Counting heads of the ACS2000 come in two different types of wheel sensors with the appropriate evaluation interface board according to requirements. Wheel sensor RSR122 comprises two independent sensor systems, which detect the wheel flange of the train wheel. The operating principle of the wheel sensor system is based on the loss of energy in an electromagnetic alternating field when damped by metal. Because the wheel sensor does not suffer any interference from the cadenced linear eddy current brake it is specially suitable for lines, where vehicles with this type of brake are in operation.

Wheel sensor RSR180 comprises two sensor systems that are placed lengthways in the rail, at a distance of 180 mm between them. The sensor operation is based on the principle of field line deflection. Because of the position tolerant mounting and the lack of any electrical adjustment at the rail, the RSR180 is the preferred choice for all common applications.

Typical characteristics of both wheel sensors:

• Quick and precise mounting at the inside face of the rail on a rail claw
• Complete absence of any other electronic components in the proximity of the rail
• No further maintenance needs in this dangerous and frequently time consuming work area of the track except for an annual checks
• Mountable onto the web of the rail; this however, requires the drilling of precise holes
• Equipped with a fixed moulded four-wire cable set, to be connected in the trackside connection box via terminal to the main signalling cable
• Wheel sensors comply with protection class IP 67, trackside connection box with IP65

Interface boards AMC and IMC optionally provide wheel sensors RSR180, RSR122 or RSR123 with the operating current necessary and imultaneously evaluate the analogue wheel sensor signals.

Additionally to the features of the EIB-OK board, these boards provide configurable digital inputs and outputs, serial interfaces for safe or non-safe communication and extended filter options for the analogue wheel sensor signals. Thus, traversing and direction pulses can be provided and the inputs can control the counting head features.

In compliance with the project, boards EIB-OK or AMC, AC8 and optionally DIOB are inserted together with a fuse board per counting section into 1911 board racks. The boards of a counting section are electrically connected at the axle counting backplane ABP. Depending on the actual configuration, a board rack with 84 pitch units can accommodate the boards of four track sections. All boards comply, without further electrical protection, with the standard EMC EN 50121-4. Thus, the board rack is but a construction element providing protection against mechanical interferences.

For applications where the track clear indication of a track section is to be made available to two signal boxes, the digital input/output board 0108 allows input of additional 16 one-channel data (optocoupler) and direct relay output at the opposite side. Transmission is bi-directional and is failsafe (EN 50159-1). For further fail-safe processing by the signalling system, data must be transmitted as equivalent signals by multiple channels.

The axle counting board ACB detects the wheel pulses of the connected counting heads of a track section and generates a fail-safe track clear indication. The microprocessor system of the ACB comprises two independent processing channels. The output information generated by these two channels is only released after two further independent comparators have confirmed that output data are identical. The interface to the signalling system is a two-channel configurable relay interface. One ACB allows simultaneous processing of the signals of six wheel sensors. For system reset there is a two-channel input. Conditions for reset and reset procedure can be configured according to railway operator needs.

For applications where the fail-safe track clear indication of a track section is to be made available for two operating units, two axle counting board ACBs can communicate by means of a serial connection (RS232 modem). The communication complies with the standard EN 50159-1 regarding fail-safe communication in closed-circuit networks. The distance between the two operating units is only limited by the efficiency of the transmission system. A second serial interface on the front face of the board allows connection of a standard laptop for read out and evaluation of diagnostics data.

In the indoor installation, each counting head is connected to its own power supply and evaluation interface board depending on the type of the wheel sensor used.

For the wheel sensor RSR122 several hundred units of the EIB-OK board, in its relay-fitted version, are already in service at DB AG in level crossing protection systems. For the specific axle counting application, output of the digital wheel sensor signals uses optocouplers in order to achieve the high switching speeds necessary. Thus, traversing speeds of 330 km/h are confirmed by trackside measurements. The board performs a two-channel evaluation of the two wheel sensor system signals of an RSR122 and uses the evaluation to generate safe digital wheel sensor pulses for further processing by the axle counting system. Evaluation processors of both wheel sensor systems of a wheel sensor are programmed with mutually independent software.

The overvoltage protection concept requires a protection at the end of the indoor installation cable run to ensure efficient discharge of the voltage peaks (transients) induced into the cable. The sensors themselves are completely insulated from ground to withstand at least 5 kV respecting the rail. Transient overvoltages are thus discharged to the signal box, where they are symmetrically limited to a maximal 1000 V by the appropriate lightning protection board BSI. Smaller overvoltages are buffered by means of overvoltage protection measures by the evaluation interface boards introduced below.