When tightening caps with a SureTorque ST-120 model, the control will increase the torque output until the measurement input is equal to or greater than the application setpoint value. Assuming that the calibration was done properly using the correct weight, pulley and calibration wire, the +/- 0.5% error comes from two main sources: measurement and control errors.
The measurement system errors:
- Transducer Error – This is defined by the full scale capacity of the transducer, usually +/- 0.1%. For example, an 88lbfin (10Nm) capacity transducer, has an error of +/-0.088lbfin.
- Analog to Digital Converter Error – For any digital system, the analog signal is sampled and stored in the device memory. The quality of the sampling is usually referenced by the resolution and speed of the AD converter. In the ST-120S, the sampling speed is 12.5ms and the resolution is 16bits. While the linearity of the AD and the limited resolution is only causing a negligible error (<0.01%), the sampling time can contribute to relatively large errors when coupled with high angular speeds.
- Other Errors – Other miscellaneous errors introduced primarily by mechanical imperfections such as slipring friction, rotary air union friction, or ball bearing friction. This part accounts for +/- 0.4% of FS error, and when added to the transducer error, we get the +/- 0 .5% FS overall measurement error.
The control system error:
This is the result of the delayed response and limited control output resolution. In the case of the ST-120S, the input (torque transducer signal) is sampled every 12.5ms. Due to limited sampling speed, it is possible that the setpoint is reached 2.5ms after the last torque signal was received. As a result, the motor will keep rotating the chuck/cap for another 10ms, which can result in substantial overshooting if the angular speed and the torque ramp is set high. The overshooting also depends on the dampening effect of the closure/liner system.
To improve the system response, Mesa Labs created an adaptive and programmable speed/torque control technique that allows high speed, low torque chuck rotation and automatic speed adjustment when resistance is building up between the threads. When the angular speed limit is kept within the 0-20rpm range, the control error falls well within the measurement system error range, and for this reason we do not list a separate error specification for the control accuracy.
Please keep in mind that the system will always overshoot the setpoint and the level of overshooting depends on the cap/bottle/liner system, angular speed and torque ramp time. For example, at a given speed, you will see a higher peak for a rigid phenolic cap on a glass bottle thread versus a polypropylene cap on an HDPE bottle. Similarly, for the same closure/bottle combination, the higher the angular speed of the chuck, the higher the application torque peak will be.
Please Note: When testing the Gold Bottle or another magnetic torque reference, the error of the magnetic standard and the error of the torque testing device accumulate.
For questions or more information, please contact your Mesa Labs representative.
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