Tc9400 v to f opinion's please

[muso52]

Hi Guys,
I am currently playing with the above 'chip' using a variable DC voltage as an input. What would be the effect of using a full-wave rectified (but not smoothed) and also a 50% duty cycle input? Yes I could smooth the incoming rectified and pulsed input and I suppose whilst on the bread board could just try it and 'suck it and see'. Obviously Values of C and R's would have to be addjusted for required Fout no problem. Any thoughts anyone?
Muso52

 

[MikeMl]

It is trivially easy to turn the input opamp inside the TC9400 into a low-pass filter. Because of the high-impedance at the input of said opamp, you can add a second low pass externally with just an RC network.

If there is any ripple present at the output of the internal opamp, there will be jitter in the pulse train output. The frequency of said pulse train is controlled by the DC level. You need to do all you can to make that a DC level; not something with ripple from the external rectifier.

 

[muso52]

Hi Mike,
Many thanks for your input.

It is trivially easy to turn the input opamp inside the TC9400 into a low-pass filter. Because of the high-impedance at the input of said opamp, you can add a second low pass externally with just an RC network.

The full wave rectified but unsmoothed voltage is nominally 0 to 12v and used to control a DC motor. This is at a frequency of 120Hz UK mains times 2. I am not sure how the low-pass filter is of benefit.

If there is any ripple present at the output of the internal opamp, there will be jitter in the pulse train output. The frequency of said pulse train is controlled by the DC level. You need to do all you can to make that a DC level; not something with ripple from the external rectifier.

Ok understand ( I think) I need to condition the above to give me a smooth DC input to the device. The obvious thing would be to use a capacitor however my concern is that the V to F conversion will 'lag behind ' the above when changing levels. Because of the cap I would look at isolating this circuit from the motor drive source also, this device effectively being in parrallel with the motor.Would a simple bridge rectifier be of any use in this respect?

Regards,

Willy

 

[MikeMl]

The "lagging behind" is a matter of degree. Anything that tries to figure out what frequency the pulse rate out of the V/F is going to be lagging several cycles of the pulse period, anyway.

 

[muso52]

The "lagging behind" is a matter of degree. Anything that tries to figure out what frequency the pulse rate out of the V/F is going to be lagging several cycles of the pulse period, anyway.

Hi Mark,
Yes take your point. Basically I am trying to use the unsmoothed voltage as an input to the TC9400. Because this voltage is variable ( 0 -12v) a capacitor would have to be calculated at mid range. This would still lead me susceptible to jitter/ripple. Secondly after
' tapping' the voltage and conditioning it it is necessary to isolate it from the later.
Can you offer any thoughts?
Would be apreciated.

Muso52 (willy)

 

[MikeMl]

Why are you using a VtoF converter? Is this for some sort of telemetry?
What frequency range are you using?

 

[muso52]

Hi Mike,

Telemetry? Sort off..........

The idea being that a dc motor is a relatively linear device and therefore voltage applied is relative to rpm or/and motion. Think of a model train engine!
By converting the variable voltage to an input for the TC9400 this can 'trigger' a relative frequency pulse that is relative to distance traveled (via the dc motor) Freq range is nominally 0 to 100hz. Although this will be divided subsequently by 10. This is done with the hopefull effect of improving linearity of the TC9400 at such a low f .

Of course I am open to other suggestions....

Willy

 

[MikeMl]

Why not just put a photo-interrupter on the motor shaft, and divide that down to get a pulse rate proportional to motor speed.

Then you are not dependent on the linearity/accuracy of the V-to-F converter. The only error source would be wheel slippage, which would also effect your scheme. Besides, the motor speed vs motor voltage is nowhere near linear.... (think about a stalled motor).

 

[muso52]

Why not just put a photo-interrupter on the motor shaft, and divide that down to get a pulse rate proportional to motor speed.

Then you are not dependent on the linearity/accuracy of the V-to-F converter. The only error source would be wheel slippage, which would also effect your scheme. Besides, the motor speed vs motor voltage is nowhere near linear.... (think about a stalled motor).

Hi Mike,

Well I was to reticent to say but the model train I alluded to is actually what it is for! So if you think of the 0-12v going to the track to run the dc engine then that is what I need to monitor. So no direct contact to the motor itself.

Willy

 

[MikeMl]

So you can hang a DC voltmeter right across the track to tell you the "motor speed" . The intergral of speed wrt time is distance, Is this what you wanted to use the VtoF for? Counting the pulses?

 

[muso52]

Hi Mike,

Yes pretty much. The applied voltage is monitored and used to trigger the V to F.
Increased voltage = Increased speed. The pulse f out is relative to speed and also by counting them we get distance wrt time.

