Strain gage circuits through a slip ring

The essential

Slip ring noise in a strain gage circuit comes from slight variations of the ring connection resistance. With all four bridge arms on the spinning side, the rings are outside the bridge and cannot affect its balance: peak ring noise stays under 0.1 % of the data signal with any Michigan Scientific slip ring. Circuits that leave one or two arms on the spinning side put rings inside the bridge and are not recommended. The chevron circuit uses fewer rings but adds crosstalk between outputs.

Why does the configuration matter?

Because it decides whether rings sit inside the bridge. A ring in series with a gage acts on the output exactly like a change of gage resistance. Strain gage bridges are full bridge (four active arms), half bridge (two) or quarter bridge (one); through a slip ring, what counts is how many arms are on the spinning side.

Why put all four arms on the spinning side?

Because the rings are then outside the bridge and cannot change its balance. This configuration works with most strain gage amplifiers, and conventional balance and calibration networks may be used.

Peak ring noise under 0.1 % of the data signal.
With 120 ohm bridges: 1 bridge on an S4, 4 on an SR10M, 9 on an SR20M, 16 on an SR36M.
With 350 ohm bridges: 1, 4, 9 and 17 bridges on the same models.
Excitation rings are paralleled to stay within the ring current limit: 2 rings up to the SR20M, 4 on an SR36M with 120 ohm bridges at 10 V.
A single active gage can be completed with dummy gages on the slip ring mounting adapter, so the whole bridge stays on the rotating side; the note gives the circuit.

What does one arm on the spinning side cost?

High ring noise: the rings are in series with the gage, so any ring resistance variation changes the output like a change of gage resistance. Its only advantage is a lot of data from few rings. Not recommended.

Use it only when large strain signals are expected and rather high noise is acceptable.
Parallel the common ring with any spare ring to reduce noise; with many gages, paralleling is mandatory to limit the ring current.
The signal to ring noise ratio does not depend on the excitation voltage; a higher gage resistance and gage factor improve it.

What do two arms on the spinning side cost?

The common excitation rings are inside the bridges, so any ring resistance variation changes the bridge balance. Noise is somewhat lower than with one arm, but the circuit is far more susceptible to ring noise than four arms or the chevron circuit. It is used only because it suits most amplifiers. Not recommended.

The data signal rings are not critical: the high input impedance of the instrumentation keeps the current through them negligible.
Parallel any spare ring with the excitation rings, even when the current is within limits.

What does the chevron circuit trade?

Fewer rings against crosstalk. The bridges share their completion arms, so all four arms remain on the spinning side and peak ring noise is again under 0.1 %. But the signal current of each bridge flows through the shared arms and shifts the common signal reference: every output picks up a part of all the others, and these contributions add algebraically.

Crosstalk depends on N, the input resistance of the instrumentation divided by the gage resistance: 4.6 % at N = 10, 0.5 % at 100, 0.05 % at 1 000, 0.005 % at 10 000.
Example: 120 ohm gages into a 12 000 ohm input give 0.5 % of crosstalk on every other output.
Capacity at 10 V with 120 ohm gages: 1 bridge on an S4, 5 on an SR10M, 13 on an SR20M, 25 on an SR36M.
To test for crosstalk: disconnect one readout instrument; the other signals should not change. Outputs sampled one after the other do not interact.

How many channels with spinning amplifiers?

Service rings for the power and control lines, then one ring per amplified channel. Strain gage amplifiers use five lines: +15 V, −15 V, power common, calibration control and signal common. Thermocouple amplifiers have no calibration control and use four.

10-ring assembly: 5 strain gage channels, or 6 thermocouple channels.
20-ring assembly: 15 strain gage channels, or 17 thermocouple channels.
36-ring assembly: 27 to 31 strain gage channels depending on bridge resistance and excitation, or 32 thermocouple channels. More than one amplifier control unit is then needed.
Limit of the slip ring: the current per circuit on its datasheet, 500 mA on most S and SR models, 250 mA on the SR36, 1 A on the B-Series. On a 36-ring assembly the ±15 V lines then need two or three rings each.

The technical notes

The original notes carry the circuit diagrams and the full tables.

To remember

Full bridge on the spinning side: peak ring noise under 0.1 %.
Quarter and half bridge completed across the rings: not recommended.
Chevron saves rings; check the crosstalk against your input impedance.
With spinning amplifiers, count five service rings, then one ring per channel.