Power Distribution

The power requirements for the robot can be broadly divided into two parts:

• Unregulated 12V for motors
• Regulated 5V and 12V.

A completely new power distribution system was constructed Spring Quarter 2010. Everything was re-crimped, re-wired and cleaned up. A discussion of the new system follows.

Goals of new system

The idea for the new system was created after ordering replacement batteries. David Baty and Mark Jacobson worked on wiring everything a few weeks before the 2010 IGVC competition. Here is a summary of goals of the system:

• Protect the batteries
• Allow the batteries to be charged inside the robot by and intelligent battery charger
• Add fuses to the subsystems
• Carefully separate motor and electronics power, even though they are connected to the same power supply
• Allow the robot to be easily powered by AC power
• Allow automatic switchover from AC to battery
• Automatic cutoff before damaging the batteries

The last two points have not been addressed yet. See Future Work below.

System Overview

Power from the batteries is connected to a 180A breaker. This is a failsafe that will cutoff the batteries from the rest of the system. Next, the battery goes to a high current manual switch. This is used to physically disconnect power from anywhere else in the system. From there, the power is split to electronics and motor power. This all occurs in the power tether case. This case includes the battery charger, AC power supply, main breaker and electronics ORing diodes.

Power exits this case as separate electronics power and motor power. The grounds are also separate, they are only connected at one point, in the power tether case. This gurantees that ground loops aren't created, which can cause all sorts of problems, especially with the spikes that will be seen on the motor rail.

Power enters the electronics case and goes directly to two separate breakers. A 50A breaker for the electronics, and a 150A breaker for the motors. The motors are also fused with a 40A auto resetable fuse for each of the four motors.

The electronics goes to a fuse block that has six ATC fuses for various subsystems. These include:

• 12V to 5V Vicor module
• 12V to 12V Vicor module
• 12V to 24V step up converter for LIDAR (LIKDAR requires up to 30W)
• Computer power
• Fan power
• Mast unregulated 12V power

This system guarantees that each system has at least 2 levels of fuses/breakers.

Batteries

After the premature failure of our previous batteries, we purchased new batteries. We have two 12V Sealed Lead Acid (SLA) batteries in parallel. The model is  PG-12V55 FR. These are larger than the previous batteries, and should have a longer life in our application.

Battery Charger

The battery charger is is an intelligent 12V, 30A charger. It is permanently wired to the batteries. It is safe to leave connected to the batteries and will keep them fully charged. When AC power is connected to the robot, the battery charger is powered and monitoring the batteries.

ORing Diodes

The ORing diodes provides "automatic" switchover from batteries to AC power for the electronics only. The inputs to the ORing are connected to the battery and the AC power supply. The ORing diodes will allow power to flow from the input with the highest potential. It also provides "seamless" switchover when one power source drops offline. This allows battery to power supply switchover without turning off the electronics.

AC power supply

A 13.8V, 30A (??) power supply has been mounted in the power tether case. This allows the computer and electronics to operate seamlessly from AC power.

Manual Power Switchover Procedure

Until and automatic swithover can be implemented, here is the manual procedure to switch power source while keeping the electronics active (this is important to keep the computer running without issue).

From battery to AC power:

• Plug in AC power
• Turn off battery switch in front of power tether case
• All of the electronics will still be powered, and the batteries will be charging

From AC power to battery:

• Turn on battery switch in front of power tether case
• Unplug AC power
• All of the electronics will still be powered, and the batteries will be powering the motors and electronics

WARNING: DO NOT leave both power sources connected for an extended period of time. As the battery voltage rises above the AC power supply voltage, power will be drawn from the batteries. The battery charger is not designed to handle this, and may over charge the batteries, resulting in permanent damage.

Future Work

The plan is to create an intelligent power switch over board that will use two very large MOSFET or IGBTs to switch the incoming power sources. These will need to handle constant currents in excess of 100A. A small control board will monitor the battery and power supply voltage, and always favor AC power if it is present. This should be very simple, a single NOT gate and two gate drivers can do this. It would also be useful to add some intelligence that will detect when the battery is critically low (the monitoring system will deal with standard battery warnings, but we want to make sure we NEVER drain the batteries below a certain point), and turn off the system. This will be rather abrupt, but is the only way of guaranteeing that the batteries are not permanently damaged. Adding some hysteresis to this will likely involve the use of a microcontroller and the built-in ADC. Choosing an ultra low power micro and powering it off of the batteries is probably the best solution, so that it will always remain powered. The incredibly small load (<1mA) should not cause a problem.

Adding capacitors to the motor power (in the electronics case) would be a good idea. This will help with some of the awful spikes that will occur as they are turned on and off.

A new busbar should probably replace the awful temporary busbar used for the motor ground.