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Unread 10-05-2012, 01:35
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Re: TI and future Jaguars

Quote:
Originally Posted by PAR_WIG1350 View Post
Here is a concept of a modular controller that was inspired by the system used for arduino 'shields'. The stack is fed power from the bottom up, with high power circuitry at the bottom on one module, this eliminates the need for high current connectors between modules. The fan is mounted just above the high current module to cool the MOSFETs. The low-current section of the stack is mounted above the high current module and fan with a gap for air flow. The signal (from the cRIO) is fed from the top down, this separation of power and signal inputs helps reduce noise. the processor is on the bottom. In some configurations, this could be a simple PWM H-bridge driver, in a CAN application, this would actually be a micro controller/processor. It is designed to accommodate 2 input modules that are each half the width of the other modules (so they occupy the same layer in the stack). One of these would be the pwm header or CAN interface. The other is for encoders, limit switches, potentiometers, hall-effect sensors, etc. On top is the auxiliary output. This is for status indicator lights and error reporting.
Even with PWM you'll need at the minimum a microcontroller. One can easily perform the functions required with a simple Atmel AVR or Microchip PIC 8bit microcontroller. You can even do it with a BASIC Stamp 2 (which is merely a Microchip PIC with a built in pBASIC interpreter) it's not really a big job.

For this application it'll probably be more expensive to attempt to tweak an input PWM signal to drive the H-Bridge directly than just putting the microcontroller on the board. This is not to say that you couldn't drive an H-bridge with PWM. It is to say that to get linearity, current monitoring, thermal monitoring, and perhaps some other features it just makes sense to use the microcontroller and a cheap crystal. Even just a few cheap operational amplifiers otherwise would approach the same cost (plus be larger and possibly more easily effected by heating).

It's not clear to me how you would be able to benefit from inputs on the PWM version. I suppose you could use them to provide basic limit switches. However, generally PWM is an output from the existing control system and an input into the electronic motor control. You'd need to come up with some way to configure the electronic motor control to get more elaborate when using PWM. With CAN I can see having optional input and output modules would add more flexibility.

The idea of stacking things on top of the fan has a slight issue with it. If the footprint of the unit is nearly that of the fan you need some way to get communications to the power module under the fan. That's why originally I had envisioned building the other way and letting the connectors extend out from the sides of the stack. After all if you look down at the fan on the Victor the footprint extends out where the connectors go. Essentially the issue of your diagram is that you've created several places where we loose PCB space to keep the footprint entirely under the fan. I do agree, however, that the headers that interconnect the boards will likely end up on the edges. Perhaps the larger issue is that the air gap between the fan and the processor actually would work against you. With the controls below the power module it's easy to put a laminated foil shield between the bottom of the power module and the top of the processor. With the fan at that spot in the assembly such a shield might become a liability as it could restrict the air flow that's already being blocked by the processor anyway.

I will say that this footprint is smaller then what I originally described. I'm just worried that using the space like that might make it hard to put indicators on the unit and with connectors for I/O in the middle you risk situatiuons with the PCB layout where the through-hole parts eat away at your space to route traces. I'm additionally not really sure that we need too many input and output modules because most of the I/O is going to be directly reflected on the processor card with perhaps the notable execption being the encoder if you install a CPLD or purpose designed IC to track that.

Course we could take the whole thing a step further and simply use an FPGA and put a small simple processor in the FPGA, along with the encoder tracking functions and skip dedicated processors or microcontrollers all together. The major advantage with that would be we could embed new features that run at raw digital speeds.

Last edited by techhelpbb : 10-05-2012 at 01:40.
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