What regulates the strength of motor signals? I've seen cognitive and robot models where the input signals from the sensors are directly used as the signal for outgoing motor control. 

This doesn't make much sense, because obviously we're able to do heavy work with our muscles quite independently from the strength of incoming signals from our sensor systems. However when I consulted several books on general cognition, I wasn't able to discover the source of these arbitrarily strong signals - I'd suspect the thalamus. 

I wonder how you could perform a task, e.g. playing a piano, by applying weak or strong force. Both the plan (how to move the fingers) and the feedback (how it sounds) signal would remain the same.

So where does the signal for motor behavior originate, and more importantly what (or who) regulates its strength? Where does the extra energy come from?
Short answer
Muscles are controlled by motor neurons in the spinal cord. The number of motor neurons that fire, as well as their individual firing rates govern the control of muscle force.

Background
Muscles consist of contractile elements: the muscle fibers. These muscle fibers are under direct control of the motor neurons in the spinal cord (Purves et al., 2001), as shown in Fig. 1. The motor neurons in the spinal cord are under direct control of the motor cortex in the brain. When the motor neurons fire, they release acetylcholine, which in turn makes the muscle fibers contract. The force in which a muscle contracts is basically governed by how strong a single muscle contracts, and the number of muscle fibers that are recruited. 

Firstly, there is the rate code that allow motor neurons to regulate muscle force. An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the associated motor unit (i.e., one or more muscle fibers) generates. When the motor neuron fires a single action potential, the muscle may only slightly twitch. If the motor neuron fires at high rates, however, the second action potential may arrive at the muscle unit before the muscle has had time to recover from the first twitch, and that second action potential will produce a greater amount of force than the first. This is due to an increase in strength of muscle contraction through action potential summation. This process has a limit. When the successive action potentials no longer produce a summation of muscle contraction (because the muscle is at its maximum state of contraction), the muscle is in a state called tetanus and is pushed to its limit (source: Neuroscience Online).

Secondly, there the recruitment of motor neurons (size principle). When a signal is sent to the motor neurons to execute a movement, motor neurons are not all recruited at the same time, or at random. The motor neuron size principle states that, with increasing strength of input onto the motor neurons from the higher brain centers, smaller motor neurons are recruited first, while the larger motor neurons are recruited only when the motor signal increases. Why does this orderly recruitment occur? Because of Ohm’s Law, a small amount of synaptic current will be sufficient to cause the membrane potential of a small motor neuron to reach firing threshold, while the large motor neuron stays below threshold. As the amount of current increases, the membrane potential of the larger motor neuron also increases, until it also reaches firing threshold. This process is called recruitment and results in more motor neurons to be activated when the brain signals for the need for a high contractile force. More motor neurons will recruit more muscle fibers (source: Neuroscience Online).


Fig. 1. Motor neurons from the spinal cord innervate the muscles. source: SE Veterinary Neurology 

References
- Purves et al. (eds), Neuroscience, 2nd ed. Sunderland (MA): Sinauer Associates. Motor Neuron-Muscle Relationships
- University of Texas, Houston Health Science Center. Neuroscience Online