Willy

 

[MikeMl]

Ok, back to the original question. You can low-pass filter the hell out of your track voltage before feeding it to the VtoF without effecting anything. In fact, if your low-pass filter time-constant matches the acceleration profile of your locomotive, you will actually improve the overall accuracy.

 

[muso52]

So your saying a low pass filter will remove the 120hz ripple?

Willy

 

[MikeMl]

So your saying a low pass filter will remove the 120hz ripple?

Willy

Does a bear crap in the woods?

Two versions of filtering. The opamp version settles much faster.

 

[muso52]

Does a bear crap in the woods?

Well no if it has a 'log' rammed up it's ****!

'Bear' with me. I'm Just an old controls guy having a bit of fun with electronics in his retirement. This old bear don't always see the wood from the trees!

Anyway, thanks for your time. Appreciated the .png.
The easiest way would of course be the use of a capacitor (filter) . However this presents it's own problems.
Of course with respect to time constant matching acceleration that is a no no. We play with the 'throttle' that vary's the applied voltage and hence speed. Said acceleration is related to change in V wrt time.

The input to the TC9400 is in effect connected to the track voltage. The reason for un-smoothed 120hz V is that DC motors ( in models anyway) perform better than if the source was smoothed DC. So unless the TC 9400 is isolated from this voltage the cap would in effect try to smooth the supply to the engine which can draw .500ma to 1 amp depending on load no?. Also the polarity to the the track is reversed when we change direction of the engine. The simplest way of rectifying this would be the use of a bridge rectifier to not only source input voltage but also isolate TC 9400 from track.

With respect to using an op-amp yes great but also similar probs exist. Although we could use a bridge again it would be advantageous to supply the TC 9400 from a regulated supply. Now I am hoping that this will be ok IF I connect ground back to source as opposed to the 'virtual' ground from the bridge. It may work ok, the bridge effectively isolating the op-amp from track supply and also changing polarity inputs when the track feed is reversed.Have I missed anything?
Any thoughts would be welcomed.

Willy

 

[MikeMl]

In the RC filter, the capacitor cannot "filter" the track voltage because it is isolated from the track voltage by the 47K resistor. The peak current that can flow ( in either direction) when the instantaneous track voltage is 18V and the capacitor voltage is say 8V can be seen by Ohms law to be I=E/R = (18-8)/47000 = 212uA. Not enough to effect the waveform across the motor...


Everything you have shown will require your "sensing" circuit to share a common ground with one of the tracks. If you want to read the differential voltage across the tracks relative to some other ground point, that can be done, but will require either optical isolation, transformer isolation, or a differential amplifier with sufficient common-mode range to span the offset voltage between the tracks and the sensing circuitry.

 

[muso52]

Hi Mike,

Ok, the bridge really is'nt needed but it do'es fulfill one usefull purpose and that is changing polarity of input when it is reversed on track. Providing I go the op-amp route I think I will be ok if, as you say, I share common ground with the track.

Ideal
Thanks,
Willy

 

[MikeMl]

Now I'm confused. Do you power your track with AC ?
...or with full-wave rectified AC (non-filtered pulsating DC) that comes from a four-arm full-wave bridge rectifier?
...or with full-wave rectified AC (non-filtered pulsating DC) that comes from a center-tapped transformer with only two rectifiers?

 

[muso52]

Now I'm confused. Do you power your track with AC ?
...or with full-wave rectified AC (non-filtered pulsating DC) that comes from a four-arm full-wave bridge rectifier? Yes

...or with full-wave rectified AC (non-filtered pulsating DC) that comes from a center-tapped transformer with only two rectifiers? No

The Bridge rectifier is a simple means of changing the polarity of the input to the TC9400. Overwise it would switch with that of the track feed supply when changing direction of the motor. It stops me having to delve into the inards of a commercial controller to connect. The accepted method of reversing the direction (of the dc and hence motor) is to use a DPDT switch which is mounted on the latter.I still think it is a useful way ( the bridge rec) of isolating the TC 9400 from the track supply whilst still monitoring it yes?


Willy

 

[MikeMl]

The Bridge rectifier is a simple means of changing the polarity of the input to the TC9400. Overwise it would switch with that of the track feed supply when changing direction of the motor. It stops me having to delve into the inards of a commercial controller to connect. The accepted method of reversing the direction (of the dc and hence motor) is to use a DPDT switch which is mounted on the latter.I still think it is a useful way ( the bridge rec) of isolating the TC 9400 from the track supply whilst still monitoring it yes?

Willy

This is even more confusing. What comes out of the commercial controller? AC or non-filtered pulsating DC.
Is there a reversing switch between the commercial controller and the track? Or is the reversing function built into the controller?

Can you post a diagram of your existing hookup (before you connect the VtoF)?