Legal Encoders

[Ouroboroz]

Originally Posted by Knufire

Remember that you're trying to measure absolute position here (the exact angle of the arm). Incremental encoders (which are most of what has been mentioned in this thread so far) measure relative postion (how much the angle of the arm has changed since the robot turned on). There are a few different solutions.

If you want to use an incremental encoder, you'll need a second sensor to tell you where "zero" (or any reference point) is. I've found the hall effect sensors from WCP made by 971 work great for this pupose. It's a little circuit board that returns a true/false based on whether there's a magnet in front of it; you attach a magnet to the arm and place the sensor board where it will trigger at a known arm angle. Now you know how much the arm angle has changed from that known position.

An alternative solution is to use an absolute encoder or potentiometer, that will tell you absolute postion. One option for this are the VP Encoder Stage (this is actually the SRX Mag Encoder mounted into a plastic block the same shape as a VP stage). You'll need a Talon SRX to interface with this sensor. If you don't want to use an SRX, one of my personal favorites is the US Digital MA3: http://www.andymark.com/product-p/am-2899.htm.

Thanks so much! That encoder might just be what my team needs.

[frcguy]

http://www.vexrobotics.com/vexpro/al.../217-5046.html

Here is the link for the VersaPlanatary Integrated Encoder. As Knufire said, it is a SRX Mag Encoder from CTRE in a case that goes in your VersaPlanatary setup. It's quite a nice form factor if you can spare the extra length of the gearbox.

[theawesome1730]

Originally Posted by frcguy

http://www.vexrobotics.com/vexpro/al.../217-5046.html

Here is the link for the VersaPlanatary Integrated Encoder. As Knufire said, it is a SRX Mag Encoder from CTRE in a case that goes in your VersaPlanatary setup. It's quite a nice form factor if you can spare the extra length of the gearbox.

If using this on the arm, use the encoder stage between your last planetary stage and your output shaft so you get your encoder counts with per shaft revolution rather than needing a ratio of counts to revolutions. For example if the encoder outputs 1024 counts per revolution, you would have 1024 counts per single revolution of the output shaft, if you have a 4:1 reduction stage between your encoder and output shaft, you would have 4096 counts per revolution of the output shaft. In an application of this sort, error in the gearbox is multiplied the through the reductions, so it is advised to use the encoder as close to the output as you can. The encoder is both absolute and quadrature so you won't need a second sensor for homing purposes.

[parry1126]

We have been using Grayhill Inc.Encoders for years. $53.93 on Digi-Key

http://www.digikey.com/product-searc...ords=GH3070-ND

[Ether]

Originally Posted by parry1126

We have been using Grayhill Inc.Encoders for years. $53.93 on Digi-Key

http://www.digikey.com/product-searc...ords=GH3070-ND

Are you recommending against using an absolute sensor for the OP's application?

[asid61]

Originally Posted by Ether

Are you recommending against using an absolute sensor for the OP's application?

On this note, I actually prefer using incremental encoders for absolute rotary applications. I don't have to worry about overflowing/turnover and they are easy to zero out if they slip in a match, which happens even when we set screw our encoder shafts.
Continuous potentiometers are cheaper than encoders, but I can make my own encoders cheaper than even those.