High Density DCC Block Detection

If you have been following along, you already know that the L&NC module 1, lower level is a complex beast. Here’s the state of the accumulated electronics as of today. I’ve talked about quite a bit of it, but there are still key areas to discuss.

Block detection on this first module turned out to be somewhat challenging because of the large number of blocks—24— in a relatively small space.

24 Blocks?

Obviously, the mainline, secondary/yard lead track and a siding are only part of the story. Two big factors in the block count are Red Bluffs Yard (5) and the Roundhouse turntable, tracks and lead in (9). Block arrangement is sometimes dictated by the Peco power routing turnouts I use and the need for insulating gaps; block 11 is ultra small (I’m rethinking the block boundaries here — adding/consolidating feeders and re-gapping is no big deal) because it is at the edge of the module and includes a turnout. The feeder map below will give you some idea of the block layout.

Module 1 LL Feeder Map. Block 1 includes all three turnouts that follow feeder1

Current sensing takes time.

Regardless of the type of sensor you use, the single most important factor is the time it takes for an ADC (analog-to-digital converter) to read the sensor. On an Arduino board with a 10-bit ADC (most boards), it takes 50 μs; if you need to do 100 reads to get an RMS reading, the total read time will be more than 5 ms; add waiting between readings plus calculation time and you need 80 – 100 ms to read a single current sensor. If you are doing only a few, that should present no problems.

But with 24 blocks you are looking at 2+ seconds to do a complete read cycle at that rate. That is an eternity if you are trying to do more than that with the microcontroller. Frankly, its even an eternity just in the context of BOD, because that 2+ seconds for a read cycle limits how fast the system can respond to changes in block occupancy.

Mayhew Labs Extended ADC Shield

I knew I would need a faster and more capable ADC, so I’ve been working with the Mayhew Labs 14 bit Extended ADC Shield. The Mayhew Labs Extended ADC Shield, which is sometimes available at Amazon, comes in 12, 14 and 16 bit flavors. Bits, in the case of an ADC, determine the smallest detectable voltage out of a given range. An ADC with 14 bits of resolution can divide a 0 – 5 volt range into 16,384 steps (2 ^ 14), making the lowest detectable current (and the detection step interval) 305 μV. In contrast, the 10 bit ADC built into an UNO is capable of only 1024 ( 2 ^ 10) steps, making the lowest detectable current (and step interval) 4.88 mV.  That is a huge difference in detection sensitivity.

Mayhew Labs Extended ADC Shield

In addition to sensitivity, the shield is fast, producing a reading in 5 µs. That is 10 times faster than the conversion rate of a built-in Arduino ADC; in fact, the conversion is completed by the time the UNO is ready to execute the next instruction after triggering the read.

The shield and the supporting Arduino Library are optimized to scan the ports sequentially. The system is at its most efficient when you read all the ports in sequence in a single pass. That requires some adaptation of my current sensing methods, although the detection algorithm itself remains unchanged.

The Mayhew Labs Extended ADC shield has eight ports that can be used in one of two fundamental ways: single ended or differential. Single ended — the same mode used by the Arduino ADC — reads each port individually; a sensor is attached to the port and to ground. Differential mode reads and compares two adjacent ports. With CT sensors, each port of the differential pair connects to one side of the sensor creating a complete circuit. CT sensors produce an AC current at the same frequency as DCC; in single ended mode the ADC only sees the positive phase of the cycle; in DIFFERENTIAL BIPOLAR mode the ADC see both phases of the cycle. Accordingly, reading CT sensors in differential bipolar mode will produce the most accurate results and require the fewest reads for calculating an RMS. However, because the DCC cycle is high frequency (8 Khz), its possible to get reasonably accurate results in single-ended mode because the ADC will see multiple positive DCC phases during each read.

It didn’t take many experiments to convince me to use the Mayhew Labs ADC in differential mode.  That means the shield is limited to 4 sensor connections. Made little difference inasmuch as I already had a problem with 24 blocks to watch from a limited number of ADC ports. I’ll concede that at $35, the Mayhew Labs shield is an expensive board; but given how cheap the CT sensors are, the overall cost of the system for 24 blocks is under $100.  I don’t think it is possible to do the job at this scale with an off-the-shelf BOD system for anything like $100. I anticipate using using the shield in one or two more areas where there are 8 or more blocks to watch.

Muxing/Demuxing to BOD Bliss

I’ve frequently talked about techniques for multiplying digital pins, using shift registers and other devices.  It is also possible to multiply analog connections to an Arduino or an external ADC using a Multiplexer/Demultiplexer IC. A mux/demux is basically a device with a common I/O port and 8 selectable I/O ports, plus 3 address lines to select which port is connected to the common I/O. Think of it as a rotary switch with 8 positions, selectable via the address line. The important part is that each connection is isolated from the others so that only one selectable port can be active and cannot by affected by the other ports.

