are you using? The pull up/down resistor should be removing most of the noise. Also, what are your tacho settings....just to sound like a broken record.....what is your S2 set to, 0,1,2 or 3?

Are you using shielded cable Simon?

Hi guys, it may be time to take some photos. My phone just died so I'll charge it up and take some. Ueee, I don't think VFD's would take too kindly to using only 1 or 2 of their phases.

I'm using one similar to this ATM: DC 6-36V 300mA 4mm Induction Sensor Inductive Proximity NPN Switch LJ12A3-4-Z/AX | eBay

It does not come with shielded cable so no I am not using shielded cable, in fact ATM i've got it connected with nothing more than alligator leads until I can get it working.

I still can't believe that noise can become a problem, we are talking very low frequencies and very strong (12V) signals. I don't see how noise of a few mV can cause such issues!

Once again, I appreciate all the help so far.

This really is trying me.

***** I forgot to add, I have the divider set to 3, but have also tried 1 and the tach/frequency setting set to 1 but have also tried 3. I no longer see these settings as being an issue ATM as even if I had them set wrong, I would still get a stable reading, even if it was incorrect. *****


Simon