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The VTi21 can be used to monitor TI21 (EBITrack 200) audio frequency track circuits.
- Supports standard power Transmitter current monitoring (Part No: VTI21-TI21-TX)
- Supports low power Transmitter current monitoring (Part No: VTI21-TI21-TX-LP)
- Supports Receiver current monitoring (Part No: VTI21-TI21-RX)
| Info |
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Please note, that presently, this VTi21 variant only supports 4-20 mA analogue output
Transmitter units and receiver units require different current transducers
No RS485 output is supported. RS485 output can be added in future. Please contact Mpec if this is of interest. |
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- The VTi21-TI21 variants are only able to monitor 1 track circuit per device.
- The current transducer must only be connected to Channel 1 of the VTi21
- VTi21 devices for monitoring EbiTrack / TI21 standard power Transmitter units must use the current transducer SCT-0400-025. (Ships with VTI21-TI21-TX)
- VTi21 devices for monitoring EbiTrack / TI21 low power Transmitter units must use the current transducer SCT-0400-005. (Ships with VTI21-TI21-TX-LP)
- VTi21 devices for monitoring EbiTrack / TI21 Receiver units must use the current transducer SCT-0400-001. (Ships with VTI21-TI21-RX)
The only difference between TX and RX models is are the transducertransducers. TX and RX VTi21 units are interchangeable if the transducer element is swapped.
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The differing sensitivities of the current transducer used for the monitoring of EbiTrack / TI21 transmitter and receiver units mean that the output scaling will differ for the VTI21-TI21-TX and VTI21-TI21-RX variants.
| Quantity | VTI21-TI21-RX | VTI21-TI21-TX-LP | VTI21-TI21-TX |
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| EBITrack Average Side-Band Current | 0 to 200 mA | 0 to 1 A | 0 to 5 A |
Increased Sensitivity
The VTi21-TI21-RX model is designed to output between 0 and 200 mA on the 4-20mA interface, as 200 mA is the largest "track clear" current that is normally seen in service.
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Item | Meaining |
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| Code | The EbiTrack 200 has 8 varieties, this letter-code (A to H) makes it easy to the reference which operating frequency you have. |
| Centre Frequency | This is the frequency equidistant between the sidebands. This frequency is not actually generated anywhere, it is an artifact artefact created by the frequency modulation of the two sideband frequencies. The VTI21 looks for these "false" frequencies in order to detect to which of the 8 EbiTrack varieties the device is connected. |
| Upper Sideband | The transmitter generates two tones, and alternates between these tones at the "modulation rate". The higher pitched of these tones is the upper sideband. The VTi21 detects the energy present at this frequency within a very narrow band with high precision. |
| Lower Sideband | The transmitter generates two tones, and alternates between these tones at the "modulation rate". The lower pitched of these tones is the lower sideband. The VTi21 detects the energy present at this frequency within a very narrow band with high precision. |
| Modulation Rate | This governs how often the transmitter hops between emitting upper and lower sideband frequencies. |
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- After calibration, the VTi21 will start to inspect the frequency content of the current flow in the monitored conductor
- The VTi21 is looking for the "loudest" EbiTrack 200 / TI21 channel present (A to H)
- The VTi21 will "lock-on" to the loudest channel, so long as that channel has an IAVE output of approximately 6 mA or more above a preselected threshold (to provide immunity to noise)
- VTI21-TI21-RX: >6 mA
- VTI21-TI21-TX-LP: >30 mA
- VTI21-TI21-TX: >150 mA
- After lock-on, the VTi21 will start to report the average side-band current of only the selected frequency channel
- The VTi21 will remember the lock-on channel until:
- The VTi21 restarts, or
- No signal greater than 6mA has been observed on the selected channel for a period of more than 2 weeks.
- In both cases, the lock-on detection process will repeat. This prevents the VTi21 from "forgetting" the lock-on channel during extended periods of track occupancy or failure.
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The graph below shows how VTi21 output varies with deviation in modulation rate from the ideal value of 4.8 Hz.
Scaling
The standard power transmitter output is scaled to be up to 5 Amps.
- Typical transmit current can be as high as 3 or 4 Amps in typical installations.
- The EbiTrack 200 transmitter features an output-protection fuse sized at 5 Amps. If drive current exceeds this, this fuse will blow disabling the transmitter. There is therefore no reason to measure beyond 5 Amps
The low power transmitter output is scaled to be up to 1 Amps.
- EbiTrack can be deployed in "low-power-mode" in a single rail configuration, typically with traditional insulating block joints delineating track block sections Typical transmit current can be as high as 0.5 Amps in these installations.
The receiver output is scaled to be up to 200 mA
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