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7. Power

Let's identify how your robot will be powered!

Most mobile robots are powered by a battery rather than tethered by a power cable connected to an outlet. Understanding power requirements for a robot can be understood by the amount of current and voltage required. Begin the robot design by documenting voltage and amperage requirements for motors, sensors, computers, and peripherals when choosing an appropriate battery. Whichever power option is used for the robot, ensure it provides enough amperage for the robot application. 



How Many Amps?

Amperage is how current is measured. Peripherals such as servo-motors require a high amount of current. Depending on size, a single servo-motor generally consumes between 1-3 amps (1,000 milli-amps) when moving quickly. When a robot has more than one servo moving simultaneously, the amperage requirement could be upwards of 5-10 amps. When planning the robot, you will need to consider this because a current shortage causes nearly every issue with EZBs restarting when servos move.



How Many Volts?

Considering the required voltage is generally an easy answer. Because most electronic devices operate at either 3.3v or 5v, the most common external power supply is 5v, and the most common battery is 7.4v. When getting a battery or power supply for the robot, get either in the range of 5v - 7.4v, which will encompass nearly all robot hardware. However, it is still a good idea to check the sensors and motors for your robot to identify what voltage they recommend.



Battery Current vs Power Supply Current (milliamps per hour)

The current provided by a battery is a little different from a power supply. A power supply limits the current it can produce (i.e., 3amps). However, a battery has an additional rating called a C rating. That rating is the capacity of energy the battery can safely discharge, represented as a multiple of its overall capacity. A battery with a higher C rating delivers more energy, which means higher performance.

Batteries are rated in milliamp per hour. So they can provide X amount of milliamps in an hour. If your battery was 5,000 mAh and the robot required 1,000mAh continuously, the battery would theoretically last 5 hours. Also, if the robot were drawing 5,000 mAh, then the battery would last 1 hour. However, the C rating means that your battery can discharge more current than the mAh rating. You see, a battery may have a 5,000 mAh capacity, but if the robot were to draw 10,000 mAh, then the battery would last 30 minutes. So a 5,000 mAh battery does not mean that is the maximum output - it means that is the current per hour. The robot can draw more, but the battery will drain quicker.

A higher C rating of 2 means that a 7.4v 5200 mAh lipo battery can deliver 10 continuous amps for 30 minutes. However, it is unlikely that any servo can withstand the heat that requires 10 continuous amps. When servos are not moving, the current is minimal and gives them time to cool down after strenuous movements.



Power Efficient Servos

Not all servo-motors are alike. Servo-motors come in many sizes and price ranges, which reflect their power consumption. The most inefficient servo-motor is low-cost analog-style hobby servo-motors, such as the popular model numbers starting with MG-xxx. The inefficiency of analog servo-motors does not contain digital logic that controls the motor's positioning with intelligent algorithms. For example, Dynamixel or EZ-Robot servos are digital and have the intelligence to use high-frequency algorithms that control the servo-motor.

For example, here is a video of the EZ-Robot HDD servos demonstrating their efficiency and built-in shutdown features. While we use this EZ-Robot HDD servo video as an example, many other servo manufacturers have similar characteristics. We didn't have a video of other servo-motors being demonstrated to the level of detail that this video does. Another servos to consider would be Dynamixel from Robotis.



Select Battery Source

Battery

This type allows the robot to be completely mobile and portable. Popular battery options are Lead-acid, LiPo, NiCAD, NiMH, and LiFePO4. The battery is charged using an external charger. More advanced configurations can have a docking system for the robot to dock with the charger automatically. Make a note of the power requirements and what size battery. For example, you may require only a general-use 7.4v 2,000 mah LiPo battery or a heavy-duty 12v 10,000 mah battery, depending on the number of servos and motors used in the robot.



AC-DC Power Adapter

This type requires the robot to be tethered to a power outlet. Power adapters called "wall-warts" do not provide enough amperage for servo-motors. If an AC-DC adapter is preferred, use a digital switching power supply with enough amperage to meet the robot's requirements. Here are some external power adapters that provide enough power for servo motors.



Battery and Power Supply Details

Because the discussion around power is such an extensive conversation, Synthiam's members made a great tutorial explaining more. We recommend reading this tutorial because it describes how voltage and amperage work. There's plenty of information on what kind of battery or power supply to use for the robot.

Battery and Power Supply Tutorial


Battery Monitor

ARC includes a battery monitor that will disable I/O outputs to reduce power consumption on supported I/O controllers. By default, the battery monitor is configured for 7v to protect the safety of LiPo batteries. Lipo (lithium polymer) batteries must never be completely drained. Therefore, the battery monitor is default configured for their usage.

The battery monitor can be disabled in the connection control configuration. Would you please read the connection control manual for additional information?