Advice on ground and shielding for inertia dyno.  - Page 1
Forum section: electronics / projects

author: adegnes
author_rank: Contributor
timestamp: January 05, 2016, 09:23:25 am
content: I've made an inertia dyno for motorcycles and are using an arduino to gather roller rpm and motor rpm.Roller rpm is registered by a Honeywell 1gt101dc hall pickup.Engine rpm by a capacity clamp on the sparkplug lead.Cables to both sensors are shielded.What I'm wondering is what is best practice for connecting the shielding?Should I connect shields to arduino ground or to dyno and motorcycle chassis? maybe both? What about the enclosure for the arduino and the rest of the circuit?Thanks!

author: adegnes
author_rank: Contributor
timestamp: January 05, 2016, 10:57:28 am
content: Capacitive clamp...

author: Ian.M
author_rank: Super Contributor
timestamp: January 05, 2016, 11:27:25 am
content: To avoid problems with ground loops, sensor cable shields should only be grounded at one end. As the hall sensor has a non-conductive housing, its shield is best grounded at the arduino end. The capacitive clamp pickup is more complicated. It needs a local ground to the chassis both for safety and correct operation. The cable shield should be grounded at the opposite end to your isolation circuit. If your input circuit ties chassis ground to Ardyuino 0V you are likely to have ground loop issues.

author: adegnes
author_rank: Contributor
timestamp: January 05, 2016, 11:37:01 am
content: Thanks!

author: adegnes
author_rank: Contributor
timestamp: January 05, 2016, 12:19:12 pm
content: Another question:I'm basically using this circuit (see picture) from herehttp://www.sportdevices.co.uk/rpm_readings/, but I had no 22pf caps(in series with the capacitive pickup) so used 100pf instead. Since my clamp around the sparkplug wire forms a very low value cap in series with 1cm2 pads on the pcb, also a low value cap, in series with the 100pf, I concluded that 100 in place of 22pf would not make much difference.Am I right? Is there other stuff than just the capacitance I should take into consideration here?

author: ovnr
author_rank: Frequent Contributor
timestamp: January 05, 2016, 12:39:50 pm
content: So you have pads on opposite sides of the board forming one of the input caps? If they are indeed 1 cm2, you've got around 2.3 pF, so it should be perfectly fine to use a 100p cap after it - it really shouldn't matter much.Just be sure to use a quality capacitor.

author: adegnes
author_rank: Contributor
timestamp: January 05, 2016, 12:51:19 pm
content: Thanks!Yep, my pcb is 1mm thick though - slightly higher capacitance, 3.5pf or thereabouts.

author: adegnes
author_rank: Contributor
timestamp: January 08, 2016, 12:19:52 pm
content: Yet another question;I found this circuit for a diy moped tachometer. In place of the low value serial capacitors into a bc547 then into the 555, they've used a larger cap in series with the pickup(which they call an inductive pickup, but actually is capacitive I think), wired in between two high value resistors that form a voltage divider between 5v and ground and the into the 555.Why is this better/worse? What exactly is wiring it like that doing to the signal? They call it a "differentiating circuit".I'm experiencing problems with spikes and interference with some of the bikes I'm testing, looking for ways to solve the issue.