Analog mixed signal front-end for Torque signal processing : A theoretical noise analysis and signal-chain evaluation

Abstract: High precision tightening of nuts and bolts together with traceability are important in many industrial assembly lines. To achieve this the nutrunner needs to accurately measure the torque which is applied to the nut, process the signal and provide the information to the controller that controls the tightening process. This is done by having a strain gauge in form of a Wheatstone bridge connected to the tightening shaft. When an excitation voltage is applied to the bridge the applied torque to the shaft can be measured in form of a voltage difference. This voltage is amplified, filtered and converted to a digital signal, this is referred to as the signal chain. Atlas Copco has provided an example of how the signal chain looks in today’s application. The signal chain consists of a gain stage and a filter stage, and the analog-to-digital conversion process is performed internally in an MCU together with several advanced digital filters. The purpose of this thesis is to analyse this signal chain, it’s performance in regards to noise and investigate if the chain can be redesigned with the same or better noise performance with other components. It should also be investigated if the conversion process can be moved from the MCU to an integrated circuit component to reduce the effects of noise from the tool motor. Hence, in addition to noise, other specifications such as power supply and physical footprint is also considered in the thesis. The goal is to define a process on how to analyse the noise, precision, speed and footprint in a front end signal chain. In addition to that, one or more alternative signal chains should be presented if the requirements of the front end can be fulfilled. A lot of effort went into researching how noise impacts a signal chain to get a thorough understanding on how to improve/rebuild an already functioning signal chain. The result was a working process on how to analyse the noise in a front end signal chain like the one described above, together with a generic noise simulation model in LTSpice which can be used if a component does not have a spice model. With the model a few alternative signal chains were considered which fulfills the noise requirements and the best performing alternative was built on a PCB in KiCAD. However, the physical PCB has not been tested due to the limited timeframe of the project and therefore only calculations and simulations act as the verifying element of the function.