Help: Continous position signal or discrete signals?

[MartinLe]

I hope this is the right forum, if not please point me into the right direction!

We plan a conveyor that will dump something into containers. A level gauge on the tip of the container will measure the heap below. The conveyor is placed on a turntable and will e rotated horizontally a piece when the heap below is at a certain height, to make use of more of the container.

The system will look something like this but only drop of to one side:

Image source:

http://www.dbtechnologiesbv.nl/products/turntable-conveyor

I need to decide between either a continuos signal for the position of the turntable, or discrete signals at every point the conveyor will be. we are talking about 6-7 position signals in that case (3 per container plus maybe one so the conveyor is out of the way when both containers are moved). From the mechanical or process standpoint, I don't see a need for a continuos signal, 3 positions per container are fine enough.

I assume that this amount of signalers is even easier to implement than a continous sensor. Is this assumption correct?

Is there an argument from the control and measurement system side of things for a continous signal?

 

[itsmoked]

I think you have the right idea. While continuous isn't real hard, specific points make more sense in that environment.

You can either use a fist full of inexpensive inductive sensors with a tab sticking off the tower somewhere to be sensed by each of the pickups or you use more complex rigid logic to keep track of sensors using dead-reckoning.

The pickups are much easier to maintain and are IP67 right out of the box. They show with built-in idiot lights what's happening for setup and future troubleshooting.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

I think that discrete is systemically harder:
> discrete implies separate sensors --> more wires, etc., as well as possibly more issues with noise, dirt, reliable signals
> discrete is harder for a control system to deal with --> a servo system needs a proportional signal, which requires continuous
> discrete will probably be slower, since there's no way to know whether you've "hit" the mark a priori, so you either go fast and overshoot, or you go slow

> continuous can usually or is usually be done as part of the inherent design of a system, like having a resolver
> continuous can accommodate any configuration changes without moving hardware --> discrete sensors will need to be moved to correspond to the configuration

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[MartinLe]

First off, we don't design the parts, we specify them and write a tender. If we specify something that's not on the market no one is helped.

itsmoked: I don't understand this: use more complex rigid logic to keep track of sensors using dead-reckoning.


IRstuff: The way I understand you we can go for a turntable configured as a servo system (which would move some complexity into the turntable and simplify 'our' control). If we find no turntable that's a servo system, we could somehow retrofit a continous sensor onto an exiting turntable but would loose a few of the advanteges you mentioned, no?

 

[itsmoked]

You can mount tabs on the moving base in unique locations that place the tabs over one of the inductive pickups. This gives a unique input for each position, a useful maintenance ability.

OR

You mount all tabs on the same plane to be seen by a single inductive pickup. This however means you need electronics to keep track of which tab has been reached while noting the direction of travel. If the power fails you need to handle 'where am I?'. Typically you would 'home' by going in a set direction until a unique tab with a second pickup is seen, denoting home. Alternatively you can put two tabs or a very wide tab or a very narrow tab that can act as an index tab for locating 'home'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MartinLe]

now I get it , thx.

 

[IRstuff]

if that's the case, then it might be easier to use an off-the-shelf digital compass in the cockpit. If you get a consumer one, it'll come with a display, and operator can steer to specific compass points. Alternately, you can get one with digital outputs that you can feed into a controller to drive the system, and provide the operator with a display so that they'll know if the system is operating correctly or not.

Since you ought to calibrate such a configuration anyway, to get rid of hard iron errors, someone can code up an app that can do the calibration and calibrate for the specific stopping points desired. This scheme would require no hardware modification if you decide to change the configuration later, and does not require anyone going on the outside of the system to install or repair sensors.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529