Hi there
Inorder to work with such sensors you will need to include an ADC IC, maybe ADC0804 or 0808, with 8051 since 8051's do not have inbuilt ADC support. This "Sharp's Distance Sensor" gives an analog output corresponding to distance (you can check out the relation in its datasheet).
I am therefore sharing some of the links that might be helpful for this thing:
1 "engineersgarage.com/contribution/interfacing-3-axis-accelerometer-with-8051-microcontroller"
2 "engineersgarage.com/contribution/dc-motor-control-using-pwm-with-adc"
3 "engineersgarage.com/contribution/interfacing-adc0808-microcontroller-8051"