CAN Bus And System Control

I like CAN (Controller Area Network) Bus for control systems. It has moderate speed (1 Mbs), it is deterministic., it has priority control for multi-master operation, and because of its use in autos it is robust and low cost. In addition it has been on the market for about 10 years so it is well established with lots of vendors to choose from. In addition there are a number of MCUs with built in CAN.

For test reactor operation I see a hierarchy of CAN buses, each devoted to a given function and then all melded into a master bus. Let me start with the sub nets.

1. Vacuum Gauge/Vacuum Pump Control Bus
2. HV Power Supply Internal Buses
3. Instrumentation Buses (Temp Monitoring, Flow Monitoring, Master Clock, Neutron Counter, Electrical Power Measurement and Control, etc.)
4. Auxiliary Control (Electrical Power Measurement and Control, etc.)

And then a master bus to send messages to/from the sub buses.

I haven't decided on an MCU yet. One of the requirements would be that it is FLASH programmable and have a built in CAN bus. I have been thinking about pressure measurement lately so I'd like to lay out what a CAN bus interface would look like.

The pressure transducers I have in mind (MKS 722 and MKS 626A) have a 0 to 10 V output. I haven't picked any other components yet. So we will just look at functions.

1. Pressure (dependent on transducer - from 1000 torr to .1 torr full scale, 17 bits)
2. Board temperature (0 to 100 deg C, nominal 30 deg C 10 bits nominal - 8 actual)
3. Board power bus voltage (20 to 60V, nominal 48VDC, 10 bits nominal - 8 actual)
4. Board power bus current (0 to 255 mA, nominal TBD, 10 bits nominal - 8 actual)
5. Various internal power supplies (TBD, TBD, 10 bits nominal - 8 actual)
6. Isolated CAN supply, Isolated Pressure transducer supply, Isolated MCU supplies.

Lets start with the transducer front end. It should have an op-amp (fully differential).

For the 24 bit converter I like the AD7767 it has an accuracy of 17 bits which matches the one part in 1e5 resolution of the pressure transducer.

The ADA4941-1 is a nice companion amplifier. And the ADR425 looks like a nice reference. Good specs, not too expensive.

That covers the main parts of the analog side. What about the computing side? I have been looking around and I think I like the Fujitsu CAN bus microprocessors the best. Its architecture is a mixture of a FORTH engine and a Z8 register bank.


Fujitsu 16 bit microcontrollers

Fujitsu 32 bit microcontrollers

Fujitsu microcontrollers

I haven't addressed the CAN interface yet. It is pretty simple: a few high speed optocouplers, a bus driver chip, a separate isolated power supply, some protection components and away we go.

Let me add that Fujitsu has 8 bitters that I have yet to take a serious look at. I'll probably remedy that in the next few days.

The Atmel CAN processors would be a good second choice. The reason I like the Fujitsu stuff better is that it has a very good migration path and I'd like to use one software model for as many of the process tasks as possible.


I also like the Infineon TC1166 32 bit processor with CAN. Digikey sells them for about $39 ea. Quantity one. Since programming and hardware design is going to be the big cost for the initial units I'm going to do what Chuck Moore suggests. Get the biggest fastest processor you can afford to start. Then reduce the foot print if volumes warrant.

The Fujitsu part is more open ended and has a very simple CALL structure. However, there are no automatic register saves with calls. Thus every CALL must have at least one PUSH and one POP if you expect nested calls. The Infineon has a more definite programming model with user space registers and system space registers. They each save registers on a CALL (but different sets). The cost is 2 to 5 clocks. Not a big hit (but they could have done better). The Infineon part also seems like it might be harder to learn due to instruction pipeline flushing requirements in some situations that require an instruction to finish before the following instruction(s) are executed. I do like the floating point and some other features of the Infineon, but I will set it aside for now. Well I read on and find that the Fujitsu Part also has a 5 deep pipeline. I guess when you need to flush it you just do a bunch of no-ops.

The Fujitsu also is more deterministic and can do tail end recursion since the pipeline is only one level deep (actually it is five deep also but it can do tail end recursion.). However the divide routines are not as mechanized as they are in the Infineon. Since I hardly ever use them except with user input to precalculate multiply constants that is not such a big hit.

