Abnormal Sensor Value Observed After Connecting to USB-1808

[Ricky Ooi]

As shown in the diagram attached, I am tapping sensor signal from a machine using USB1808 DAQ and the DAQ is connecting to a CPU through USB cable. In normal situation, the sensor reading should display 0. However, when my CPU is shut down (DAQ is not power ON), the sensor reading become negative value (approx -10). When CPU is turning on back (DAQ is powered ON), sensor reading back to normal (e.g. 0). Is there any method we can channel power directly to the USB1808 DAQ instead of using power source through USB cable? Or do we have any method which can avoid this problem through any circuit connection solution?

[Fausto]

Hello @Ricky Ooi.

Does your sensor have a display?  Do you have a datasheet for your sensor?  Is the CPU a laptop or standard PC? What reading does the sensor display if you disconnect the USB cable?  How is the sensor connected to the machine?

Try using an externally powered USB hub between the USB-1808 and your CPU.

Regards,

Fausto

[JRys]

Hey Ricky,

When the USB-1808 electronics are not powered, some leakage path to the ground could distort the sensor output. We make no claims that our devices will appear inert to a circuit when turned off. If it's a big problem, use several relays connecting the sensor signal to the USB-1808 when powered. For example, a small 5-volt relay could get away with using the +VO 5-volt terminal, which is active when the device is powered.

That said, if the machine sensor is isolated, the analog input low-side ground connection makes sense; if it's not, then I don't see the need for the analog input low-side ground reference. 

Best regards,
John

[Ricky Ooi]

@Fausto
1. Does your sensor have a display? 
Yes, it does. We are using the display to check for the sensor reading normality after tapping its signal using USB1808.

2. Do you have a datasheet for your sensor? 
No.

3. Is the CPU a laptop or standard PC?
It was a standard PC. 

4. What reading does the sensor display if you disconnect the USB cable? 
When USB cable to disconnect from CPU, USB1808 will not have power. At this case, the sensor reading deviate from a normal condition.

5. How is the sensor connected to the machine?
The sensor was originally connect to machine's controller (PLC) with Vcc to 24V, GND to Ground and Sig to PLC controller. We are actually connecting an extra cable directly to Sig pin to measure the voltage value using USB1808. The connectivity between DAQ and sensor are shown in the diagram above.

6. Try using an externally powered USB hub between the USB-1808 and your CPU.
We are actually power the USB hub using power source from CPU. However, in some cases, the CPU might shutdown by itself due to OS update, etc. This might affect the machine performance because the sensor reading running away from normal condition when CPU is power off.

[Fausto]

@Ricky Ooi

Thank you for your responses.  Please consider using an externally powered USB hub (not powered by the CPU) or consider @JRys suggestion to use a relay.

[Ricky Ooi]

@Fausto,

If does this, I not able to connect CPU with DAQ for data collection. Was thinking to directly source in power to the board through pin, is that possible?

[Fausto]

@Ricky Ooi ,

Which option does not work for you?  We have many customers using externally powered USB hubs with MCC devices.  With this scenario, the USB-1808 will still have power even when the CPU shuts down.

[Ricky Ooi]

@Fausto,

USB-1808 only have one USB Type B port right? If the port had been connecting to CPU for data collection, how to have external power channel to USB1808 through USB?

 

[Fausto]

@Ricky Ooi ,

The USB hub has an external power supply.  Connect the USB hub to the CPU.  The USB-1808 is connected to the USB hub.  When the CPU is shut off, the USB hub continues to provide power to the USB-1808 module.