I have a linear motor working in force control mode. I want to have a rough math model to this motor from reference current to the position. I used a first order linear model for the force control loop, and double integrator in order to get the position signal, both integrator has a zero initial value. This model has some problem: the output position will drift away when applying a simusodial input. But, this is not observed in practice. What could be the correct model for the force control loop of this linear motor?