voltage modification... newbie question, please help!...

[catboy]

Hi, firstly sorry if this is a really simple question, but I have no idea about electrics or electronics... Im a mechanical engineer by trade (sorry! )

Anyway, I have a voltage signal which varies depending on speed output of a motor, and need a way to alter this voltage. Its 0-12v and low current (a couple of amps max; I can confirm this if its important, but as its a signal, I dont think its very high)

I need something I can put inline with this voltage signal so the output reads zero until the input reaches a pre-defined value, then it can read as normal from there... ie the output reads 0v when the input reads 0 to say 3v, then it jumps to the input value (ie 3v-12v input = 3v-12v output).

Im not sure if this is a good description, but I hope someone can understand it and point me in the direction of a part, or circuit of parts that would do the trick...

I dont know the exact voltage I need it to switch at currently, but I can find this out, or better yet, if someone could tell me what I need to calculate in order to "size" the components, I could do it myself...

Thanks in advance, and sorry if this is a really stupid question...

b

 

[JimB]

Not a stupid question.

But first, what are you going to do with the 3 to 12 volt signal after the first 3 volts has been clamped to 0volts?

Or, are you really wanting to just switch something at the 3volt point?

JimB

 

[catboy]

the output should be the same as the input from 3v upwards, just basically want an output value of zero upto that point...

I have a lathe which has a relay to energize a coil on the cover into the locked position. This is a safety feature that prevents you operating the lathe with the cover open, however its beneficial to be able to operate the lathe at low speeds for inspection and for hand-cleaning, de-burring, etc with the cover open. The signal to the relay is fed from a speed sensor on the back of the motor, which also goes to the digital display on the lathe which gives readings for rotational speed, etc... This is powered by a 12v supply so Im pretty sure this is what the signal will range between, however I can confirm if this is important to the answer.

This is a signal, its not looking at the voltage across the motor! I know which pin on the relay this applies to, so just want something to give the relay a value of zero for low-speed operation, without removing the wire from the motor to the display or completely removing the signal to the cover, as I would like this to operate normally at higher machining speeds

I dont know the exact values at the moment, so the 3v and 0v-12v is just a value plucked from the air... Im really wondering if this is possible and if so how difficult it is to do. Is this something I could do at home (Im fairly practical and pretty handy with a soldering iron) or does it require some kind of programmed chip? If anyone can give me more information (like how to do this, and what I might need to calculate to find the right components, this would be a massive bonus! )

b

 

[eblc1388]

Two problems here:

1. If the lathe has digital rpm display, 95% chance that the tachometer is producing varying frequency pulses instead of voltage level pulses.

2. This is a public forum and no one in their right mind can advise you how to bypass a mechanical safety interlock as that would result in possible personal injury or equipment damages. But we would be happy to advise on how a circuit can be made to do something instead. How would you prevent the lathe from operating at higher speed when the cover is opened?

 

[catboy]

1) i can check this, but I have been told its a variable voltage; I dont know for sure, I havent had time to check this yet...
2) thats why I posted my initial post as vague as possible... Im not asking anyone to make this mod for me, and obviously I wont be holding anyone responsible for the circuit...

so, to clearup any issues my previous posts might have created...

What can I use that will condition a voltage in the following way

Input Output

0v - 3v 0v
3v - 12v 3v - 12v (as input)

Also, if this happens to be working on variable frequency rather than variable voltage, how could I go about something similar (ie giving an output of zero upto a certain frequency, then giving an output the same as the input from there up)

Thanks in advance

b

 

[eblc1388]

There is no need to modify the speed signal. What you need a a signal detector(either voltage level triggered or frequency triggered) circuit using the speed signal as input. The output from this detector is simply a relay contact that you place in series somewhere in the existing circuit. Note this would defeat the mechanical safety interlock as you are awared.

Both voltage/frequency detector options are not difficult to implement, electronically.

 

[catboy]

Weird really; just works like a lock to prevent you opening the cover when its running, you can start it with the cover open and use it at any speed... I dont want to modify it too much and would like the ability to slow it down, open the cover, and re-close, so want to make this kind of mod rather than just snipping the pin off the relay or stop lathe, open cover, re-start, etc..

Would something like the signal detector you mention be available off the shelf or as a single component, or is it something I would need to build from other bits? If I need to build it, any ideas where I can find a circuit diagram, and how I would need to calculate component "sizes" to get it to switch at the desired speed? This wouldnt need to be adjustable, just to operate at the low speed I use for inspection (about 60rpm)

b

 

[JimB]

As elbc said, you dont need to modify the speed signal to implement the low speed "interlock override", but it makes a big difference on the way the speed signal really works.

If the lathe has a digital readout for speed, the obvious way to implement this is with a variable frequency pulse train from an optical encoder to a frequency counter.
However, it is possible that the measurement is done with a tacho-generator and digital voltmeter.

The solution to your problem depends on how the speed signal is really derived.

JimB

 

[catboy]

I will check this out tonight and get back to you guys in the morning.
So the information I need to get is...

What is the signal (variable voltage or frequency)
What is the required value to use as a switching point (either V or Hz)

...also...

Current of signal?
Voltage of signal?

If theres any other information I can find out by then, I will post it tomorrow (ie how the signal is generated)

Thanks for the responses today peeps

b

 

[eblc1388]

I will check this out tonight and get back to you guys in the morning.
So the information I need to get is...

What is the signal (variable voltage or frequency)

You'll have a hard time telling which is which because while the signal frequency is always a fixed multiple of the machine rpm, the tachometer output voltage always increase along with rpm. That's why good design always use frequency to derive the rpm as one can amplify/condition the signal without changing the frequency.

On the other hand, you can simply use voltage level of the speed signal to switch the interlock if the switching point is not required exactly.