I am busy with a project which will track the sun on a single axis (I am building a segment of a prototype Linear Fresnel Collector).
I will use an algorithm (SPA algorithm) to determine the position of the sun and then will passively control the position of the mirror surface.
However, I want to add a sensor in the form of a photodiode to provide an error signal as a means of feedback to compare a more active control strategy to the passive one.
I am looking at using two BPX 61 photodiode’s (data sheet: http://docs-europe.electrocomponents.com/webdocs/08b4/0900766b808b4432.pdf). Although this is just a starting point and can be varied (cost is not a huge factor, although cheaper is better).
I want to connect them as shown in the attachment.
I don’t necessarily want to know absolute readings, but I would like to know one photodiodes output relative to the other (then I would know how much to move in which direction).
My thoughts so far are that the outputs of the photodiodes would be relatively noisy and would be difficult to compare as is. I am thinking that I need to first filter each individual signal (band pass, combination of high and low pass rather?) then amplify them and finally compare them (subtracting one from the other and if a certain threshold is reached then movement is necessary).
I am aware that a virtual ground will probably be needed.
I am looking at using LM351 op amps (data sheet: http://www.ti.com/lit/ds/symlink/lf351-n.pdf), or otherwise I have LF353 and LF347 at my disposal - although this can change.
What I am unsure about is what frequency I should design the filter for? I don’t need ridiculously fast measurements (the sun only moves at 15 degrees an hour, which is 6.9e-4 rpm), but i would like to sample the photodiodes at 10 Hz potentially (this is pretty flexible).
I realise I have given quite a lot of info, but the crux of my question is: do I need a filter and for what frequencies?
I have a moderate amount of theoretical knowledge but hardly any practical knowledge so any practical advice would be really awesome!
Thank you in advance for the help!