On robust control of transmission squawk in a medium-duty vehicle

Torsional oscillations can pose a significant challenge in automatic transmissions, including those stemming from instabilities induced by friction in the clutch system during shifts. Examples include chatter, squeal, shudder, judder and squawk. Transmission squawk is more than just an annoying noise; it is a symptom of underlying issues that, if left unaddressed, can lead to significant structural failures. Squawk is a high-frequency torsional oscillation, predominantly induced due to a negative friction slope and the presence of a weakly damped oscillating mode in the transmission system. Although numerous passive methods are available to prevent the squawking of the clutch in automotive transmission, the significant drawback of passive methods is the limited duration of effectiveness. This paper particularly focuses on the mitigation of squawk using active control techniques. Since squawk occurs in clutch output, therefore, the output speed is used as the measured signal and clutch clamping force as the control action. The primary objective is to develop a control strategy that effectively dampens squawk oscillations while also minimizing control effort, a crucial aspect that has been overlooked in previous research on robust control of friction-induced vibrations (FIV) in context of automatic transmissions. The effectiveness of the designed controller is tested on an experimentally validated non-linear vehicle-level simulation model of a 9-speed automatic transmission. With the designed controller, the squawk oscillations are successfully suppressed. Comparisons with industrial routine PI controller are made to demonstrate the performance of $\mu$-optimal controller in terms of minimal control effort and smoother clutch engagement.