Adhesive wear and frictional characteristics of UHMWPE and HDPE sliding against different counterfaces under dry contact condition

The current work evaluates the wear and frictional performance of ultrahigh molecular weight polyethylene (UHMWPE) and high density polyethylene (HDPE) sliding against different metal counterfaces, stainless steel(SS), mild steel (MS) and aluminium (Al), under dry contact condition. The experiments were conducted using pin on disc machine at different sliding distances (0–40.32 km), 15 N applied load and 2?8 m s21 sliding velocity. Interface temperatures and frictional forces were measured simultaneously during the sliding, while specific wear rates were determined for every 1.68 km sliding distance. Based on the optical microscopy of the worn surface and wear track, frictional and wear results were analysed and discussed. The experimental results showed that the type of counterface material significantly influences both frictional and wear performances of the selected polymers. This was mainly due to the film transfer characteristics. Higher temperature and friction coefficient for UHMWPE and HDPE were evident when sliding took place against Al counterface. Sliding the polymers against stainless steel showed low friction coefficients compared to other counterfaces

Dynamic hysteresis torque band for improving the performance of lookup-table-based DTC of induction machines

The magnitude of hysteresis torque band has a considerable effect on the performance of two-level Direct Torque Control (DTC) of induction machines. The conventional DTC fails at low speed due to the poor flux regulation. In this paper, two dynamic hysteresis torque band (DHTB) strategies for the conventional DTC are proposed to solve this problem by dynamically altering the amplitude of hysteresis torque band based on a flux error range. In this way, the simplicity of DTC algorithm is retained since it only requires a minor modification on its structure. The paper also presents further analysis on flux degradation, particularly the droop in between the flux sectors. In addition, the switching frequency effect of reverse voltage vectors for both DHTB schemes is investigated on the performance of the speed-sensorless DTC drive. The effectiveness of the proposed schemes is confirmed by simulation and experimental validation. Results show a significant enhancement in the flux regulation and dynamic torque response from zero motor speed

Sliding mode observer-based MRAS for direct torque control of induction motor

This paper proposes a sensorless sliding mode observer based model reference adaptive system applied to a direct torque control of the induction motor drive. The reference model is a sensorless sliding mode observer and the adaptive model is a typical current model. The proposed scheme provides speed and rotor flux estimations with the derivative of rotor flux set as the state of the adaptive mechanism. The simulation results show the effectiveness of the proposed observer compared with the conventional model reference adaptive system

An improved flux regulation using a controlled hysteresis toque band for DTC of induction machines

This paper presents a dynamic hysteresis toque band strategy for improving flux regulation at low and zero speeds in a direct torque control (DTC)-look-up-table-based induction machine. The hysteresis torque band is controlled based on speed variation to solve the stator flux magnitude droops which normally occur at low frequency. The main benefit of the proposed approach is its simplicity, since it only requires a minor modification to the hysteresis band value and does not require an alteration on the lookup table. The simulation results of the proposed system showed a significant enhancement in speed and torque estimations at very low and zero speed and at the same time removes the problem of flux droop in the conventional hysteresis based DTC

An optimized extended kalman filter for speed sensorless direct troque control of an induction motor

This paper presents a new approach of optimizing the performance of an Extended Kalman Filter (EKF) using particle swarm optimization (PSO) for speed estimation of an induction motor drive. The development of the EKF algorithm and selection of the filter covariance for the EKF based speed estimation are presented and discussed. The effectiveness of the optimization technique is verified through Matlab/Simulink simulation of a direct torque control (DTC) drive system under various operating conditions

Improved stator flux estimation for direct torque control of induction motor drives

Stator flux estimation using voltage model is basically the integration of the induced stator back electromotive force (emf) signal. In practical implementation the pure integration is replaced by a low pass filter to avoid the DC drift and saturation problems at the integrator output because of the initial condition error and the inevitable DC components in the back emf signal. However, the low pass filter introduces errors in the estimated stator flux which are significant at frequencies near or lower than the cutoff frequency. Also the DC components in the back emf signal are amplified at the low pass filter output by a factor equals to. Therefore, different integration algorithms have been proposed to improve the stator flux estimation at steady state and transient conditions. In this paper a new algorithm for stator flux estimation is proposed for direct torque control (DTC) of induction motor drives. The proposed algorithm is composed of a second order high pass filter and an integrator which can effectively eliminates the effect of the error initial condition and the DC components. The amplitude and phase errors compensation algorithm is selected such that the steady state frequency response amplitude and phase angle are equivalent to that of the pure integrator and the multiplication and division by stator frequency are avoided. Also the cutoff frequency selection is improved, even small value can filter out the DC components in the back emf signal. The simulation results show the improved performance of the induction motor direct torque control drive with the proposed stator flux estimation algorithm. The simulation results are verified by the experimental results

Toxoplasma gondii infection among pregnant women attending maternity clinics; multi center experience from Riyadh Region, Saudi Arabia

Introduction: Toxoplasma gondii infection in pregnant women is linked with higher risk of spontaneous abortion or premature infants. This study aims to investigate the seroprevalence of T. gondii infection in pregnant women in Riyadh, Saudi Arabia. Material and Methods: 1000 blood samples were collected from women visiting various regional laboratories of Riyadh. Serum was tested for toxoplasmosis using ELISA technique. Results: The result revealed that 93% pregnant women were serologically negative and only 7% were positive. The study concludes that youngand aged women who have T. gondii infection should have more awareness about this infection and its complications. Conclusion: The study would be valuable in improving pregnancy care by encouraging the health workers to implement clinical and protective care plans side by side

Hematological and Biochemical Parameters Among Obese Students at the PSAU, Alkharj, KSA

Management of obesity represents a global problem that challenges the provision of healthcare services in most countries. Saudi Arabia ranked number 29 on a 2007 list of countries with 6% of its population being overweight (BMI > 25).In a university setting, we studied hematological parameters (including whole blood counts, haemoglobin and platelets), the presence of basophilia, iron levels and lipid profiles in obese students, and also in non-obese student controls. We found a significant increase in whole blood count in obese compared to healthy individuals, and also found a high level of basophilia compared to healthy controls.  We also report that the obese student group suffered from low iron levels, and also a reduced total iron binding capacity, as compared to healthy controls. Levels of cholesterol and triglycerides was significantly higher in obese students compared to healthy controls. This study can be interpreted that universities across the Kingdom, and beyond, should consider targeting obesity management in their students to try to reduce the prevalence of obesity and associated disorders, and to support such healthcare programs by offering a variety of environmental, physical exercise and nutritional interventions