How eBIRST Switching System Test Tools Work
eBIRST switching system test tools are a set of tools that allow users to identify faulty relays in Pickering Interfaces switching systems, primarily those with PXI, PCI or LXI (Ethernet) control interfaces. It measures the path resistance between pins of the switching system connector(s) under controlled settings of the switching system. Provided the fault is caused by a failed relay (by far the most common cause) it will find relays that fail to open, fail to close or simply have high contact resistance which indicates the contact is beginning to wear. If the fault is not a relay the fault pattern will give a strong indication of the cause of a switching system malfunction.
An eBIRST tool may have an adaptor part which converts the eBIRST measurement connector to the switching system. The eBIRST tools are controlled from a USB2 interface and use USB power to power the tool. An application program runs on Windows PC to manage the tool, collect results and display the resulting test results.
The controller presents the results primarily as a graphical output which shows the location of all the relays in the switching system and colour codes them as to their condition. Relays that need replacing are highlighted in red.
The tools and adaptors have to be purchased from Pickering Interfaces, the application program is free to download and contains files that define the test process used by the tool.
eBIRST Functional Diagram
Controlling the System
Inside each eBIRST tool there is a small controller that provides the USB2 interface, controls the measurement engine and sets what path is being measured. The software interface from the eBIRST controller to the Windows PC is proprietary.
Setting the Path
The eBIRST tool uses solid state multiplexers to select which of the pins of the measurement connector are connected to the measurement engine. The path can be set between any two pins of the measurement connector and provides a full four wire measurement to remove any contribution from the solid state MUX's path resistance.
The measurement engine injects a voltage via a resistor into the path selected, for relay testing this becomes the equivalent of a constant current source. The tool includes various current options to suit different measurement targets. The measurement engine measures the voltage drop across the selected path which allows the eBIRST tool to convert to a resistance measurement. Any DC offsets caused by the measurement engine or thermoelectric effects are removed by the measuring method used. The resistance measurement range can be scaled by changing current sources and by adjusting the voltage gain of an amplifier before the analog to digital converter (ADC) makes the measurement.
Each tool includes a set of precision resistors, by measuring these resistors the application program can scale the ADC response from the measurement engine into resistance. The tool is therefore dependent on the precision internal resistors remaining stable with time, errors are very unlikely.
In addition the tool can be calibrated by connecting to an external fixture that also contains precise resistors, these resistors can be independently checked by a DMM so for the minority of users that require a more robust traceability route the calibration fixture provides this.
Pickering interfaces does not state the tool accuracy. In measuring paths that include relays the temperature stability of the path is typically dominated by copper tracks and wires, copper has a stability of 0.39% per C. The tool accuracy is sufficient that results can be influenced by a relatively minor change in path temperature. The ADC used provides 14 bit measurement resolution. The eBIRST tool has much better accuracy and resolution than is required for the purpose of evaluating path resistance in a switching system. Pickering Interfaces does have the information on tool accuracy but it is dependent on tool settings used, it is an unnecessary complication to state it as the results are dependent on the settings Pickering Interfaces use to measure a specific switching system. The eBIRST tool has significantly more capability than required to diagnose faulty relays.
Switching systems that can be tested
The tool can measure any path resistance between two pins of a switching system connector, which means that as long as that tool is supported and the switching system is DC coupled (almost all are) then the paths can be measured. The application program needs information on the switching system design (the relays, where they are located physically, their interconnections) which is contained in a definition file named Test Definition File (TDF). The application program uses the TDF to determines what paths are to be tested and uses the results to identify the relays at fault.
The TDF contains a graphical image of the switching system relay layouts, faults can be clearly identified by layout location.
The application program is being continually added to as new Test Definition Files are created, once you own a tool you can update the application program at any time and you will receive all the latest files. There is no cost for new files.
Why the 2A limit?
The 2A limit is a rough guide. eBIRST uses a low level signal to derive its path resistance measurement—the test voltage is 5V and the maximum current is 30mA. Mechanical relays at 2A and lower generally use sealed reed relays or gold contact electromechanical relays which have consistent path resistance at low signal levels. This tends not to be true of higher current mechanical relays where contacts can have materials such as silver and nickel alloys to increase the robustness of the contacts under hot switching conditions. Relays with high hot switch capacity may have a gold flash present but this is used to protect the relay contacts during manufacturing test and subsequent storage - it tends to be removed following sustained hot switching (on very high capacity relays just one event may remove the gold flash). Once the gold is removed then environmental effects (including vapours from plastic cases) can impact the contacts and cause non-linear contact resistance (variable with hot/cold switching and carry current). Use of BIRST on relays of this type could lead to some indicated relay failures which are not faulty. The same will be true if faults are found from a DMM measurement, which typically operates at just 1mA so is a less capable tool from this perspective than eBIRST.
There are a few types of switching systems which have a rating of 2A or less which we would not recommend for use with eBIRST, These might for example be designs where higher capacity relays are used but the design is limited by other factors such as relay spacing, tracks or connectors. Relays of this type may have stated minimum switching capacity specification of 5V, 10mA , however our experience of these relays is that often the performance is variable, and the manufacturers do tend to place cautions on such claims to cover different environmental conditions or relay usage. There are also some relays which use contact materials we do not recommend for eBIRST, for example those used in high voltage switching.
There are no restrictions on the ability to test DC coupled solid state switching systems.
Pickering Interfaces will provide advice on what relays can be supported, but the 2A rule is usually a very good guide. On the web site if the landing page for a product (and the data sheet) does not have a specific statement that it is supported by eBIRST then the assumption should be it is not available for technical reasons
More information on minimum switching capacity can be found here Minimum Switching Capacity
Power Supply Management
USB power provides the supply for the eBIRST tool, however this voltage is dependent on the current draw and the voltage from the PC USB connection since the connection is by cable. Each eBIRST tool therefore has a DC to DC converter that regulates the tool power supply voltage in order to ensure no USB voltage dependency. The tool is therefore independent of the USB power connection.
What happens if I replace relays and the fault persists?
Most faults in service are caused by relay abuse which in turn causes a relay to fail. Relay abuse can be caused by programming accidents, use beyond rated specification or connection to a device which has a fault which creates a load condition which it cannot support. As the switching system ages lifetime issues may become an issue. For more information see what causes relays to fail.
If replacing relays (the most likely cause of a failure) does not solve a problem then eBIRST still has a role to play as the fault pattern is likely to give strong clues as to the cause of a failure. Contacting Pickering Interfaces support team with a set of eBIRST results will enable us to help diagnose the cause of the fault.
Further help on fault finding where relays are not involved can be found here.