options for precise small-scale positioning and sensing - Page 1
Forum section: electronics / beginners

author: woodslanding
author_rank: Newbie
timestamp: December 18, 2021, 12:19:52 am
content: Hi all. I am new to this forum, and I have a crazy idea I'm trying to prototype....I'm looking to build an electro-mechanical pedal steel guitar.A traditional PSG uses a completely mechanical system of pedals moving rods and levers to change the pitch of a particular string. This allows for different chords. Then a metal bar held in the left hand is used to pitch these chords to the correct key.Virtually all PSGs have a particular variation on a pedal setup designed over the past 50 years for the singular purpose of playing country music. However, the capability of sliding one chord to another different chord in another key is something that can't be accomplished by any other instrument ever designed! So the potential of the instrument is tragically under-utilized. This idea is a first step in rectifying that issue.Someone built a similar system for a traditional guitar, using rotary servos:http://imgur.com/a/a0ZlWAnd here's a little test bed of using a stepper motor to do the same thing:The deflection required for a particular string to achieve the largest practical pitch change is on the order of 3mm. A lever at the bridge for each string can bring that up to around 10-20 mm, depending on the geometry.The tension of the string can be counter-balanced with a return spring, to reduce the force required to move the lever and re-tune the string. You can see that clearly in the second video above, although he has not completely counter-balanced it, and is pulling, as opposed to positioning.What's needed for each string is a means of moving its lever over 1-2 cm of distance under microprocessor control. It needs to respond very quickly to changes in the input, something on the order of <100 ms over the whole range. 30 ms would be nice. I don't think I can move my foot much faster than that, although I haven't measured.... The precision needs to be on the order of 10 bits. The maximum pitch range would be about 4 or 5 semitones, and with a 1/2-cent precision (which I think is sufficient) that's a total of 800-1000. Most pitch-changes would only use a portion of that range, 1 or 2 semitones. 9 bits, or 1 cent precision would probably be okay. I don't think 8 bits will cut it, but it might barely, if that made things simpler.The other aspect of the issue is measuring the position of the lever, to insure the instrument stays in tune. This precision needs to be similar, around 10 bits. I think the analog inputs on arduinos are 10 bit, right?I've looked at linear actuators (expensive, short on power, and not as small as I'd like) rotary servos (not ideal for a linear application, although there are workarounds) and a simple lead screw driven by an electric motor. The latter seems most appropriate, although I don't know how much precision one could get over such a small distance. And there are questions of power, size and rpm that I'm not at all sure about. And this application requires a means of detecting the position of the tuning lever.For position sensing there are so many options, and I'm not clear how precise they might be over this distance. I've looked at:rotary encoding, via either optical or hall sensing tech. This allows you to know how many rotations the motor has moved, but doesn't give an absolute position. I'd prefer absolute position sensing if it's practical.For absolute sensing, there are hall effect, inductive, LVDT, and who knows what else. Unfortunately, it's hard to know what ranges these sensors work well at, and what accuracy they can provide.I'm looking to create a 1-string test bed, as above. Some things I think need to change from that version:1. The stepper motor seems needlessly large and cumbersome. By using a stronger return spring, you should be able to manage with a much smaller motor.2. There needs to be continuous control from position A->B with a continuous input. A pot would be fine. Eventually that would be mounted in a foot pedal. In the first video above, he used a hall effect sensor. That might be fine too, although since my setup would not necessarily have end-stops, (different pedals might pitch the string different amounts, and most pedals are unlikely to use the whole range) the min and max values coming from the pedal would need to be consistent and repeatable. [Edit: actually, the end positions could be translated in software, I imagine]3. My mechanism will pivot on a knife edge, which should eliminate the mechanical imprecision issue he has in the video, which keeps the string from returning to pitch properly.4. Finally, if I'm going to scale this up to 8 or 10 strings, the parts need to be relatively inexpensive.What do you think? Looking forward to any thoughts...Thanks in advance!=eric

author: wizard69
author_rank: Super Contributor
timestamp: December 19, 2021, 09:39:47 pm
content: Wow! I don't think I understood any of that music related talk.As for linear actuators there are all sorts of stuff already on the market. You also have the possibility of using rotary actuators with cams. However the first thing that comes to mind is this, how do you keep all of the electrical noise and mechanical noise out of the music? The next thing that comes to mind is how does the human interface, with the traditional approach the human drives the pedals directly so I'm not sure why you want to do this electronically.In any event to size a servo solution properly you would need to know how much energy is needed to get your deflection. You will likely need to measure this as I have no idea not even a guess as to the required torque. Speed is also an issue, WWW,Newport.com has some tiny actuators but they are slow. Then there is:https://linmot.com/products/linear-motors/for a compact linear motor solution. Could they deflect a string as you ask - no idea as again I don't know what is required. In any event Linmot comes to mind due to previous knowledge but realize that there are hundreds of players in this field. That is small linear actuators. I wouldn't dis miss a rotary solution either.The same thing goes for rotary position detection, there are literally hundreds of companies offering solution in this sphere. Probably the simplest is a potentiometer feed back and going up in cost and complexity from there.

author: MarkLiu
author_rank: Newbie
timestamp: December 24, 2021, 02:59:55 am
content: Maybe you could try a linear actuator stepper motor?