You spin me right round, baby, right round

by Ivan Hamilton 4/2/2008 3:05:00 PM

So, I'm trying to build a bi-directional motor controller. The standard format for such a beast is the H-bridge. There are 5 essential states to a H-bridge: Freewheeling, forward, reverse, and braking to supply or ground.

It's simple enough and I've known this for a long time (it was shown to me as a young boy by a father's friend using a DPDT knife switch). What I think I've done is underestimated the importance of the braking modes (not something achieved with the DPDT switch). I'd heard before about braking in H-bridges before, having seen them in application notes, and listed as a feature in H-bridge driver semiconductors, but hadn't seen it in practice.

Seeing how useful it is, was an accident. I was testing one of the motors I have, and during an unpowered spin down... I knocked its terminals together. The result was immediate and obvious.

I presumed the amount of braking force applied when the terminals were shorted would be related to the amount of current the motor would generate. Since these were designed as motors, and not generators their generation ability wouldn't be much, and therefore braking by shorting the terminals would be minimal... I love to be surprised.

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My mill mach' brings all the boys to the yard...

by Ivan Hamilton 4/1/2008 3:36:00 PM

I finally did it. I've been thinking and looking and scheming for over 12 months. And here it is...

Now I just need to work out how to use it. If you're looking for a machine like this, I can highly recommend Titan Machinery. Stuart from Titan has been very helpful, and continues to be so.

I'd looked at mini-mills like the Sieg Super X3 series, but they have only 300mm x 160mm x 350mm travel and are priced around $2200.

For only a little more, I got one capable of 500mm x 200mm x 450mm. Oh, and this one had a stand, vice, drill chuck & arbour, face cutter, clamp kit, lamp, collets and collet chuck, and chip tray (several hundred dollars to buy elsewhere).

This is the largest machine I found before they become floor standing and suddenly 700kg! Without a permanent workshop to place it, I need something a little easier to move. At about 350kg total, this machine is "man-handleable" when disassembled.

What next? Well, I've got the guitar, I just don't have any strings yet... and sheet music is a long way off. Huh? I have a $30 power drill... I don't mind cheap power drills since they take a fair bit of abuse and I consider them a consumable item. When dragging them around, they're eventually going to get destroyed (full of sand, dropped from height onto concrete, burnt out by stupid use, etc). The small collection of drill bits I have probably cost 10 times what the drill cost. Good drill bits make all the difference when drilling solid material, and will out last cheap ones many times over. I assume milling cutters will be the same; there will be many cheaper alternatives available... but in the end a good quality set is well worth it. But cheapies are good to practise with.

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Name of author Ivan Hamilton
"My inner nerd can beat up your inner nerd."

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