{"id":441,"date":"2011-11-02T10:55:55","date_gmt":"2011-11-02T09:55:55","guid":{"rendered":"http:\/\/www.nobugs.org\/blog\/?p=441"},"modified":"2011-11-02T10:55:55","modified_gmt":"2011-11-02T09:55:55","slug":"high-precision-wheels","status":"publish","type":"post","link":"https:\/\/www.nobugs.org\/blog\/archives\/2011\/11\/02\/high-precision-wheels\/","title":{"rendered":"High precision wheels"},"content":{"rendered":"

This evening I was truing one of my bicycle wheels. I started out with a truing stand – literally, a metal bolt identifies high spots on the rim by rubbing against them. I managed to get the wheel pretty true. But then I remember that I also have a DTI<\/a>, accurate to 0.01mm. It’s like a red rag to a perfectionist. After a bit of futzing with that, I could get the wheel true within ~0.1mm and so I stopped there. The first time I hit a bump it’ll blow that precision away! But it got me thinking … how you could automate this (rather laborious) wheel truing task.<\/p>\n

After a bit of thinking, I realised you can utilise the fact that the wheel is made of conductive aluminium, and therefore can act as one plate of a capacitor. Sure enough, wikipedia agrees<\/a>. Unfortunately, the sensors in question<\/a> are quite pricey – $120 – so I doubt I’ll ever get one.<\/p>\n

Could I make a homebrew version? I think it’d be hard. Given how small a wheel rim is, you’d probably only manage a plate 10x10mm. If you got the wheel true within 1mm, your plate distance would be maximum 1mm. So that gives a capacitance of 0.88pF. That sounds pretty small to my ears. Charged up to 5v and draining through a 1M resistor, it’d drop to around half its voltage in a mere 0.88 microseconds. <\/p>\n

The typical low-tech way to measure unknown capacitances is to stick it in as part of the timing circuit for a 555 astable and then measure the resulting pulse frequency. Fine in theory, but I don’t own a frequency counter .. and also the frequency would be in the megahertz range.<\/p>\n

Microchip have a nice technote<\/a> describing how to use a PIC microcontroller to measure small capacitances – and when they say small, they mean sub-pF. I’m still digesting that.<\/p>\n

I’m tempted to skip all the tricky stuff and just build a larger scale version to test the concept … just to see if I can make the basic method work at all. Whilst I started off thinking about measuring distances, it occurs to me that a large-scale variable-capacitance controller oscillator is … a theremin<\/a>!<\/p>\n","protected":false},"excerpt":{"rendered":"

This evening I was truing one of my bicycle wheels. I started out with a truing stand – literally, a metal bolt identifies high spots on the rim by rubbing against them. I managed to get the wheel pretty true. But then I remember that I also have a DTI, accurate to 0.01mm. It’s like […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-441","post","type-post","status-publish","format-standard","hentry","category-general"],"_links":{"self":[{"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/posts\/441","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/comments?post=441"}],"version-history":[{"count":7,"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/posts\/441\/revisions"}],"predecessor-version":[{"id":449,"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/posts\/441\/revisions\/449"}],"wp:attachment":[{"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/media?parent=441"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/categories?post=441"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.nobugs.org\/blog\/wp-json\/wp\/v2\/tags?post=441"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}