Measuring the tension in a cable

[CptAvocado]

I am building a tendon-driven gripper for handling soft produce. It will comprise of three silicone fingers, each actuated via a tendon. The three tendons are all pulled by a single stepper motor and lead screw.

I would like to have force control, but it must use an indirect method of sensing the force at the fingertips. This is because I don't want to have to mount a hard fsr on the fingertips as that would defeat the purpose of using soft silicone fingers - the hard sensor is what would be in contact with the delicate target and could cause damage.

I would therefore like to measure the tensile force in the tendons, and work out the force at the fingertips based on that. I am unsure how to go about this. Tensile sensors all seem to be very large and very expensive.

My current idea is to attach a spring to the end of each tendon. With a magnet on one end of the spring, a hall sensor on the other could measure the extension of the spring and I could then find out the force using F=kx?

Is this a good idea or is there something easier that I could do? Would it be possible to measure the current in the stepper motor and get a force from that like you can with a dc motor?

Thanks

[JohnLincoln]

@CptAvocado

The three tendons are all pulled by a single stepper motor and lead screw.

It is possible to calculate the force in the tendons from the torque of the stepper motor and the pitch of the lead screw.

If you calculate it for the maximum torque that the stepper motor can provide and that is acceptable then you don't need to do anything else.

[dougp]

What about a small air bladder in the fingertip connected to a pneumatic pressure sensor? The bladder being flexible it would not damage the fruit if only squeezed gently enough to lift an item.

.02

[hmeijdam]

If you already design to have the springs at the end of the tendon, why not connect the moving part of the spring to a linear potentiometer and measure its value to calculate how much the spring is extracting?

They cost no more than a few Euro's each.

[CptAvocado]

Looking online I cannot see any cheap and small pneumatic pressure sensors but I might be searching for the wrong thing. I like the idea though.

[jremington]

mount a hard fsr on the fingertips

Have you tried a "soft fsr"?

Force Sensitive Resistor 0.5" - SEN-09375 - SparkFun Electronics

[camsysca]

How about the small air bladder, mounted on top of an FSR? That should spread the pressure of the grip uniformly, allowing non-uniform object pick up while using a simple FSR on the 'body' of the finger.

[dougp]

Small enough?

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[JCA34F]

How about some mechanical drawings with dimensions, speed and force numbers?

[xfpd]

Tensiometer/tension meter.

Vehicles with spokes (threaded rods to hold a rim laterally and radially true) use a spoke tension gauge. It is triangular in shape, "pulls up" on the ends of the triangle while the central spring "measures" the tension in the middle. The measurement is a scaled deflection that will need to be cross-referenced to the material used, which needs raw data to create the "real tension" curve. The example of the spokes uses material (steel, stainless steel, aluminum), shape (round, flat, butted, double-butted) and width (1.6, 1.8, ... 2.6mm).