The easiest way to prevent things becoming trapped is to take reasonable steps to prevent things getting near the "danger zone", so a barrier 2 metres around the barrel would be a good start. Probably it would be an impractical idea, but...
A physical barrier of some sort would be the best idea, because that is less likely to suffer a failure than an electronic system (well assuming it has been designed right). If you are using an electronic system then it needs to "fail-safe", when it suffers a failure it must adopt its safe state (barrel open).
You could surround the lid with IR lights and receivers in the lip. They would need to be spaced at a distance where an arm/finger couldn't get between the lid and lip without breaking a beam. This solution would only work over the last few inches, because of the angle of the beams. If all receivers had a signal the it would be safe to close. The problem is the receivers would need to be close together to prevent transgression.
Monitoring the current drawn by the actuator and the position of the actuator might be another. I am assuming that there is an increase in current draw when the lid actually closes, and a similar increase in draw when something is trapped. If the lid isn't fully closed when the current increases then something is trapped.
The final way I can think of is probably the most effective. As the barrel closes don't close it the last few inches, allow it to 'fall' as the actuator descends. This means if there is an object in there the actuator isn't pulling the lid down and the object will hold the lid against gravity. This would be done by having a channel in the mounting block for the actuator 'pin'/ connection point to slide up and down.
OK Its not an electrical solution.