Understanding power managment for mobile robots
Autonomous mobile robots are free to wander around your facility. Therefore, they have to “carry their power on their backs”. All mobile robots are battery powered, and the choice and implementation of a battery for a mobile robot is critical to the safe and effective operation of the system. Mobile robot battery management is critical to the efficient operation throughout a work shift.
In addition to battery chemistry and capacity, the other important deployment decision for mobile robot battery management is that you need to make is how many charging stations to deploy and where to place them within your facility.
In this section, you will learn about:
- Mobile Robot battery management
- How to rate power requirements for your application
- Differences in battery chemistry and power management
- How to determine the number of charger stations required for your application
- Battery chemistry
- Charging concepts
Factors in power management
The vast majority of mobile robots operate on battery power, with the exception of agriculture and mining vehicles which operate on normal fuels. Battery chemistry and battery size is critical to the operating time of a mobile robot between recharging. There are several factors which can impact the length of run-time between recharging:
- Distance driven
- Payload power consumption
- Payload/cargo mass
Just like your car, the further you drive, the more fuel your vehicle consumes. An AMR operating in a large facility with long paths between pickup and drop off, will rundown its battery faster than a vehicle with more idle time and shorter runs. It will be hard to estimate how this impact your actual power consumption until the AMR has been in actual operation within your facility. Distance driven is one of the key factor in proper mobile robot battery management.
Any AMR payload with active components such as additional sensors, cameras or computers will consume the robot battery, unless a separate power source is provided. Likewise, the most power hungry payload components can be manipulator arms placed on top of the AMR or additional lifting devices for pallets or other operational needs.
Payload and Cargo Mass
Any AMR payload with active components such as additional sensors, cameras or computers will consume the robot battery, unless a separate power source is provided. Likewise, the most power hungry payload components can be manipulator arms placed on top of the AMR or additional lifting devices for pallets or other operational needs.Mass and velocitySimple physics answers the question about how much energy is required to move a mass. In a nutshell, the heavier the payload and cargo being carried by the AMR, the more energy that is required to move it. In the same way that it’s easier to push an empty wheelbarrow up a hill. Moving heavy masses will drain the battery faster. Also impacting this is how often the vehicle needs to stop/start along it’s path (to navigate or avoid obstacles). Again, it’s difficult to predict the impact of this without actually running the robot in your facility and completing the tasks that you demand of it.
There are many varieties of battery chemistry available today, and this technology is rapidly evolving. However, there are two primary battery type families in use today. The first is lead-based battery chemistry. This is state-of-the-art battery chemistry which derives it’s heritage from deep-cycle marine batteries or batteries used in automotive applications. These batteries are designed to be drawn down until they are almost dead and then recharged without any damage to the battery. Many AGV’s in use today, use some form of lead-acid battery pack.
The newer technology for battery chemistry is to use lithium-based battery chemistry. Lithium based batteries can hold much more energy than a lead-based battery of the same weight. Your cellphone has a lithium-based battery in it. The downside to lithium batteries is that they are more susceptible to explosion/fire, especially if they are damaged. The other downside is that lithium-based batteries can’t be run down as deeply as other battery chemistries. However, lithium-based batteries are continuously evolving and with proper battery management algorithms (on the robot), they are considered a safe and effective answer to providing power to mobile robots.
Proper battery management comes down to three different strategies to keep the vehicle powered up and operational:
- Opportunity charging
- Deep charging
- Inter shift battery replacement
To deal with the issues of recharging the battery on the AMR, vendors have implemented a variety of charging processes for their vehicles.
The most basic charging process is for the AMR to autonomously find its way to a charging station and then position itself so that it can recharge the batteries. Most vendors require that the charging stations be located in a permanent spot in the facility and that the robot is taught where this location is during mapping or map editing. In addition, the robot typically track the charge level of their batteries so that they know when it’s time to stop working, locate a charger and initiate the charging process. Running a robot too long may require that the vehicle be offline for an extended period of time which the battery goes through a deep cycle recharge to bring it back to full charge.
Some battery chemistries, such as LiPo batteries, are able to be recharged opportunistically. This type of opportunity charging doesn’t damage the battery. Being able to opportunity charge the battery means that a robot with a few minutes to spare between delivery tasks can hop on to an open charger and grab a few electrons. Some battery chemistries don’t support opportunity charging, so make sure that you ask your vendor about this feature.
In facilities where 24/7 operations require that the robots are always in operation, and always in motion (with little to no chance to deep or opportunity charge the battery), vendors have implemented the idea of “inter-shift” battery swap. Most AMR vehicles today have been spec’d and designed to operate a minimum of 8 hours (continuous) operation. This enables the robot to cover the typical 8 hour shift. To allow the robot to remain in continuous operation, it’s possible to have an operator swap the battery pack at the beginning or end of each shift. The discharged batter pack is then manually put onto a charge to fully recharge before the next shift/battery change.
Finally, another strategy for dealing with extreme battery draining operations is to specify enough robots in the fleet so that a percentage of the fleet is always charging and able to readily replace a robot which is nearing the end of it’s discharge cycle.