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Turtle Theory of OperationFrom SPCoast
[edit] Overview[edit] MainBoard
The Turtle is a multi-board set made up of a mainboard and 4 daughterboards. The main board contains
[edit] DaughterBoards
Each of the four (4) remote boards mount on a Circuitron Tortoise switch machine controller; they provide:
[edit] OperationThe Turtle provides a LocoIO-compatibile interface to <a href="http://www.digitrax.com">DigiTrax</a>'s LocoNet. It responds to turnout commands and generates occupancy and turnout position feedback messages as required. Though it was designed to be used with [PanelPro], it should also work with other layout control programs. The daughterboards are intended to be mounted on Circuitron Tortoise machines (available at many places online, see [Google(circuitron tortoise)]) They contain the DCC power connections (both from the DCC booster and to the track), occupancy detection and position feedback circuits. The position feedback can be taken from the tortoise contacts, or, for the truely prototypical (paranoid?), from external contacts driven by the turnout points themselves. Turnout control packets from the Loconet are decoded by the onboard LocoIO device interface and used to change the position of a turnout. As the points move, the position feedback generates a Loconet packet indicating that the points are out of position; the frog polarity is changed, and finally, the position feedback generates another Loconet packet that indicates that the points are now aligned in the commanded position. In addition, as detectable rolling stock fouls the turnout, Loconet block occupancy messages are generated. [edit] Tortoise Turnout Motor controller
The Turtle is designed to control four low-current stall-type switch motors, such as the Circuitron Tortoise. One digital control line is used to select between Normal and Diverging positions. When the Motor Control line is high (not grounded), "Normal" polarity is applied to move the switch motor; when low (grounded), the polarity is reversed to "Diverge" the turnout. The Tortoise-mounted daughter board has jumpers to set the physical routes associated with these logical positions (so "Normal" is normal :-). [edit] Switch Machine Controller theory of operationThe LM324 Quad Op Amp generates a differential signal that drives the stall motor. When the circuit is in steady state, the Op Amps are held at 5v on one input (via a pull up resistor) and 1.4v (the reference voltage generated by the R3/R4 voltage Divider) at the other. Since the two op-amps inputs are connected + to - (See schematic, refer to pins 2 and 5 & 3 and 6), one op amp's output is driven to ground while the other's is driven to Vcc. When the Motor Control input is grounded, the situation reverses, and the op amps reverse state - the high output becomes low, and the low becomes high. Since the stall motor sits between the two outputs, it effectively sees its polarity reverse, and so drives the Tortoise to the other direction. (The rectified and filtered input voltage (12v-16v) powers the OpAmps, and so controls the voltage seen by the stall motor.) LM324 OpAmps U1 and U3 each drive two tortoises; U2 and U4 are used only to provide optional visual position indications via bi-color LED-T1, -T2, -T3 and -T4. If this visual feedback is not desired, U2, U4, R6-9 and LED-T1 through LED-T4 can all be omitted. The stall motor driver is protected by bypass capacitor C2 in the daughtercard that suppresses the little inductive "bounce" when the stall motor reaches the end of its travel and stalls. The daughtercard has provision on it to mount a bicolor (red/green) LED in series with the Tortoise motor; in addition, a set of external LEDs can be used to show the intended turnout position. This can be used as a debugging aid while working under the layout, as a control panel indication, or as simple track side signaling, as desired. More details are given below. [edit] Daughterboard
The turtle "panel" PCB has four snap-off daughter boards that contain the control interface, an occupancy detector, Tortoise connections, track connections, a "simple signals" interface, frog power (current limiting and polarity), as well as configuration jumpers. These daughterboards can be attached directly to Tortoise switch machine controllers. These boards are little more than a platform for a detector and a bunch of connectors, but they make wiring tortoises extremely easy.
[edit] DCC Isolated DetectionThe detector is a modification of the original design done by [Rob Paisley]. It uses a pulse transformer that completely isolates the detector circuit from the track power circuits. Because it relies on an AC signal being coupled through the transformer, it is for use with DCC systems only - it will not work with pure analog DC throttles. The Coilcraft VT-5 Current Transformer was available from [All Electronics Corp], but as of Nov, 2005, was out of stock and unlikely to be reordered. Other similar pulse transformers may/should work as well, but they have not been tested. The low cost of the VT-5 ($0.35 in quantity) made this circuit much more economical than traditional diode type detector circuits.
Features of the pulse transformer detector include
[edit] Detection theory of operationWhen no trains are detected, the transistor does not conduct, the capacitor charges to 5v over a period of about 2 seconds, and current stops flowing. With no current flowing, the TRIGGER voltage is pulled to ground by the 1Meg resistor, causing the open collector output (!DETECTED) to go high and the Occupied feedback LED to go dark. When current flows in the detected track section, a voltage is induced in the pulse transformer. When a sufficiently large voltage (> 0.7v) is induced into the secondary of the current transformer, a pulsed current will flow through the Base to Emitter junction of the transistor and it turns on. This shorts out the capacitor, which causes current to flow thru R1 and the 1M resistor. This pulls TRIGGER close to 5V, which causes !DETECTED to be pulled to ground and the Occupied LED to light. The sensitivity of the detector can be changed by increasing or decreasing the number of turns on the primary side of the transformer. The default suggestion of "2 turns" gives a sensitivity of less than 1mA. Keep in mind:
NOTE: Block occupancy detectors for DCC systems do not need to sense current flow in both directions due to the high rate of direction change (thousands of times per second). The time delay [given by R(1Meg) * C(2.2uF) = 2.2seconds] is more than enough to compensate for the few miliseconds when the track current is flowing in a reversed direction. [edit] LocoIO Loconet Interface
John Jabour's LocoIO provided 16 digital I/O conections that could be accessed via Digitrax's LocoNet. Selected Loconet packets could control outputs; alternatively, inputs would generate Loconet messages. John made kits available for a while; after he went on to other pursuits, Hans DeLoof took up the project, revised the circuit board, and produced updated versions of the firmware. Both these boards required cables of some sort (ribbon or telco) to connect them to sensors and devices. Since the Detector project was designed to utilize all 16 I/O lines on a LocoIO, it was possible to reduce the cost by eliminating 8 telco connectors, jacks and cable assemblies and including a "prewired" LocoIO core on the same circuit board.
This is a simple PIC design - it uses a LM311 Comparator to read the Loconet bitstream and a NPN transistor to write to it. There are a couple of pins that are used to monitor configuration jumpers, one to drive a "Loconet Activity" LED; the rest are directly tied to the "Occupied" outputs from the detection circuitry. See the LocoIO documentation for more information. |
