Torque Sensor

One of the tests appeared to demonstrate the resistance of a spring varying under extension and then there were 4 springs in what looked like a bridge arrangement. I just did a quick manual test with a small spring and measured 236mR at rest and 238mR at extension. I'm not sure this could be consistent under all conditions of temperature and loading etc. etc. Not for precision performance perhaps.

In another scene of the video there were MEMS devices being used. Analog Devoces make MEMS parts MEMS | Analog Devices. These are precision parts that you can integrate into a mechanism to measure torque.

Can you get a translation of the commentary?

I measured a spring too (one of a box of 200 tension springs I bought from Aldi years ago !).

I also detected only a very small change in resistance (once all the coils were separated)  - probably of the order of 0.1 - 0.2% at big extensions.

No surprise there - metal foil strain gauges typically have a gauge factor of about 2 which means that for 1000ustrain (increase in length of 0.1%) you get a change in R of 0.2%. When a tension spring is stretched the change in strain the metal wire is nothing like as big as the increase in length of the spring so it all fits together.

Real load cells usually use four gauges in a bridge arranged so that their temperatures will track very closely - the 4 spring bridge would be much worse for this.

AFAIK none of the major players makes MEMS load cells - the problem is that using large chunks of bulk silicon would be very expensive - a few people have tried making small stick on silicon strain gauges but although you get bigger gauge factors and bigger electrical signals the temperature drift and mis-match to a metal substrate mean that over all they work no better than metal foil.

Some wide range postal scales (not the ebay for a tenner kind !) use a resonant quartz sensor under tension and can achieve very good dynamic range so that they can do letters with an error of less than 0.5g while still being capable of weighing 20 or 30 kg.

If the OP only wants to measure/control torque he might do as well by controlling the current in a real DC servo motor (or even a brush-less one but it's much harder to do).

MK

I measured a spring too (one of a box of 200 tension springs I bought from Aldi years ago !).

I also detected only a very small change in resistance (once all the coils were separated)  - probably of the order of 0.1 - 0.2% at big extensions.

No surprise there - metal foil strain gauges typically have a gauge factor of about 2 which means that for 1000ustrain (increase in length of 0.1%) you get a change in R of 0.2%. When a tension spring is stretched the change in strain the metal wire is nothing like as big as the increase in length of the spring so it all fits together.

Real load cells usually use four gauges in a bridge arranged so that their temperatures will track very closely - the 4 spring bridge would be much worse for this.

AFAIK none of the major players makes MEMS load cells - the problem is that using large chunks of bulk silicon would be very expensive - a few people have tried making small stick on silicon strain gauges but although you get bigger gauge factors and bigger electrical signals the temperature drift and mis-match to a metal substrate mean that over all they work no better than metal foil.

Some wide range postal scales (not the ebay for a tenner kind !) use a resonant quartz sensor under tension and can achieve very good dynamic range so that they can do letters with an error of less than 0.5g while still being capable of weighing 20 or 30 kg.

If the OP only wants to measure/control torque he might do as well by controlling the current in a real DC servo motor (or even a brush-less one but it's much harder to do).

MK

Aldi sells springs!? Were they spring onions? Or spring greens?