Update 09 Jun 1534z

I have been looking at the Freescale MPC551x MPUs. I like them. Plus Freescale gives away an assembler good enough to get a FORTH up on the chip.

I like the Atmel ATA6660 CAN bus physical interface. There are later and greater chips out with more functionality , however I like to keep the interface simple and electrically isolated. You can do that with three high speed optocouplers and an isolation supply. A 48 V to 5 V job at about 1 W would be good.

Fusor Vacuum Pump Choice

I have been neglecting general education on vacuum pumps. So here is Technical Notes on Various Vacuum Pump Types. And of course the wiki on Vacuum Pumps. All for the purpose:

I'm trying to decide on a vacuum pump set up and have come up with two candidates based on their compression ratios for H2:

Adixen ATH31+. 1E11 N2, 1E5 H2 Compression Ratio
Aprox Prices:about $4,700 pump, about $1,700 controller, about $600 required accessories. 4 1/2" CF High Vacuum connection.

Pfeiffer TMU 071YP >1E11 N2, 1E5 H2 Compression Ratio
Aprox Pricesabout $5,200 pump, about $1,300 controller. 4 1/2" CF High Vacuum connection.

	l/s	CR	TU	Wt	IF	OF	FP	FPS	$Pmp	$Cnt
ATH 31+	14	1e5	5e-10	2.7	4.5" CF	KF16	45	1	$4,700	$1700
TMH071P	42	1e5	5e-10	8.4	4.5" CF	KF16	18	2.5	$5,208	$1,298

Abreviations:
l/s = Pump Speed in liters per second For H2
CR = Compression Ratio H2
TU = Ultimate pressure in mbar
Wt = weight lbs
IF = Input (High Vacuum) Flange
OF = Output (Fore Pump) Flange
FP = Max Foreline Pressure mbar
FPS = Min Foreline Pump Speed m^3/hr
$Pmp = Cost of the Pump
$Cnt = Cost of the Controller

The Pfeiffer looks like a better pump for the money. A lot will depend on the fore pump. I'm leaning to a roots blower or some other oil less type pump. So let us look at some. First some education: Scroll vs Rotary Lobe Pumps.

IDP-3, technical [pdf], dry scroll, 3 m³/hr rate, 2.5e-1 torr. ultimate, $2,650

ACP15, technical [pdf], rotary lobe, 14 m³/hr rate, 3.8e-2 torr. ultimate, $5,134

1 torr = 1.33 mbar

Vacuum Flanges

DN	NW	Tube ID. mm	CF Flange O.D. mm	CF Flange O.D. (inches)
DN16	16	16	34	1 1/3"
DN25	25	22	-54	(2 1/8")
DN40	35	35	70	2 3/4"
DN50	50	47	-86	(3 3/8")
DN63	63	57	114	4 1/2"
DN100	100	98	150	6"
DN150	150	146	203	8"
DN200	200	197	254	10"

Taken from 5Pascal [pdf]

Fusor Vacuum Pumps

I'm going to start with turbo pumps and then add roughing pumps later. I'm going to add some compression ratio numbers which will determine final pressure. Plus prices for pumps and controllers if I can find them.

Adixen ATH31+. 1E11 N2, 1E5 H2 Compression Ratio
Aprox Prices:about $4,700 pump, about $1,700 controller, about $600 required accessories. 4 1/2" CF High Vacuum connection.

Varian Turbo V81M. 5E8 N2, 7E3 H2 Compression Ratio
Aprox Prices $5,900 Pump + Controller. CF 63 High Vacuum connection

Pfeiffer TMU 071YP >1E11 N2, 1E5 H2 Compression Ratio
Aprox Pricesabout $4,700 pump, about $1,200 controller,

Oerlikon Leybold TURBOVAC 50 2E6 N2 Compression Ratio
Aprox Prices:about $3,300 pump, about $1,500 controller. CF 63 High Vacuum connection

Small Vacuum Vessel Suppliers

Here is a handy list of vacuum vessel suppliers suitable for Fusor Construction. I will be adding to the list from time to time.

Meyer Tool and Manufacturing Oak Lawn, Illinois

Kimball Physics Wilton, New Hampshire

Kurt J. Lesker Company Clairton, PA

MDC Vacuum Products, LLC Hayward, California

Nor-Cal Products Yreka, CA

Trinos Vacuum Systems, Inc. Chicago, Illinois

A&N Corporation Williston, FL

Atlas Technologies Port Townsend WA

Sci Quip - Used Eqpt.

