Torsion Factor; Angle Validation - Stanley QB Expert Instruction Manual

Torsion Factor

10 Torsion Compensation
10.1 Torsion Factor
For all STANLEY electric assembly tools, the angle information is based on the rotation of the resolver, which
is directly attached to the rotor. This information is used for motor commutation, and it also serves as an angle
encoder. The rotation of the tool output can be determined by dividing the rotor angle by the total gear ratio for
the tool.
All things can deflect when loaded. Just as a long steel bar attached to a socket to produce high torque will
deflect, likewise the gears within an assembly tool will deflect when subjected to the torque loads. In effect, the
gears act as a torsion spring between the rotor and the socket, and it is the deflection of this spring that can give
false angle data. In addition to the angular deflection within the gears of the tool, there can also be deflection of
the parts of the joint.
Whenever this deflection is present in the tool or the joint or the tool mounting device, the angle information
derived from the resolver will indicate a larger angle than the tool output actually rotates. This error is directly
proportional to the torque level. That is, the deflection at 40 NM will be twice that at 20 NM.
In a torque vs. angle curve of a fastening cycle, at the end when the torque reaches its maximum value, the
angle will also be at its maximum value. After shut off, as the torque falls to zero, the angle should remain at
its maximum value. But in the typical torque vs. angle curve, as the torque falls to zero, the angle also appears
to fall some amount. This is not because the fastener is being loosened. It is actually the resolver indicating that
the angular deflection of the gears is relaxing to the neutral position. In this case, the maximum angle indicated
at the maximum torque was incorrect. The resolver indicated more angle than the tool output actually rotated.
To correct for this slight error in angle data, the Alpha controller has a STANLEY-exclusive solution. The Torsion
Factor allows the user to input a value that compensates for the torsional spring rate of any part of the fastening
system (the gears of the tool, the joint components, or the tool mounting device), and this factor is used to correct
the angle reading throughout the fastening cycle. This factor is entered as Degrees per NM, and its default value
is zero. If the default value is used, there will be no angular correction. If a value of 0.1 is used, each angle data
point (every millisecond) will be modified by subtracting 0.1 times the torque value. For example, at 15 NM, the
controller will subtract 1.5 degrees from the angle reading for that sample. At 30 NM, the controller will subtract
three degrees for that sample.
The easiest way to determine the correct value for the Torsion Factor is to look at a torque vs. angle trace
with Torsion Factor set to zero. The amount of degrees that the socket appears to loosen after the maximum
torque, divided by that maximum torque is the Torsion Factor. For example, consider a torque vs. angle trace
that indicates a maximum torque of 40 NM, and the maximum angle at this torque of 50 degrees. But the angle
appears to loosen by four degrees as the torque drops to zero. The Torsion Factor can be determined by dividing
four degrees by 40 NM to arrive at a Torsion Factor of 0.1 degrees per NM. When this value is entered into the
Torsion Factor parameter, each angle reading will be corrected by this factor. When this factor is set correctly, any
torque vs. angle trace will now indicate no apparent loosening of the fastener as the torque drops to zero after
shut off; which is exactly as it should be.

10.2 Angle Validation

Now that the angle can be indicated with great precision, the other challenge is to validate these results against
an external torque/angle transducer with monitor. This is not as simple as setting both the controller and the
monitor to the same snug torque and comparing the resulting angle.
It has been found that a tool's torque trace will never track exactly the same as the external. The calibration is only
the average of a number of readings, generally at a high torque near the maximum capacity of the tool. When
any individual torque reading from the tool's controller is compared to a torque reading from the external torque
monitor, it can easily have a difference of several percent higher or lower. This means that the tool's controller will
start counting angle at a different point than the external torque/angle monitor starts counting. This could be five
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QB Expert Alpha Controller