Bridge Board Problem

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Bridge Board Problem

Postby madjoge » Wed Jul 25, 2012 5:29 am


I'm made a project with Phidget 4-Input bridge to measure PT100 RTD. In fact I use 5 Boards (Total 20 Sensors) + 8_8_8 Phidget Board for relays and 4-20 Sensors. Interface is Visual C2008.

in regard with 8_8_8 the board is working fine. But we experience several problems using Bridge Board. The problem is very important, because we are going to use this soluton over 50 Piers at Spain to measure temperatures in storages (So it means about 300 phidgets).

My problem is with source code: I made that in usual way than 8:8:8
1- Create Iface for each Bridge Board
2- Open It
3- Wait to attach
4- The on Attach enable the 4 Bridges on The Boards & Configure Gain1 and sample 1000ms
5- Then in a Loop (every 1000 Seconds) y call to CPhidgetBridge_getBridgeValue
to get the actual value.

It's works fine for a while, but suddenly (I don't know way) CPhidgetBridge_getBridgeValue return code 9 (UnkNown value) and Board stop to respond (always return "9"). The only solution we found at this moment is to repeat the step from 3:
3- Wait to reatach
4- On Attach Reconfigure Phidget

then it turn to work again, until it fail again with "9". The rate of failed is not fixed. Sometimes
it take a while (1 hour) before failed, somethimes just 5 seconds.
I try to use a "handle call" aproximation, then the callback is "call" every second in regular way, but again somethimes it stop to call and we need again to do reatach-reconfigure.

If it's needed we can provide source code. It's very important for me try to understand the problem, because we start by monday to deploy the system in real situation, and if it work we proceed with next piers, but i'm very worried about having problems because it will be work in 24x7 schema over industrial computer

Thanks a Lot

Jorge Contreras

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Re: Bridge Board Problem

Postby Patrick » Wed Jul 25, 2012 8:38 am


The bridge value will return unknown if the input is out of range, or if the bridge input in not enabled - most likely I think the bridge went through a detach-attach cycle and became disabled. This could be caused by a voltage drop, EMI, etc.

You should enable logging in the library - the output will shed some light:

CPhidget_enableLogging(PHIDGET_LOG_INFO, "filename");

Something you can do to recover from detach-attach cycles is to use the attach event, and set your gain/data rate/enable inputs there - so they are all done right away.

If you are getting detaches - you will need to look at how the PhidgetBridge is attached to the computer. Make sure you use a thick/short USB cable with ferrites and either connect directly to the PC, or use a powered hub - not an unpowered hub - this all maximizes stability against EMI and voltage drops.

You should also listen to the error event - this will report any over/under range conditions.


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Re: Bridge Board Problem

Postby erik » Wed Jul 25, 2012 9:13 am

How are you connecting the RTDs to the PhidgetBridge?

We have a short app note describing how we connected an RTD with great results:

We used these resistors in order to get high accuracy:

The resistors are necessary since it will limit the amount of current that flows through the RTD, in turn limiting self-heating. Also, they reduce the inrush current on each input port when they are switched in during sampling (which might explain why the board is resetting randomly).

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Re: Bridge Board Problem

Postby madjoge » Wed Jul 25, 2012 11:13 am

Thanks a lot for your fast reply

I use a 8_8_8 usb board with hub and power supply of 12V. All the others bridge board (5) are connected within this hub. No other is present in the hub rather than phidget items.

I think, probably the origin of the problem is due to out of range values. So my question is, if I have such kind of error (out of range), the bridge is automatically disconected, and then we need to set again the values?.

At this moment I'm making that, and it seem work perfectly. I suppose doing that is similar to re-enabling bridge at handler error callback (doesn't it)?

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Re: Bridge Board Problem

Postby madjoge » Wed Jul 25, 2012 11:18 am

For made the bridge, I'm using 1Khm resistor of 0.01% tolerance and 1/4watt. Doing that, PT100 is reading without any problem.

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