# Energy Sources of Mobile Robot Power Systems: A Systematic Review and Comparison of Efficiency
## 2 Power Requirements for Mobile Robots
### Mobile Robots' Actuation Systems  
So far, a number of ways to actuate mobile robots have been developed, i.e., mechanical, electrical, hydraulic, or pneumatic. Purely mechanical systems are not common today due to their complex design and limited control capabilities. Combinations of mechanical and electrical systems or mechanical and pneumatic/hydraulic actuations are the most widely used due to controllability and relative simplicity of the design.  
In electro-mechanical drive systems, usually, rotary motion created by a motor is transformed using gearboxes and linkage systems to usable mechanical actuation. In the case of hydraulic or pneumatic actuation, sliding motion is the primary result of those actuators; therefore, mechanical linkages are the easiest solution to transform sliding motion into more complex 3D joint rotations [15].  
Rotary motion in fluid power can also be achieved and hydraulic motors offer even ten times higher power density (100 W/kg to 20 kW/kg) than their electric (2 W/kg to 500 W/kg) or pneumatic (11 W/kg to 8 kW/kg) counterparts [15].  
In each of the cases described, the control and detection systems need electricity to operate. In the case of hydraulic and pneumatic systems, servo valves are electrically controlled to achieve fluid flow to the actuator from a pressure tank and to release pressure from the actuator's other end to enable motion. In the case of electrical drive, the control of current and voltage applied to the motor windings provides ways of controlling speed, acceleration, and torque produced by such actuator.  
This implies a need for electrical supply in any modern robotic system configuration. In a purely electrically actuated robot, the same supply can be used for the actuation system and control system; however, care must be taken to ensure the power rail for the control and sensors is free from interference from the noisy motor drivers. This effect can be provided, for example, by using an additional power source for the control and detection system. Pneumatic and hydraulic systems require pressure tanks and/or compressors/pumps to generate fluid power. This implies that the main energy source for such systems could be purely fluid pressure storage, a pump/compressor powered by a combustion engine or electric motor, or a combination of hydraulic storage and pump/compressor supply operated with some prime energy unit. In such systems, energy for control and sensing can either be provided from individual electric energy storage or by energy conversion from the main energy source.  
[PERSON] et al. [16] summarizes the use of various energy sources in robotics. The proposed division of sources by the way they generate and store energy includes the following functional categories:  
* Energy storage--including batteries, capacitors, and super capacitors;
* Energy generation--including classical electromagnetic generators, fuel cells, and solar cells;
* Energy harvesting--including electrochemical, wireless, thermoelectric, photovoltaic and nano-generators.