If the Ultrasonic Sensor can’t avoid bars or table’s legs, don’t trash it, or install bumpers because I have the solution. My solution is not something like a 180Degree Ultrasonic sensor that only existed in Robocop (it is an old show, older than many people in this forum) I feel old now :((. It is basically another sensor.
Robots follow your orders and they can’t deal with what they’re not taught to deal with. So, if you want a sensor that can feel the world around it, then you would need to make the Ultrasonic sensor rotate 180Degree to cover the front side of the Robot. Basically, hook the sensor to a motor that moves to the right 90Degree starting from the center, and then it goes back to the center and moves 90Degree to the left.
Now a lot of questions will be raised in your mind regarding this method. For example, what if the sensor is pointing in the wrong direction when the robot is heading toward the wall? To solve such problem you would need to have another Ultrasonic sensor. We use it in the Petroleum Industry to know the RPM (round per minute) of rotating equipments. It doesn’t get the distance like the VEX Ultrasonic sensor, but it can tell if something is in front of it or not. Therefore, by pointing the sensor perpendicularly on the wheel it will read either (1) or (0), but when the wheel is moving the reading will be a continuously changing variable of (1) & (0). Thus, using this knowledge you can tell if the Robot is moving or not. I posted some pictures for you to see how to hook it up. Positive with the red wire, Negative with the black, and the white wire with the third wire.
At normal conditions the robot is moving forward and the first Ultrasonic sensor is pointing forward, but once the Robot stops moving the second sensor detects the problem. Then the First sensor starts moving 180Degree to know where the blocking object is. From there the robot can navigate to a unblock itself.
I think my solution is a bit confusing and not clear. It also requires some coding skills. If you need help executing this idea, I would be happy to help you.