I use the Texas Instruments CD4051B CMOS Single 8-Channel Analog Multiplexer/Demultiplexer [ Digi-Key ]. This functional diagram of the CD4051B show how it works and includes the address bits required to select an I/O channel. [Note: other CD405XB variants have different port arrangements that are useful in other situations.]

TI CD4051B Functional Diagram

To use these, you have to supply power, ground, three digital address lines and an I/O line back to your microprocessor—two power connections and four logic connections. So you are getting a 8 to 1 improvement in analog capacity, at the cost of three digital address lines, which is OK; with connection sharing (sounds odd, but you’ll see it works), my three address lines can control multiple banks of 8 sensors, further improving pin efficiency.

Round Robin Reads

Since I have 24 blocks, and each CD4051B IC supports 8 channels, three groups of 8 sensors seemed like the optimal approach. The idea is to do an optimized read of all three boards on each pass, using the address lines to select which sensors are read. I think of this as a “round robin” technique. I’ll get into the details of the reading technique in second part of this two part post.

I set out to create sensor boards and the very first question that arose was this: how wise was it to have one side of each CT connected to a shared I/O line, with only the other side of the CT’s isolated through the mux/demux? I’m not an electrical engineer, but I had a feeling that connecting all CT’s together on one side might not be a good idea.

I decided to follow my instincts and build two boards using two 4051B’s to fully isolate each sensor. Here are pictures of that version (annotated as much as possible for those who are interested in building their own):

Top of CT Sensor Board

CT Sensor Board, Top View with ICs

Bottom side of CT Sensor Board.

A couple of notes about this board:

  1. I start with a Busboard Prototype Systems B1 Solderable Breadboard. The board has 6 rails that, with good planning, can cut the number of jumpers you have to use. Use the outer rails for power (+5v on one side, Ground on other); the other two rails on each side can be used to connect sensors to the mux/demux ICs. I didn’t get it quite right on any of them and left some unused rails as a result.
  2. Use high accuracy resistors at the CT’s so that they are essentially identical. I use 100Ω resistors with .1% tolerance [Digi-Key].
  3. I get CT sensors in quantity from Digi-Key. I use universal jumper sets that I get on Amazon.
  4. I have a fetish for power indication LED’s so I know a board is live. I’m starting to get wise about the brightness of today’s LEDs, though, and am using 1K resistors to cut the brightness back (skip the calculations and use a bigger resistor!).
  5. Once I decided on a connection protocol (see the pictures), building the board is a matter of patiently matching connections per the IC pin-outs — like many IC’s this one will make you wish the designers had shown some mercy in arranging the pin-outs! But basically its this simple: connect channel 0 of mux1 to one side of sensor 0, then channel 0 of mux 2 to the other side of sensor 0. Do that for 7 more CT sensors and you’ll have an eight sensor detection board that is as effective as anything out there. Be consistent about which side of the sensors are connected to which mux.

    CD4051B Pin Outs NOTE: INH, VEE and VSS will be grounded; supply +5 volts at VDD. A, B and C are the three address lines. COM OUT/IN is the common I/O channel that connects back to the ADC.

  6. I use 22 gauge wire for feeders. I’ve learned to cut feeder leads for each sensor when I build the board — about 12″ long. The lead needs to wrap 3 times around the sensor, with a tail on each side for splicing to the feeders and connecting to the power distribution board. This makes the installation process a whole lot easier. The “wrap three times” thing is almost a catechism; 2 times will work but the signal is significantly weaker. This is an induction system, so the more times wrapped the better.
  7. I secure the wrapped feeder on the CT coil with a dab of hot glue.
  8. I’m sure you color code your feeders. I recommend that you always read the same rail on all CT sensors.

That 8 sensor board installed on the layout.

How NOT to do it.

But, since part of the point of this exercise is to learn and apply knowledge, I also made a single mux/demux version with one side of the CT’s on a shared line. That would simplify wiring and omit an IC; a good thing, right? Reduces your IC count and the number of connections you have to solder. Initial tests on the bench seemed to show that it worked just fine, thus making me further question my sanity.

But then I installed the whole system, and wouldn’t you know that under load my pessimism was rewarded.  The odd board produced odd readings, clearly off the norms of the other two boards. Lots of cross-talk was evident; occupancy of one block would cause other blocks to show false readings. I’m sure a real electrical engineer would find that pretty funny and predictable.

Here is a picture of the errant board.  I put this here as a warning NOT to do this or you will be profoundly disappointed.

Bad board with all sensors connected to a common line on one side. Looks attractively simple. Don’t do it!

The Interface board.

To consolidate the three boards and their connections, I had to create an interface board.

Interface board linking detector boards to power and microcontroller.

The trick here is that the address lines are shared; address lines set the same address on all three boards at the same time.  So, when a “round robin” read cycle occurs, the same channel is read on each board on each pass: the first pass is channel 0, next channel 1 and so on.

To Be Continued

I had to modify my current sensing algorithm to accommodate the “round robin reads.” That was easy enough. But when I got around to integrating all the functionality on an UNO, I ran into the proverbial brick wall.

More in the companion post that follows.

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