Model Boat Mayhem

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: PWM to Analogue (0-5V) Output Card  (Read 3439 times)

Phil_C

  • Guest
PWM to Analogue (0-5V) Output Card
« on: December 16, 2010, 02:05:19 pm »

Hi All,

Not sure if this is the right place to post or not but;

Does anybody know of a box of tricks that I can plug into an "off the shelf" reciever that will convert the PWM out signal into a proportional 0-5v signal that I can then feed into a voltade follow-up device? I need to do this so I can drive a much larger linear actuator, instead of a servo.

Ideally I'd like to find a UK supplier, as the device I've found in the US is prohibitavely expensive to ship over.

Thanks in advance!

Regards... Phil

Logged

Subculture

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 4,187
  • Location: North London
    • Dive-in to Model submarines
Re: PWM to Analogue (0-5V) Output Card
« Reply #1 on: December 16, 2010, 05:32:21 pm »

I'd have thought a simple forwards only ESC would do the the trick.
Logged

Davy1

  • Guest
Re: PWM to Analogue (0-5V) Output Card
« Reply #2 on: December 17, 2010, 05:39:50 pm »

It couldn't be much simpler actually. All you need is a low pass filter which in practice can be just a resistor feeding a capacitor which has one end grounded.

David

Logged

malcolmfrary

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 6,027
  • Location: Blackpool, Lancs, UK
Re: PWM to Analogue (0-5V) Output Card
« Reply #3 on: December 17, 2010, 07:36:24 pm »

Combine the previous two, the signal from the RX is a bit thin, and liable to produce a rather unstable output if its just integrated and you then try to use it for something.  The ESC would convert the .5mS to 1.5mS per 20mS signal to a 0 to 100% PWM signal.  Its just a question of integrating this, or averaging and smoothing it with an R/C network, or such a network and an Op-Amp. A suitable instruction book would probably be on
http://babanibooks.com/bb7.htm
BP335 , operational amplifier users handbook.   The Bernard Babani books are usually written in English rather than technical, for the first few pages anyway.  Its amazing what you can do with a 741 op-amp, even more so when you use quad version such as a LM324.  Come to think of it, at least one pre PIC ESC was made using such a chip.  Just a question of careful reading of the data sheet and doing lots of arithmetic to get the right components.
Is the actuator just measuring voltage and responding, or is it using power from the 0-5 volt terminal?
Logged
"With the right tool, you can break anything" - Garfield

flashtwo

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 521
  • Location: Sevenoaks, Kent, England
Re: PWM to Analogue (0-5V) Output Card
« Reply #4 on: December 17, 2010, 09:58:16 pm »

Hi Phil,

I assume that your linear actuator has a positional feed back potentiometer giving a signal of 0 to 5volts back to a motor controller that receives an external position demand signal of 0 to 5volts.

Malcolm and Subculture's idea of using a low power ESC to give pulses with less gaps than in the original PWM, would give a faster response than just trying to integrate the 1 to 2ms pulses every 20ms. If it was a programmable ESC, you could calibrate the output to your exact need of 0 to 5 volts range for 1 to 2ms input.

I needed a 0 to 5 volts signal derived from RPM pulses for a motor control system and ended up programming a PIC micro-controller to do the job. It was very accurate and had a fast response. The PIC has its own high frequency PWM output much like an ESC, which I integrated with a simple resistor-capacitor network and fed the signal back into the PIC's analogue input. The PIC then ensured that the output was correct. I also have some PIC software that measures the pulse length of a PWM signal from a radio-receiver and stores the signal as a 0-100% value. This percentage value could then be used to control the analogue output voltage.

The PIC route is a totally over the top for your application and I would think that a cheap servo driving a potentiometer (say 10k linear) through a shaft will be the most practical solution.

If you had a supply that was higher than the 5 volts required, then a small trim potentiometer could be placed in series with the servo-driven pot and adjusted to give exactly the 5 volt range required.

Ian
Logged

flashtwo

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 521
  • Location: Sevenoaks, Kent, England
Re: PWM to Analogue (0-5V) Output Card
« Reply #5 on: December 17, 2010, 11:20:59 pm »

Hi Phil,

I'm just thinking that a standard servo has about 90deg movement for 1-2ms PWM input and that a standard potentiometer has about 270deg movement.

Therefore, to obtain a 0-5volt output, without gearing, the potentiometer would need about 15volts across it, so that the first 90deg of movement would give 0-5volts.

To obtain a 15volt supply without batteries you could use a 5 to 15 volt DC-DC convertor such as the Rapid Electronics part no. 84-2512; see

http://www.rapidonline.com/sku.aspx?tier1=Electrical+%26+Power&tier2=Power+Supplies&tier3=DC-DC+Converter+ICs&tier4=1W+Single+output+DC+to+DC+converters+-+RA%2fRB+series&moduleno78067&catref=84-2512

I've used these on projects and they are quite neat units and don't consume much power.

Ian
Logged

flashtwo

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 521
  • Location: Sevenoaks, Kent, England
Re: PWM to Analogue (0-5V) Output Card
« Reply #6 on: December 17, 2010, 11:24:05 pm »

Hi Phil,

You could use the 84-2542 instead since the 10k pot would require so little current.

Ian.
Logged
Pages: [1]   Go Up
 

Page created in 0.092 seconds with 21 queries.