Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!pt.cs.cmu.edu!cive.ri.cmu.edu!gerry From: gerry@cive.ri.cmu.edu (Gerry Roston) Newsgroups: comp.robotics Subject: Micro rovers vs. Stompers Message-ID: <9792@pt.cs.cmu.edu> Date: 2 Jul 90 20:47:35 GMT Reply-To: gerry@cive.ri.cmu.edu (Gerry Roston) Organization: Carnegie-Mellon University, CS/RI Lines: 70 Keywords: All of this talk about small rovers strikes me as being quite silly. Let's get real folks! small rovers can not accomplish meaningful scientific experiments. Let's consider a basic issue, power. One person suggested the use of solar panels. The most efficient solar panels avaiable for commercial use, can deliver approximately 150 W/m^2. If we define a small rover to have an area of 1/4 m^2, we can get ~40W of power HERE ON THE EARTH. If we now move this solar panel to Mars, the amount of power that can be extracted will be approximately 43% of the power on the Earth. Now, let's allow for wiz-bang solar panels which are twice as efficient as current ones, and we arrive at a total energy of about 34W. These same panels have a mass of about 15 kg/m^2. For our rover, this amount to a mass of 3.75 kg for the solar panels ALONE. Since this is to be a simple rover, we will assume minimal computing requirements, say the equivalent of a single MC68020 processor. To get ball park figures, we can look up the power consupmtion for a single board computer. A typical number is about 25 W. Again, let's assume that advanced technologies make this available at 13 W and a weight of 1 kg. To continue the analysis, let's assume that we wish for the computers to remain active during the night. If the vehicle is stationary, then we must simply supply 13 W times 13 hours. However, we must also allow for dust storms which will block the light, so let's up the 13 hours to 25 hours. So, we need a capacity of 325 watt hours of power. Using silver-zinc batteries, which are among the most efficient, we find that we need about 22 Kg of batteries, using about 0.01 m^3 of volume. Now, if the batteries can be slowly charged over the 13 hours of daylight, we must store (assuming 100% efficiency and no leakage) about 13 W/hr. Keeping with the notion of a small rover, let's assume that the rover is 1/2 meter tall, giving a total volume of 0.125 m^3. So, where are we now? We have a rover with solar cells, batteries and a computer. We can supply 34 W/hr of power and we are using 26 W/hr. We also have a mass of 26.75 Kg and have consumed almost 10% of the available space; AND WE STILL HAVE NO LOCOMOTIVE SYSTEM OR EXTERNAL SENSORS! If we assume that half of the available space in our rover if filled such that the average density is that of water, the total vehicle mass would be in excess of 60 kg. If interesting items do indeed occur in areas of rough terrain, the ability of the rover to scale slopes with 30 degree inclines is important. Given the power avaiable for locomotion and assuming to losses during movement, the vehicle could maintain a steady speed of 0.07 m/s going up the incline. I think that this back-of-the-envelope discussion should be sufficient to pursuade folks that the idea of solar powered micro-rovers is a pipe fantasy. This is not to say that they can not be made, but rather that they would be incapable of doing anything meaningful. The size of JPL's Robby and the CMU Ambler reflect this reality. Both vehicles are sized as they are so they can carry out meaningful scientific experiments, overpower the terrain they are in, and carry with them a power supply such as an RTG (radio-isotope, thermo-electric generator). -- gerry roston, field robotics center robotics institute, carnegie mellon university pittsburgh, pennsylvania, 15213 (412) 268-6557 gerry@cive.ri.cmu.edu