Last week I discussed my latest project, an accelerometer based dual axis solar tracker (that blog is here). When starting a project its easy to get lost in the details. For example, this project has a whole host of possible control functions and interfaces. I’ve always found it useful to start my work on the schematic and hardware. So that’s what I did.
Since this is an R&D project I decided to use some components I haven’t used before. This design will control a 24VDC brushed motor, and a 12VDC brushed motor. I’m going to use two St Microelectronics PN: VNH3SP30-E. They’re rated for 40V and 30A (although there’s no way they can handle 30A without melting off the PCB). One feature I like about these controllers is that they only need a single PWM channel for proportional control. Two other digital channels are used to provide direction. Here’s a section of the schematic that shows the connections between a PIC16F1789 and two of the motor controllers (click the image for a better view).
In the end, this control system will be used to control our dual axis controller that uses a worm gear for horizontal movement, and a linear actuator for vertical movement. Here’s a photo of the assembly in front of the old college text books we could never bring ourselves to throw away. Both the worm gear and linear actual came with feedback. The worm gear was very expensive and has an encoder on the motor. the linear actuator was pretty cheap and came with a potentiometer that failed in about a month. That was one catalyst for attempting that has the position feedback on the control board via an accelerometer.
The circuit board ended up being about 2.5” x 3.5”. Here’s what the top copper layer looks like. The 6 dots on the upper and lower right-hand side of the board are for mounting automotive style blade fuses.
Here is the column from my bill-of-materials.
There are a couple of interesting findings. First, I have an 8MHz oscillator on the design that has the smallest range of operation (-20C to 70C). I’m actually going to use the internal oscillator available in the microcontroller, and I don’t see clock timing to be an incredibly important issue in this design. I added the oscillator part to the schematic as kind of a back-up plan but don’t plan on using it. So I’ll ignore that problem for now.
The next lowest temp. range part is the PDV-P7002, a photocell I will be using to detect daylight. For a part like this, whose resistance changes with light, I would guess that it goes “out-of-specification” outside of its operating range. I doubt it quits working. I’ll have to research that some more, but since I’m using the photocell as a yes/no type input I can accommodate a wide resistance variance.
That leaves me with some ceramic caps that don’t meet my operating temperature range, and I can certainly select a similar part with an extended temperature range, so I’m probably good there.
I guess there was one other issue. The small metal buttons I have on this design (E-Switch parts) had no temperature rating. I thought that was interesting. These are not the kind of buttons you would use in an outdoor design, but no temperature rating?
And now for a sanity check. Am I really going to run this design between –40C and 85C. Nope. This is R&D, it’ll spend its life in my office. If this were a consulting contract we would design for this temperature but suggest our clients place test fixtures in the intended environment to collect operating data and/or make use of a temperature chamber for extended temperature testing.
That’s as far as I was able to get this week. I’ll try to take some time over Christmas break to order the circuit board. I need to panelize it with some other designs so it doesn’t cost an arm and a leg. Hopefully I can begin writing code in January.