To calculate the ZMP you have basically two options. To use a force sensor to the ankle (usually very big in size and price) or, by observing accelerations of all joints. There are two papers I recommend:
- ZERO-MOMENT POINT - Thirty five years of its life (Vukobratovic and Borovac)
- Planning walking patterns for a biped robot (Huang, Yokoi, Kajita, Kaneko, Arai, Koyachi and Tanie).
The second one, in the “Appendix A” has the equation to calculate the ZMP.
There is a third option though and that’s the Center of Pressure (COP). If the foot is stationary then ZMP and COP are the same exactly point. And you can measure the COP by using some pressure sensors on the bottom of the foot.
I am guessing you need COM (of the whole bot) and COP/ZMP in order to use a simplified “Inverted Pendulum” model?