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Simplest way to monitor current with a Prop? — Parallax Forums

Simplest way to monitor current with a Prop?

T ChapT Chap Posts: 4,223
edited 2014-07-26 22:43 in Propeller 1
I have an electromagnet clutch and I want to know if it is engaged or not. The only thing I can think of is to put a .5ohm on the 24VDC path to the clutch, use a diff amp on both sides of the shunt and amplify the gain to a comparator to provide ample signal. Then set a threshold on the other input of the comparator with the Prop DAC. If the clutch is engaged it will be pulling around an amp. 24VDC 28W. This method requires a differential amp plus a comparator, but I am trying to look for a simpler solution(less cost, less parts). The Prop needs to watch for clutch engagement after it is told to engage before continuing. Any ideas?

Comments

  • pmrobertpmrobert Posts: 673
    edited 2014-07-26 15:09
    Old school reed switch positioned by said electromagnet so it trips when field is present?
  • T ChapT Chap Posts: 4,223
    edited 2014-07-26 15:11
    A reed switch would add a mechanical failure potential to the equation.
  • pmrobertpmrobert Posts: 673
    edited 2014-07-26 16:19
    OK, two of them each monitored separately. A lack of agreement in state or lack of field on both throws error and prevents progression of your process. A likewise check after disengagement would prevent the case of both being stuck closed. You already have a single point mechanical failure situation with the clutch. A powered actuator != mechanical actuation.
  • T ChapT Chap Posts: 4,223
    edited 2014-07-26 16:42
    Good ideas, the clutch is spinning and mounting relays near it would be some work. The clutch will not fail except by a bad connection between it the controls. Controls are easily accessed, the clutch and assembly is not. The current sense if more just part of a diagnostics to insure that everything is connected before operation. I am looking for something to give a 10 year life without access, and my experience with relays is that even under light use they can go bad. I am hoping for a solution at the controller end that is accessible.

    I can likely remove the comparator out of the circuit, and program the gain of the diff amp so the output falls clearly away from 3v3/2 in either state(on or off). This will save one part and a few passives. The ideal is no extra IC's but that may too much to ask for the Prop alone.
  • pmrobertpmrobert Posts: 673
    edited 2014-07-26 17:01
    You're right about relays, that's for sure. They seem to fail at the worst possible moment, I see you have another post about a rotary encoder - is that for a shaft downstream from the clutch?
  • JonathanJonathan Posts: 1,023
    edited 2014-07-26 17:02
    It sounds like you just need to know if it is off or on, right? In that case a simple opto-isolator would work. No mechanical parts.

    Jonathan
  • T ChapT Chap Posts: 4,223
    edited 2014-07-26 17:03
    One encoder is on the motor for PID. The other is on the shaft at the pulley. I need to resync the motor position to the assembly position if the clutch is ever manually disengaged( de-synced).
  • T ChapT Chap Posts: 4,223
    edited 2014-07-26 17:03
    Can you explain how the opto knows there is about an amp being pulled? I just need to know if there is current being drawn which means the clutch is engaged and there was no cable failure. So it is On or Off, no need to measure any current.
  • JonathanJonathan Posts: 1,023
    edited 2014-07-26 17:25
    The opto will only know if there is voltage present, not if it is actually drawing current. I misunderstood your needs. The same is true of a reed relay, mechanical failures aside.

    So, maybe a current transformer to an opto? With the right resistors you get a prop pin to read a 1 if enough current was drawn and 0 if not. Current sensor to a sigma delta pseudo ADC? Allegro makes some good ones.

    Jonathan
  • T ChapT Chap Posts: 4,223
    edited 2014-07-26 18:12
    The Allegro ACS714 would produce an output of 2.5V at 0amps. 2.7 at 1amp. Even if using a divider on the output, this would be pushing it I think to fit the Off at .1V under the Prop input threshold, and On a .1V above. I currently use the Analog Devices AD628 which is perfect in terms of functionality as it has programmable gain output to give a wide range output. But it is pricey and requires the shunts, plus a number of passives. This may be the solution if I can't find a better trick.
  • pmrobertpmrobert Posts: 673
    edited 2014-07-26 18:29
    I seem to recall some Melexis Hall Effect devices used in a competition vehicle I was peripherally involved in. They were being used to quantify and qualify various pumps and solenoids current consumption.
  • JonathanJonathan Posts: 1,023
    edited 2014-07-26 19:12
    Why not use the sigma delta DC trick? It takes two pins, two resistors and two caps. That with the Allegro sensors should work. I didn't look at the datasheet for the allegro unit you mentioned, but I think 0 amps the output is VDD/2, and you can use 3.3VDC IIRC.

    Jonathan
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-07-26 21:15
    If you can sense on the low side (i.e. sense resistor to ground), it should be a piece of cake with just a sigma-delta ADC. That's the way motor current sensing on the S2 robot works.

    -Phil
  • kwinnkwinn Posts: 8,697
    edited 2014-07-26 22:43
    A 1.5 to 2 ohm resistor in series with the clutch and an optoisolator with current limiting resistor in parallel with the 1.5 to 2 ohm resistor should do the trick.
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