Injector Control Options

[TonyS]

Is this close? -

Hoon.sch (Schematic Design).pdf

(7.83 KiB) Downloaded 882 times

Any advice on how to get the contents of a pdf to display in the message box?

- Huff

[jharvey]

Nice, I like hoon uC. I might like to add some stuff. Is the source file something that can be shared? If so what did you draw it in?

Using Rsense has pro's and con's. One concern is that Rsense creates a tank circuit that can cause the voltage on that side of the drive silicon to jump up drastically. I think it's better to measure that current without inducing a resistance. I see Alegro has a chip for under $2.50 USD that can do just that.

http://www.allegromicro.com/en/Products ... /index.asp

It appears digikey has several of the 5 amp and 20 amp devices in stock, for around $1.75 usd.

http://octopart.com/partsearch/#search/ ... a&q=ACS714

The D1 diode isn't required if using OV drive silicon. However it might be handy for those that are familiar with that snubbing technique. Hmmm I see that 3rd line from D1 to the uC, are you thinking of a pin diode? Those are typically used as RF switches.

[jharvey]

Here's a reference design for measuring inductance with a PSoC

http://www.circuitcellar.com/psoc2002/winners/h3.pdf

[Fred]

Is this to be an "add-on" board (remote?) or is to be a "populate parts" option on the main board?

I don't think it matters much, does it?

The reason I asked is because I initially assumed that an external box application was a given, but when I read "Configuration of the micro should be done over SPI or I2C by the primary micro (FreeEMS)" I wasn't sure, as these protocols are more typically used for intra-board communications, while board - board communications tend to use more environmentally "robust" protocols (485, CAN,...).

Solid point! I contradicted myself...

To clarify this:

For standalone use, code could be loaded with desired parameters preprogrammed.
For integrated use, the master cpu could modify these on the fly, and/or get feedback from the device, but this would be optional and the device would function without this interaction.

As for the schematic, perfect. The 8 IO requirement is really a 8 digital inputs requirement, but in practice these are one and the same.

Is there any reason to go to 8 straight away? The circuit could be duplicated from 4 to 8 or 12. Probably 8 is more common anyway, though. If it made sense it could be one micro per channel, but that is unlikely.

The PWM could be software too (8 digital outputs), it would be easy to do with this little programmatic load.

Fred.

[TonyS]

Nice, I like hoon uC. I might like to add some stuff. Is the source file something that can be shared? If so what did you draw it in?

I use a package called Easy-PC (terrible name - but works well in my opinion). I'm not sure how we can share at this point, but I will try to update the drawing as suggestions, requests, specs,... come in.

The Allegro ACS714 is quite an interesting part (good find!). Because of the bi-directional current sensing capability (0 to +/-5A), it has a 185mV/A response centered at Vcc/2. I haven't dug deep into the data sheet to look for any "gotchas" (I did notice that it has a +/-40mV, 0A spec). With the 185mV/A response spec, would a 10bit A/D be required on the uC to get the resolution that is needed?

The D1 diode isn't required if using OV drive silicon. However it might be handy for those that are familiar with that snubbing technique. Hmmm I see that 3rd line from D1 to the uC, are you thinking of a pin diode? Those are typically used as RF switches.

I really think that I will have to come up with a circuit simulation to explain/show why two levels of snubbing are needed. The "3rd line" is to show that this snubber function needs to be controlled so that it is "in-circuit" during pwm and "out-of-circuit" at injector shutoff.

Is there any reason to go to 8 straight away? The circuit could be duplicated from 4 to 8 or 12. Probably 8 is more common anyway, though. If it made sense it could be one micro per channel, but that is unlikely.

The PWM could be software too (8 digital outputs), it would be easy to do with this little programmatic load.

I don't really have an opinion as to the number of driver channels that should be accomodated. Design for 8 and populate the board for 4, 6, or 8 as needed? If you think that the PWM requirement for the uC is not needed, well we will just take it out and add another batch of I/O.
-Huff

[Fred]

You don't have to explain the bi-level snubbing to me.

PWM would be nice, but not 100% required, probably. It'd make it trivial to implement if you did have it, though. Leave it in, I guess, but keep that in mind.

Fred.

[jharvey]

The Allegro ACS714 is quite an interesting part (good find!). Because of the bi-directional current sensing capability (0 to +/-5A), it has a 185mV/A response centered at Vcc/2. I haven't dug deep into the data sheet to look for any "gotchas" (I did notice that it has a +/-40mV, 0A spec).

I wonder what the sources of tolerances are. We may find the actual tolerance is tighter than we expect, or looser. For example, we'll probably have a fairly consistent temperature, so that source of tolerance is minimized. Either way, I suspect it's tight enough. I don't think it's all that important to measure the peak, and we can probably handle a fairly large tolerance in the 1 amp range. The big heat savings is when we go from 4 to 1 amp. If it's 1.01, it's not really the end of the world. So 8 bit is probably good enough, however if we have 10 bit, might as well get the better accuracy.

I'd like to chat about the "bi-level snubbing" I can't seem to find that term in google. What advantages does it have from a normal snubber diode or just an OV drive?

[jharvey]

Interesting breakout board for that hall sensor http://www.pololu.com/catalog/product/1185

[Fred]

I'd like to chat about the "bi-level snubbing" I can't seem to find that term in google. What advantages does it have from a normal snubber diode or just an OV drive?

He actually explained it in an earlier post, have a read back through them, perhaps?

[TonyS]

Did a little update of the block diagram -

Hoon.sch_001.pdf

(8.17 KiB) Downloaded 796 times

More feedback is needed to make progress on this design (7 downloads of the original block diagram, but only two people commenting).

So 8 bit is probably good enough, however if we have 10 bit, might as well get the better accuracy.

With an 8-bit A/D, you will have resolution of about 1 count for every 100mA of current (based on using the ACS714). Is this enough?

Any better alternatives (circuit topology wise) to the "Active Clamp" that is used on the Mega?

flyback-damp.jpg

(24.9 KiB) Not downloaded yet

- Do the injector specifications in the block diagram represent a reasonable / targeted range?
- Any alternatives that should be considered instead of the Allegro ACS714 for current sensing?
- Besides ON Semi, IR, and ST, are there any other mainline manufacturers of "protected" power MOSFETs (I have found it difficult to search Digikey and Mouser for "protected" parts as the manufacturers have different trade names for this type of part)?
- Is there any exertise among the forum members in selecting the "proper" MOSFET based on the injector load, voltage clamping, and PWM parameters or do we just go with a "brute force" solution and select a big MOSFET (any part number suggestions?)?

-Huff