Oerlikon Leybold Flanges and Fittings

Vacuum Pumping

I'd like to go a little deeper into the subject of vacuum pumping and easily attainable ultimate pressures.

Since this is a paper exercise we can pick any pumps we want. I'm going to look at 3 pump mfgrs. and pick their highest capacity turbo molecular pumps.

Adixen - Mag Lev [pdf]
Pfeiffer - Mag Lev
Varian [pdf]

There are two critical specifications for us. H2 pumping speed in liters/second and H2 compression ratio.

Turbo Molecular Pump Comparison

	H2 Speed l/s	H2 Compression Ratio
Adixen ATH-2300M	1200	3.0X103
Pfeiffer HiMag 3400	2850	4X104
VarianTurbo V 3K-T	2300	1.5X104

It looks like the Pfeiffer HiMag 3400 all the way.

Next lets look at a Roots fore pumps. I really like the Adixen RSV [pdf] series. The link has a great look at pumping capacities vs vacuum pressure and shows the limitations of roots blowers. Let us look at the RSV-1002. The largest standard pump. It has a pumping speed of 800 M³/hr with the model 2100 SD [pdf] roughing pump. That is about 225 l/s at a pressure of 3E-1 mbar (which for out Rough Order of Magnitude (ROM) purposes can be considered equal to 3E-1 torr). At 1E-2 mbar (where you can turn on your turbo pumps) it is about 100 l/sec. At 1E-3 mbar it is about 40 l/s. Ultimate pressure is 2E-4 mbar.

The Roots fore pumps basically stops pumping at 2E-4 torr. Given a turbo pump compression ratio in the neighborhood of 1E4 that gets us down to 2E-8 torr. Not enough. In addition the closer you get to the Roots ultimate pressure the lower your capacity. To get us lower we are going to need another turbo pump in series with the chamber pumps. Tom Ligon in an e-mail suggested using one pump to service all the chamber pumps. Brilliant idea. This should work fine as the volume of gas to be moved will be 1/10,000th of the amount (in liters) of the gas being pumped out of the chamber. Even with 10 or 20 chamber pumps you would still have a lot of excess capacity if you used a turbo pump with 1/100th the capacity of the total of all your chamber pumps. A pump of 600 l/s should be more than adequate. We leave the choice of that pump as an exercise for the reader.

So let us look at the chain of pumps. Say 6 or 10 reactor chamber turbo molecular pumps. A smaller turbo pump drawing from those pumps. A Roots blower next followed by a roughing pump. This all has to be properly sequenced to avoid damage to the pumps and systems. Then you have to sequence the various pressure reading devices depending on pressure. There should be enough work to keep the vacuum guy busy for at least a couple of weeks. Especially if s/he is budget constrained.

Update 21 Nov 007 0642z

I messed up pump volume calculations so let us go over them.

20 turbopumps X 3,000 l/s = 60,000 l/s. 1/100th of that is 600 l/sec.

Corrected in the text.

Turbo Pump Ratings

An education is a wonderful thing. I'm getting an education. I hope to distill it into a couple of useful paragraphs.

Turbo molecular pumps (TMPs) are rated for gas flow at a certain chamber pressure. A high volume pump I am looking at is rated at 2,000 liters a second. How many ccs a second is that at STP (not Space-Time Productions - Standard Pressure and Temperature, which is 760 torr and 273.15°K) at a chamber pressure of 1E-7 torr?

2,000 l/s * 1,000 cc/l * 1E-7 torr * 1 atm/760 torr = 2.63E-4 cc/sec @ STP

Turbo pumps have a compression ratio rating. Let us look at what that means in terms of ultimate chamber pressure in terms of the worst case gas - molecular Hydrogen. The particular pump I am looking at is rated at an ultimate pressure (on the low side) of 1E-9 torr. If the pump has a compression ratio of 10,000 for hydrogen that means the outlet of the TMP must be held at a pressure of 1E-5 torr to reach a final pressure of 1E-9 torr.

We leave the design of an actual system as an exercise for the reader. Dig out them catalogs and get the slide rules slipin.