# Rpm sensor pulse width problem

919 4
I've encountered problem with my conversion. There are two bolts on the motor/gearbox adapter (2signals per revolution) as seen here.

I calculated the pulse width to be:

6-10mm /(200mm * π * 120hz) = ~ 79-132us. ( I don't know the precise diameter of the bolt head, I used 7200rpm as the max. value)

The controller wants the signal to be at least 200us. I used the Evnetics logger and it shows rpm up to 2300 and then void. (With 6mm bolt the pulse width is ~250us at that rpm.)

The "easy" solution would be to increase bolt head diameter. In my case, I would need to remove gearbox and motor. I just can't take the adapter/coupling away from between the two because there is so little space. And then take the adapter to a machinist who makes approbative changes. ..and balances the whole thing. Bolt head diameter should be at least 20mm so the pulse width would be around 260us.

So my question is, is there any other, preferably easier, way to increase pulse width? Electronically? Or could I wire the tach signal (from the Omron hall -sensor) directly to an external rpm gauge with shift light? And then use the shift light as a throttle limit via Solitons inputs? I don't need external rpm gauge, but working rpm limit is a cheap insurance.
1 - 5 of 5 Posts

#### kennybobby

· Registered
Joined
·
1,953 Posts
How about using some electronics to solve this instead of removing the hardware for machine work.

But first we must know for what function does the controller use the pulse width? Is it measuring speed by counting pulses, or counting pulse edges, or actually measuring the width to calculate a speed?

It sounds as though it requires a minimum width in order to detect that an edge has occurred? The problem with inductive pickups such as seen in the video is that the amplitude of the pulse can be reduced at higher RPM--the quicker passing of the bolt head leaves less time for the induced signal to rise to full voltage. If that is the case then some sort of signal amplifier may be needed to boost it to a level to be detected in the controller.

But if it is just the width, then a digital logic "OR" gate could be used to add some delay to the pulse falling edge and give you a longer pulse width. This would be in a logic chip such as a 4071 quad OR gate, seen here:

https://en.wikipedia.org/wiki/OR_gate#/media/File:CMOS_4071_diagram.svg

The amount of delay or extension to the pulse width is set by the R*C time constant, so it can be adjusted to suit the situation. For example to make a delay constant of 200 usec use a 2k resistor and a 0.1 uF capacitor.

#### Attachments

• 44.4 KB Views: 148

#### Tony Bogs

· Registered
Joined
·
746 Posts
Another basic circuit for lenghtening positive pulses with a wide VCC 4093 schmitt-trigger and a diode-rc combo.

#### Attachments

• 29.8 KB Views: 143

#### piotrsko

· Registered
Joined
·
2,049 Posts
IIRC, the sol1 uses the step between states to trigger whatever it does for measurement. Tesseract has a formula somewhere in the manual for how to make a trigger for various rpms out of bolt heads.

Have you tried reversing the sensor from hold high to hold low via the shunt resistor?

#### Tomppa

· Registered
Joined
·
34 Posts
Discussion Starter · ·
I've read the manual many times but I just couldn't find that (if you meant min. pulse width?) formula there. Am I blind or sthg Nevertheless, this thread has more info.

Here's what Tesseract wrote in that thread:
Finally, the Soliton controllers need the pulses from the tach sensor to be at least 200us wide (whether starting from ~12V and dropping to 0V when the target approaches or vice versa) or the hardware filter/squaring circuit will ignore them. In other words, the sensor targets need to be a certain minimum width relative to the circumference of the circle they scribe as they rotate. For example, let's say you want to read up to 7200 rpm so you can effectively limit the motor to 6000 rpm (the controller needs to be able to read a higher rpm than the desired limit). If you make a disc out of aluminum and cut two notches in it for an inductive prox to sense, the notches need to be at least 1cm wide if the disc is 12cm in diameter.
At that time I didn't have a resistor, but now I have bought 1k ohm resistor for pull-up or pull-down but I haven't installed it yet. I don't own a laptop and have to borrow it every time I need to see the logger. Yet I'm quite certain the main problem is the pulse width. As stated in the quote for minimal requirements; 1cm and 12cm disc vs mine <1cm and 20cm adapter/coupler disc.

Thanks for your replies. I'm going to try kennybobby's solution as it is pretty straightforward and easy. If it works, you have saved me a ton of work and time !

1 - 5 of 5 Posts