How do I prevent output of opamp summing amplifier taking negative values?
There is a problem with this design since, with the series diode, there is no path for the op-amp output to sink current.

Consider the case that both inputs are positive.  The effective circuit is:







simulate this circuit &ndash; Schematic created using CircuitLab

Then, assuming \$V_{out}\$ is connected to a high enough impedance, we have:

\$V_{out} = \dfrac{V_{s1}R_2 + V_{s2}R_1}{R_1 + R_2} \ne 0\$

This problem is essentially due to the fact that you've lost negative feedback when the diode is reverse biased and thus, there is no virtual ground at the inverting input.

Take a look at an inverting precision rectifier to see what you need to do to fix this problem: