Thinking About Future Posts

Two areas that I'm going to be researching for future posts will be locomotion and robot muscle; specifically those types of each that will be adaptable to the needs of a robot harvester/tender.

Regarding modes of locomotion, I've already concluded that a walker will be the best all-around option for a harvester/tender [1],[2].  I've been looking at both hexapod walkers and robot lizard walkers and comparing their motion to that of actual lizards.  Three things stand out immediately.  First, real lizards have feet; hexapod walkers don't. Second, a real lizard's elbow moves horizontally, while the elbows on hexapod walkers move vertically.  And third, hexapod walkers have rigid torsos while a real lizard bends its spine from side to side as it moves.  What does all this mean?  I don't know.  Hopefully, with some research, thought, and time, I can arrive at some answers.

Once the mode of locomotion for a robot harvester/tender has been decided upon, the next step in design is to specify the type of actuator (robot muscle) that can be used to implement this motion.

Regarding possible candidates for robot muscle, to date there are no good candidates!  But the first step in narrowing down the set of most likely candidates is to start a list of the constraints that a viable candidate for a robot harvester/tender’s muscle must satisfy. 

Next, the question of our robot harvester/tender’s brain needs to be addressed.  Whatever computer system ends up being used to control a robot harvester/tender, it will still be held to the same constraints, in terms of size, weight, power consumption, cost, and etc., that the rest of the robot design will be held to.  But here lies the design challenge; there are no currently available computer systems that fit these constraints! 

Lastly, the question of fabrication methods: if/when robot harvester/tenders take their place in the farming industry, they will need to be produced by the 100K’s quantities.  They will need to be fabricated in a way that makes them cheap to make but still robust in use.  The modern auto industry has been addressing this exact issue for years, so a good place to start would be to look at how cars are designed and built.

[1] Agricultural Robots: Locomotion, Walker, Crawler or Wheels?
[2] Dancing Robots vs Robot Dancing

Piezoelectric Actuators

Electromagnetic driven actuators will be off the table for agricultural robotics because of their high current draw and low force/current ratios.  On the other hand, because of their high force/current and high force/size ratios, piezoelectric driven hydraulics has the potential to be a good candidate for "robot muscle" for portable and field deployed robotic systems.  The other nice thing about hydraulics is they are very abuse tolerant, work in the mud and dirt, and even under water. 

This is going to be an ongoing area of research for me.  As I find new information, I'll post it here and then bump this post to the head of the queue again.

This isn't hydraulics, but it is an interesting use of piezoelectrics to drive a linear actuator.  It uses a ‘walking” motion to generate 8-Nt of force with a 20-cm/sec velocity. http://www.linear-actuator.net/Ultrasonic-Motors_Linear_Actuators.php

Here is a piezoelectric driven hydraulic pump capable of 2500-psi output pressure and a flow rate of 40-cc/sec: http://www.kineticceramics.com/piez_fluid.html

Also see: http://aml.seas.ucla.edu/research/areas/piezoelectric/pump-non-mech-valves/2002-12_lee.pdf

Micro Gas Turbine Engines

While researching gas turbines, I came across a fascinating new area in high-tech R&D that bills itself as "micro turbines".  These are gas turbines on the order of 1 cm in size and turn at 500,000 to 1,000,000 RPM and higher.  Here are a few images from the links included below.

 

This combustion chamber only measures 20 mm in diameter and is 2.7 mm thick.
Supplied by a hydrogen-air mixture, it produces a power of up to 1200 W.



How about a 50-100 W gas turbine generator with

fuel tank the size of a pack of playing cards!

Here is a list of micro turbine links.  Some are news posts, but most are journal published, research papers.

Engine on a Chip

Micro-turbines, Maxi-difficulties

PowerMEMS Project - Microsystems for power generation

ULTRA MICRO GAS TURBINE GENERATOR

A SILICON-BASED MICRO GAS TURBINE ENGINE FOR POWER GENERATION (PDF)

 

DEVELOPMENT OF MICROMACHINE GAS TURBINES AT TOHOKU UNIVERSITY (PDF)

MILLIMETER-SCALE, MEMS GAS TURBINE ENGINES (PDF)

POWER MEMS AND MICROENGINES (PDF)

THE USE OF MICRO TURBINE GENERATORS IN HYBRID ELECTRIC VEHICLES (PDF)

DEVELOPMENT OF MICROMACHINE GAS TURBINE FOR PORTABLE POWER GENERATION (PDF)

ULTRA-HIGH-ENERGY-DENSITY CONVERTER FOR PORTABLE POWER (PDF)

POWER ELECTRONICS INTERFACE FOR A 100 W, 500000 RPM, GAS TURBINE PORTABLE POWER UNIT (PDF)

AN ULTRA-HIGH-SPEED, 500000 RPM 1 kW ELECTRIC DRIVE SYSTEM (PDF)

MICROFABRICATED SILICON CARBIDE MICROENGINE STRUCTURES (PDF)

HIGH POWER DENSITY SILICON COMBUSTION SYSTEMS FOR MICRO GAS TURBINE ENGINES (PDF)

Piston vs. Gas Turbine Engines, Efficiency, Final Thoughts

Pulling together all of the research and thinking I've done on the question of engine efficiency over the last couple of months, here is my conclusion...

If I were to write a design spec. for the power source for my "dream-team" agricultural robot harvester/tender it would specify a hybrid design consisting of a single-shaft gas-turbine generator combination driving a super-capacitor bank.

It would come in a drop-in package that could be changed in/out with a 1/2" socket set and 3/8" flare nut wrench.  The engine-generator set, the fuel tank and the super-capacitor bank would each be mounted as separate units.

But such a design is not realizable yet.  To date, there are no small 1-2 kW gas turbine engines commercially produced.  Though, the work being done on micro gas turbines makes one optimistic that someday they will be.

For now, if I had the machine shop to do it in, what I would do, would be to mount one of the Honda Gas Engines (showcased in a previous post) on a testbed and try to run it continuously for a few months and see how it holds up.  If it does, then it would make a good second option for an agricultural robot harvester/tender's power source.

If more information comes my way, I may yet change my mind on this subject.  But for now, I've gone as far on this subject as I can.  So it's time to put this aspect of robot design to bed for awhile and start on something new.

Piston vs. Gas Turbine Engines, Efficiency, Global View

The Global View  The issues of local and mid-range views of engine efficiency overlap the global view in the areas of maintenance and repair. 

Based on my experience, something that electronics and software engineers, working in the area of agricultural robotics, don't seem to appreciate is the high cost of machine maintenance.  Everyone is familiar with the high cost of auto repair these days, so why isn't the engineering community making the connection that robot repair is not going to be any less expensive?

Engine efficiency often comes at the expense of higher temperatures, higher pressures and greater mechanical complicity.  All of these are factors in decreased engine life, more stringent maintenance requirements and higher cost of repair.

Let's try some order-of-magnitude estimates, OME's, for the service costs of a robot harvester/tender and for the fuel budget savings for a 20% increase in engine thermal efficiency. 

First, an example repair situation...
   $100.00  2 hours labor
   $200.00  parts
   $100.00  tow-truck to pick-up/put-back our robot from/to the field
   $100.00  cost for a fill-in-for-the-day replacement robot
   $500.00  total cost to farmer for a typical repair  

Now look at the cost savings for a 20% increase in fuel efficiency.  Assume a fuel consumption of 2 gal/day.  At $4.00 a gallon, a 20% increase in fuel efficiency yields about 17% savings in fuel costs, which translates to about $1.35 a day.  At this rate it would take a year's worth of operation to balance out the cost of just one typical repair.   

From the point of view of the farmer's pocketbook, the bottom line is that reliability and low maintenance costs will always trump considerations of engine thermal efficiency .

Electromagnetic Actuators? Why Not?

Regardless of the novelty of a design concept, the laws of physics will always put limits on what any actuator can do. That good-old F=ma equation tells you that in order to move an actuator mechanism quickly, large forces are required to first accelerate it up to speed and then to de-accelerate it back to a stop.

If electro-magnetic forces are going to be the driver of an actuator's motion, then the equation F=iLBsin(θ) tells you how much wire(L), current(i) and static B-field you need to generate a given force level.

Given a possible 2000-8000 gauss range for permanent magnets, the currents necessary to generate any kind of usable force levels (with speed equivalent to human hand and arm motions) will be on the order of 10's of amps.  Then, with these current levels, the resultant resistive losses from the actuator's "windings" will generate copious amounts of heat that will have to be "dumped" somehow.

I worked on this concept a couple of years ago and for the above reasons, came to the conclusion that it will be impossible, using standard-technology electro-magnetic actuators, to recreate a robotic hand/arm system that mimics the action of a muscle fiber/tendon bundle.

Muscle fiber can only pull, not push. So to accomplish the finger, hand and wrist joint movements that we can, several muscle groups have to act simultaneously, pulling either together or in opposition to each other. Because of this fact, we have control over not just the position, force, and velocity of our hand movements, but also the overall mechanical stiffness of our hand/arm system as well.

Another thing to consider is the force level our muscles are capable of. If you look at our body's muscle-bone attachments and their attendant mechanical disadvantage in terms of forces and lever-arms, you'll see that our muscles don't just generate ounces of force, but 10's and 100's of pounds of force. And in the case of exceptional individuals, like an Olympic power-lifter for example, 1000's of pounds of force! I believe, the only thing in the same category, in terms of force per unit volume, is hydraulics.

The skeletal-muscular system that Mother Nature has evolved is truly extraordinary.

Now back to the topic of this post, if the goal is to create a robotic hand/arm system and do it using actuators that mimic the action of a muscle fiber/tendon bundle, then the following observations should be noted.

First, since a muscle fiber is a pull-only actuator, several different muscles, some in opposition to each other, have to fire smoothly, simultaneously and continuously together to reproduce the range of joint movements we have.

Second, the accuracy to which we can control our finger movements is tied to the rigidity that we can hold our hand/arm system to. There is no "brake" in the human hand/arm that allows it to be locked into a fixed position. The way we do this is to fire opposing muscle groups, in a full-on mode, and thereby force our hand/arm into a state of ridge tension.

These two observations together imply that for a robotic hand/arm system that mimics the action of a muscle fiber/tendon bundle, most, if not all, of the actuators will be in some state of "on"-ness all of the time. If one assumes voice-coil type actuators, operating in the 10's to 100's of watts power range, then multiples that power load by a factor of 10 to 30 (the approximate number of muscles in a human hand and forearm that might be engaged at any one moment), one finds that the total power load skyrockets. And of course along with the accompanying waste-heat cooling problem.

And this is just one hand/arm we're talking about.  Now multiply this by two hand/arms, then add in four to six legs for locomotion, then add in the power to run the harvester/tender's computer systems, and the power requirements for our robot become greater than any reasonably sized "portable" power source will be capable of delivering. 

Or to say it a different way; the reason factory floor mounted robots run just fine using electro-magnetic actuators is that they don't have to carry their power source with them.  Actuator power consumption is not a design constraint when your robot can plug into the power grid.  But once a robot is required to be self contained power-wise, then any excessively large current requirement will become a design show-stopper.  
 
Which is why, IMHO, that if a robotic hand/arm system is going to be made using actuators that mimic the action of a muscle fiber/tendon bundle, then it won't be done with electro-magnetic based devices.

Topic for a future post  The technological step I‘m looking for in hydraulics is to have a small fast-acting piezo-electrical driven pump that would mount directly onto the cylinder it is driving; thus forming a self-contained unit, a kind of "point-of-load" hydraulic supply.  This would get rid of the need for an external engine-driven hydraulic pump with its accumulator and rats nest of hoses that is typical of current hydraulic systems.

Piston vs. Gas Turbine Engines, Efficiency, Mid-Range View

The Mid-Range View  Fortunately for agricultural robotics most harvester/tender duties will be very even and predictable in their power requirements.  This makes a hybrid design a very workable choice for a harvester/tender’s power source.

Once the choice of a hybrid design has been made, the choice of actuator technology follows.  Since energy in a hybrid design is stored as electric charge, the most efficient pathway to convert that energy to mechanical work will be with electrical based actuators.  That is, once energy has been stored as electrical charge, it would be a detour in inefficiency to first use that energy to power an electric motor that in turn drives a pneumatic or hydraulic pump that then is used to drive the corresponding actuator type.

Topic for an upcoming post:  Conventional electromagnetic actuators have serious, even fatal, drawbacks when it comes to their use in agricultural robotics applications.  At this point, some thinking out of the box is in order.

Piston vs. Gas Turbine Engines, Efficiency, Narrow View

The Narrow View   With a few hours of web searching one can find a number of fascinating and far ranging discussions regarding thermal efficiency, covering everything from the theoretical realm of physics to the mundane world of practical mechanical issues.  

As an example of what is attainable in terms of thermal efficiency, here is a clip of a gas-turbine engine combined with a co-generation steam turbine that can attain a 60% thermal efficiency. 

Unfortunately for those of us designing robotics, large utility power plants, like the one featured above, enjoy an economy of scale that designers of agricultural robots won't have. 

One of the main design constraints for industrial/agricultural robotics is the KISS principle; that is, systems must be field serviceable by workers with a minimum of tools and expertise.  This means that designs for robotic power plants must consist of a minimum of moving parts.  Which means that the added complexities that come with co-generation schemes are off the table as design options.     

Fuel efficiency for an internal combustion engine is generally measured in  Brake-Specific-Fuel-Consumption.  This is a ratio of fuel burned to energy on the engine's output shaft and is given in units of pounds per horsepower-hour, or in units of grams per kilowatt-hour. 

The one aspect that is most important for our discussion is the concept of an efficiency “sweet spot” in the torque versus RPM graph; that is, a region in the torque versus RPM graph where an engine achieves its maximum efficiency [1]. 
 

BSFC for a Saturn DOHC engine with 20-HP operating curve marked in red.
Source Link

I've looked without success on the web for an equivalent graph for a gas turbine engine.  Based on the information I've come across, the "sweet spot" for a gas turbine would be smaller in area but higher in efficiency than a comparable piston engine; with efficiency dropping off dramatically as one moves away from that "sweet spot" region.    

Again, based on my readings, gas turbines have a higher efficiency over a narrow region in their torque/RPM graph, while piston engines have a "reasonable" efficiency over a broader area in the torque/RPM graph.  

In practical terms, if a robot has a narrow and well defined power requirement, then a gas turbine could be a good choice for its power source.  On the other hand, if a robot's power load is going to vary, then a piston engine will give the greatest efficiency when fuel consumption is averaged over all of the robot's working conditions.  This is one of the reasons that Boston Dynamics' "Big Dog" is piston engine driven.

The Hybrid Engine Concept  The idea is to decouple the engine from the machine it's driving by using an intermediate energy storage reservoir.  The engine drives a generator that charges either a battery or super-capacitor bank.  The robot then pulls its power from there.  This allows the engine to charge up its associated energy storage system while running at its point of maximum thermal efficiency.  It can then throttle back to a fuel saving "idle mode" when a sufficient buffer amount of energy has been stored-up.  This effectively decouples the narrow view from the mid-range view of thermodynamic efficiency.  

[1]. The "sweet spot" for the graph above is centered at a speed of 2500 RPM and a torque of 124.8 Nt-m.  The red 20-HP line represents the average power needed to move the Saturn car under typical driving conditions.  The closed curves represent lines of constant BSFC.  A lower BSFC means less fuel used for the same power output, that is, lower BSFC is equivalent to a higher fuel efficiency.

Piston vs. Gas Turbine Engines, Efficiency

What efficiency measures is a function of how broad or narrow one's view of a system's thermodynamic universe is.

A narrow view would be to look at just the thermal efficiency of an engine itself.  That is, what percentage of the chemical energy contained in an engine’s fuel gets converted to power output on the engine’s driveshaft.

A mid-range view would consider efficiency as the ratio of the work done by one’s robot to the amount of fuel its engine consumes.  In this case, one has to consider, not just the thermal efficiency of the engine itself, but also how efficiently that power is coupled to the work the robot is doing.  This is the impedance matching problem from before.

At a global view, the system's thermodynamic universe becomes an economic one.  That is, to the farmer efficiency is now the amount of profit generated per robot versus its cost of operation.  In the end, this is the most important figure-of-merit to the farmer.

It would certainly be advantageous if one could break down the question of engine efficiency into three such well-defined categories.  But such is not the case.

The first complication is that an engine's thermal efficiency is dependent on the power load and RPM it is running at.  This effectively couples the local and mid-range views together.  One can’t analyze an engine’s thermal efficiency without considering at the same time the mechanical system that engine is driving. 

The second complication is that the laws of physics put hard limits on the maximum thermal efficiency any heat engine can attain.  Ultimately what physics tells us is those higher efficiencies require higher operating pressures and temperatures.  These lead to higher mechanical stresses on an engine’s design, leading to more frequent breakdowns and required maintenance.  And it is in the area of maintenance costs where the local view of thermal efficiency crosses over into the global view of economic profitability.

Dawn's Science Class

1: Amateur Rocketry

This is a launch at the Black Rock Desert in Nevada: LiveLeak.com - Homemade Rocket Launch.  This launch went to 121,000 ft; approximately 23 miles up.  At this altitude you can start to see the curvature of the earth.  As you can see, amateur rockets are not model rockets.  These are the real things and have to be treated accordingly. 

The Experimental Rocket Propulsion Society is an amateur rocketry group located just over the hill in Silicon Valley.  Their static test facilities are at the Rocket Ranch, which is an amateur rocket test facility in the hills east from San Jose.  I'm not sure how active they currently are. 

Friends of Amateur Rocketry is probably the most active amateur rocket group in the world today.  They operate out of the Mojave Desert in Southern California.  The video below shows a static test of a rocket built by an engineering team at San Diego State University.  Static testing is when a rocket is strapped down so that it can be instrumented to measure things like thrust, pressures and temperatures during operation.  One thing that this video doesn't do justice to is the incredible loudness of a rocket test. 

The X-Prize Foundation sponsors multi-million dollar prizes that are offered for engineering achievements.  I followed this particular contest because one of the entries was a father/son team Unreasonable Rocket.  They just missed sharing the prize because their rocket ran out of fuel a few seconds short of the prize's required time of flight.  But they were competing against two commercial operations that were backed by millions of dollars each. 

2: Amateur Astronomy

The Fremont Peak Observatory is located just south from San Juan Bautista, CA.  I think it might be the largest amateur telescope in the world.  It is open to the public once a month or so.

3: Amateur Radio Astronomy

The Society of Amateur Radio Astronomy lists a number of interesting tech projects that a student can take on; things from listening to the planet Jupiter, gamma ray burst observations, to project SETI.

 4: Amateur Robotics

Something that should have gotten a lot more press than it did was Aptos High School's robortics team taking first place in this years Marine Advanced Technology Education(MATE), competition.  This is an international competition that includes college teams as well as high school teams. and was hosted this year by the Neutral Buoyancy Lab (NBL) at the NASA Johnson Space Center in Houston 

Another fun area is RoboGames.  These events are held just up the road at the San Mateo Fairgrounds.  The most intense to watch and the most demanding, from an engineering standpoint, are the combat robots. 

Here is just a sample of the action from this year's RoboGames.  If you've never been to one of these events, they are impressive!  If you fast foward to about 25:50 you'll see The Great Pumpkin vs.Raging Scottsman.  Raging Scottsman is a high school team from Piedmont High School, Piedmont, CA.

  

5: Medical Device Engineering

Boston Scientific is a major manufacturer and supplier of medical devices.  A fun exercise is to go to their web site, click on the "View All Products" link, and take a look at the wide variety of devices that are used in medicine today.  Most of the devices listed contain additional entries with accompanying detailed descriptions.  And for some of the devices, case studies are also linked to.   Below is a case study for a device I helped design for a company called CryoVascular Systems.       

Acute Popliteal Case Study

Most medical device design is done by small startups like CryoVascular Systems or contract engineering houses such as Circle Medical Devices.  Because the costs of getting a new medical device to market are so enormous, all small startups like CryoVascular Systems are eventually acquired by a large firm such as Boston Scientific.  Only firms the size of Boston Scientific have the resources and international presence necessary to get a new product through all of the reviews and acceptance processes that all of the various nations of the world require before a new device can be licensed, sold and used in them. 


6: Engineering Challenges in the Future

Just to tweak the imagination, here are some engineering challenges that young people in school today can look forward to in the future.  The first is a robot mule from Boston Dynamics called AlphaDog.  The guys you see in the video are probably some of the engineers that designed the AlphaDog.  They get to do fun things like kick their mule as part of its motion stability testing.

This video shows just one example of a whole new area of research and development in the field of prosthetics.  By surgically tying into a patient's remaining nerve endings, the mind can be retrained to use a bionic replacement as if it were just another part of their body.  And it's not just arms, hands and legs being replaced this way, but sight and hearing, too.  The exciting thing for the future in this video is that our patient is controlling the hand with only four nerve contacts being used.  The muscle control nerve in the arm, the radial nerve, has thousands of individual axons.  Imagine what could be done if even a few hundred of these nerve axons could be accessed instead of just four?

This example shows an engineering development that some might find a bit unsettling in its implications.  This robot is being controlled by a culture of rat brain cells being kept alive on a matrix in the researcher's lab.

This last example shows an engineering development in the field of nano-fabrication.  Engineers now have the ability to fabricate structures on an atomic scale, one atom at a time.

Nano-Car on a tarmac of copper atoms.

What is the difference between a Motor and an Engine?

This next post was intended to be about engine efficiencies.  But in the process of thinking about the subject of efficiency, it occurred to me that there is no clear engineering definition of what an "engine" is in the first place.  The reason this is a problem is the following: a measure of efficiency only has meaning when applied to a well-defined system.  And by well-defined, it is meant that there exists a boundary that unambiguously separates all relevant factors into those that are included in the system and those that are not. 

So before I start a discussion of engine efficiencies, I thought I should do a post on just what exactly an “engine” is in the first place.  And a good exercise to start with seemed to be contrasting our usage of the term "engine" with our usage of the word "motor".

If you put the words "engine versus motor" into your favorite Internet search engine, you'll find any number of discussions addressing the various meanings given to these two words.  It would appear from the web-discussions I've looked at, that for most cases the terms motor or engine can be used interchangeably.  But, while saying "rocket motor" or "rocket engine" is equally acceptable, one never says "electric engine", only "electric motor".  This would seem to imply that there is some distinction between these terms. 

It appears from use, that motor is a more general term than engine.  A motor is something that drives/runs a machine/mechanism; while an engine is a particular kind of motor.  But if you then look at all of the various devices that get referred to as "engines", there doesn't seem to be any consistent pattern as to what is or isn't one.  At which point, one's quest to find a useful distinction between the terms motor and engine ends empty-handed.

The reason this is important for a discussion of robotic power sources is that how one defines "efficiency" depends on the nature of the power source one is using.  If one starts with the general notion that a motor is a device that takes energy from some source and converts it into mechanical work, then "efficiency" is a measure of the "completeness" of that conversion process. 

But different laws of physics are going to kick in depending on the nature of a motor's conversion process.  So one always needs to be mindful when comparing efficiency of one choice for a robot's power source versus another, that one isn't making the mistake of comparing apples to oranges.  That is, one always needs to make sure that one's comparison is being made between systems that encompass the same thermodynamic universe.

A perfect example from current events is the fact that an electric motor can always be made more efficient in converting electrical energy into mechanical work than an internal combustion engine can ever be at converting the chemical energy of its fuel into mechanical work.

Those marketing "green technology" use this fact to advertise electric cars as being far more efficient than a comparable sized gas-powered car.  But a true apples-to-apples comparison would have to include into the electric motor's thermodynamic universe the efficiency of the power plant generating the electricity, any losses in the power grid's network, any losses in the battery charger circuit, and etc.

So, in an odd turn around, even though "motor" is used in a more general sense than "engine", the term engine encompasses a much broader system-wide view of the total energy conversion process between the original energy source and the final mechanical work output.

The moral of this tale is that, from an engineering point of view, the term "engine" is ambiguous, so any discussions regarding engine efficiencies will always be required to also include a well-defined description of the thermodynamic universe encompassing one's arena of comparison.

Piston vs. Gas Turbine Engines: Reliability and Maintenance

Reliability:  In my recent searching through a number of aircraft engine related web sites, I came across numerous comments to the effect that gas turbines were more reliable in operation and went longer between routine scheduled maintenance cycles than piston engines.  I have no background in this area, so I only have this anecdotal information to go on.  But it does make sense, since a gas turbine has only one main moving part versus the dozens found in a piston engine.  Also, a gas turbine runs in a continuous smooth manner versus the intense internal pounding that a piston engine is subject to. 

Maintenance:  Any service and maintenance requirements for a 'bot's power source will be inherited from those same requirements for the harvester/tender as a whole.  So with this in mind... 

It would be a violation of the WID Rule  having to add an additional workforce to service the special repair/maintenance needs of a team of robots when there are already farm machinery mechanics on the payroll.

WID Rule, Corollary 2: Regarding operation, service and maintenance, K.I.S.S., since the workforce that will be responsible for these jobs in the future will be the same people that now operate, service and maintain the existing farm equipment.

First, the most likely place for a robot harvester/tender to break down will be out working in the middle of a field.  For this reason, field repairability is a must.  The way to meet this design constraint is to break a 'bot's design into modules that can be easily swapped in/out in the field by someone with a few hand tools and minimal experience.

Second, a field supervisor has enough to worry about already, making sure the crop is picked, packed and moved out of the field on time  The added responsibility of dealing with daily 'bot break-downs is not something they need.  Given that there might be anywhere from 10 to 20 'bots in the field at any one time, the mean-time-between-failure for a 'bot needs to be on the order of 200-500 hours, so that a field supervisor can expect to go, at least day-or-two, without having to deal with yet another 'bot breakdown.   

Lastly, most crops are seasonal.  The ideal situation is for the length of the farming season to coincide with the time for a harvester/tender's major services.  This way, the winter off-season, when most 'bots would be out of service anyway, can be used to do the tear-down and rebuild maintenance necessary to get the 'bots ready for the next season.  This implies a 2,000 to 3,000 hours run time for our 'bot's power source.  

In the end, I don't know if any of these requirements are accessible to either a piston or gas turbine engine.  So I have some homework to do for the future.

Piston vs. Gas Turbine Engines, Cost

The small 1.3 HP Honda GX35 engine pictured in the previous post retails for around $250.00.  But no company makes a comparable commercially available, mass-produced, small gas-turbine engine.  The only small 5-50 HP gas turbines that fit this description are the after-market turbochargers made for car engines.  A quick check on eBay shows these items for sale at under $200.00. 

I’m not ready to consider this crude price comparison a valid cost OME (order of magnitude estimate), but it does make me wonder.  It does seem to indicate that, contrary to popular wisdom, a gas turbine might be cheaper to make than a piston engine of comparable power output.

The one area where a valid cost OME comparison might be made is in the area of aircraft engines.  Both piston and small gas turbine engines are used to power private planes and small commercial planes.  So with this in mind, I spent several days searching the web, looking for information on piston versus gas turbines.  It seems the accepted wisdom in the aircraft industry is that, at the small end of the power/size scale, pistons are always less expensive than gas turbines.  But for all of my searching, I never ran across any hard numbers to back up this accepted wisdom.

What comparisons I did find, always struck me as apples and oranges.  Here is the problem: the smaller the turbine, the faster it spins and the bigger the gearbox needed to couple the engine to the propeller.  Why is this so?  A turbine blade’s ability to compress/expand airflow is a function of its speed through that air.  As the radius of a turbine gets smaller, its rotational velocity has to go up in order to keep the blade’s speed constant. 

So as a gas turbine is made smaller it will require an increasingly higher gear reduction to couple to the machinery it is meant to drive.  So it would seem to me, that at some point, it would be the size/complexity of a gas turbine's associated gearbox that drives the cost of a gas turbine engine, not the cost of the gas turbine itself.

Impedance Matching is a term that has specific and quantitative expressions when applied to various problems in physics and engineering.  But it can also be used as a general term to describe the problem of coupling any power source to its load.  For the case of mechanical systems such as an engine driving some mechanism, impedance matching often takes the form of a gearbox. 

For example, if you look at the picture of the Solar Turbines GS-350 shown in the last post, you will notice that the gearbox looks to be about the same size as the turbine itself.  An industrial generator is constrained to work at a 60 Hz frequency; hence the requirement for a gearbox to couple the high speed rotation of the gas turbine to that of its much slower, by comparison, attached generator. 

Gearboxes? We don’t need no stink’n gearboxes!  But unlike industrial situations that require a 60 Hz AC source, our robot harvester/tender is going to be DC powered.  Since our gas turbine’s AC generator’s output current is going to be converted to DC, this means that we can let it turn at any speed it wants too.  Which further means that one can build our small gas turbine generator set as a single shaft connecting turbine rotor, compressor rotor and generator assembly [1].  

And without the cost of a speed reduction gearbox, it appears that the cost of a gas turbine might actually work out to be less than that of a comparable piston engine.

[1] Noting that an added benefit of this configuration is that the generator can be used in reverse as the turbine’s starter-motor.

Piston vs. Gas Turbine Engines, Power/Size Ratios

When it comes to power/size and power/weight ratios, gas turbines are the undisputed winners over piston engines.  Tanks, heavy lift helicopters and high capacity portable power plants for industry are applications where having the highest possible power/size and/or power/weight ratio is a must.



The M1A1 Abrams uses a Honeywell AGT1500 gas turbine engine [1]

 

Robinson Helicopter Co. R66 gas turbine [2]



Solar Turbines, GS-350, 225 kW generator set [3]
Note: the generator is bigger than the turbine!

The first question that needs to be answered is, what are the size and weight constraints that the design of our robot harvester/tender puts on a possible choice for its power source?   We can get these design constraints by remembering that every design problem always brings with it its own size and weight scales.  So as long as the size and weight of our 'bot's power source is less that 5-10% of its overall size and weight, there is going to be no marginal benefit to making it smaller.

The target size and weight for our 'bot, that we estimated previously, was on the order of 6-8 ft long and less than 300-400 lbs.  This puts a size limit of 1-2 cubic-ft and 10-20 lbs on our 1-2 kW power source.

Here is a 1.3 HP gas engine from Honda that fits our design constraints easily.

Honda GX35, commercial lawn and garden engine [4]

Compared with a small gas turbine from BladonJets that also fits our design constraints. 

Bladon Jets, Micro Gas Turbine Engine [5]

So it seems that either choice of power source, piston or gas turbine, will easily fit into our robot harvester/tender's size and weight design constraints.

And just for comparison, here are a few spec's for a comparable reformed methanol fuel cell.
UltraCell's XX55: 50 W, 3.5 lb, 12.3" 3.2" 8.6" l/w/h

[1] M1A1 AGT1500 spec's
[2] Robinson Helicopter Co. R66's spec's
[3] Solar Turbines
[4] GX35 spec's: 1.0 kW, 7.6 lb, 8" 9.2" 9.4" l/w/h
[5] Bladon Jets

Piston vs. Gas Turbine Engines

The last few days I've been searching the web, without success, for a nice concise comparison between piston engines and gas turbine engines.  In hindsight, it now appears that my lack of success comes about because both types of engines have their advantages and disadvantages.  And how one engine type compares to the other depends entirely on the particular application.

The main points of comparison between engine types are as follows:

power/size and power/weight ratios,

thermal efficiency,

cost, and

reliability and maintenance. 

Future Posts: I started this inquiry assuming that a gas turbine would be the clear choice for a small 1-2 kW robotic power plant.  But now I’m not so sure.  In the next few posts I’m going to separately explore each of these areas of comparison with a robot harvester/tender as the target application.

Agricultural Robotics, BigDog

BigDog is a gas powered robot produced by Boston Dynamics [1].  This robot design is interesting because it overlaps in many areas with the design of an agricultural robot harvester/tender.  There are some critical differences, though, which, by way of contrast and comparison, can help us better understand some of the design issues facing agricultural robotics.

Here are some spec's for the BigDog cited on Wikipedia.
_Dimensions: 2.5 ft tall, 3 ft long
_Weight: 240 lbs
_Engine Size: 15 HP go-kart engine
_Computer: PC/104 stack, Pentium 4, QNX real time operating system [2]
_Speed: 8 mph
_Carrying Capacity: 340 lbs 

Size: One of the main reasons to do order-of-magnitude estimates (OME's) is to get a feel for the size of a design problem before you start.  But doing OME's requires a good engineer's intuition to start with.  The BigDog is a great working example of a robot that fits the profile of a robot harvester/tender.  As such, it offers a great test case to exercise one's OME intuition skills.  

In physics and engineering, every problem brings with it its own size, mass and response-time scales.  For the case of a robot harvester/tender, it has to be bigger than the plants it will be working on, yet it will have to be light enough to be "wrangled" without the use of extra equipment to pick it up and/or move it around.  This indicates something the size and weight of a motorcycle; that is, dimensions of 3-5 ft and 300-400 lbs or less.

So, if the BigDog can be used as a good example, then it appears that our attempt at an OME has proven valid. 

Engine Size: Here is where the design spec's for a robot harvester/tender will differ from those of the BigDog. 

The BigDog, by the nature of the tasks it's designed to do, will experience high peak loads with large swings between low and high continuous loads.  This calls for a power source that is both "throttle-able" and has a reasonable power response over a wide range of loads.  The engine that fits this requirement is a piston-driven ICE. 

On the other hand, a harvester/tender ‘bot only needs to move along at a steady speed of 2-20 ft/min.  The tender ‘bot, only needs to carry is its own weight.  While, the harvester ‘bot, will need to carry the extra weight of the produce it’s harvested and the weight of any packing boxes it will need.  Either way, the total work load for a harvester/tender will be lower and much more even than that of the BigDog.  In which case, one can sacrifice the requirements for "throttle-ablity" and wide power-band for a power source with a much greater efficiency than a piston-driven ICE.    

Next Post: Piston vs. Turbine Engines.

[1] I would recommend that people check out their website and look at the various robots they build.  Boston Dynamics has its own YouTube Channel , too.
[2] This is actually not a very large computer core by embedded-systems standards.

Agricultural Robots: Power Source, Batteries, Why Not?

There are no doubt new battery technologies, being worked on in the lab these days, that will out-perform the Chevy Volt's Li-ion battery.  But for now, the Volt's battery can be considered a good representative of what is "state-of-the-art" for a "light-weight" large capacity battery, manufacturable in production quantities. 

Chevy Volt Battery, without cover

It's a "T" shape, roughly 5-ft long and 3-ft wide.  It has a weight of 435-lb (197-kg), and volume of approximately 15.0 cubic-ft.  Its capacity is rated at 16-kWh, but for reasons of longevity, its usable capacity has to be de-rated down to about 12-kWh.

The Chevy Volt's listed curb weight is 3750-lb, (1750-kg).  The weight of the Volt's battery is not a factor in the car's overall performance because the battery represents only 12% of the car's total weight.  This means that the marginal increase in rolling friction this extra weight brings is only 12%. 

At 35-mph, the aerodynamic drag for a typical car is comparable to its rolling friction [1].  So the extra drag on the Volt, caused by the extra battery weight, will amount to less than 6% of its total energy consumption when driving down the road at freeway speeds.   This is the reason that the Volt can get away with being battery powered. 

Now look at the situation for our robot harvester/tender.  Its speed of travel, for most cases, will be on the order of 2-20 ft/min.  Aerodynamic drag will never be a factor.  The overwhelming amount of the energy it uses for locomotion will be expended moving its weight around.  In this case, the weight of the robot will be the dominant factor in determining its power needs.    

The lighter the robot, the quicker it can move, and the faster it can perform its harvester/tender duties.  Adding a 400 lb battery to a robot that is going to weight less than 300-400 lb to start with, will compromise its performance fatally.

One could put a smaller battery in our robot harvester/tender, but then it could only run for an hour or so before needing a recharge.  Now imagine trying to run a cluster of ten to twenty robots in a field at one time, each of them needing to be taken out of service every hour or so for recharging.  Our field supervisor would need to bring in extra help to service the 'bots that needed charging, and also bring in extra 'bots to fill in for the 'bots being cycled out of service for recharging.  This scenario becomes a major violation of the WID Rule.       

So what is the long term outlook for battery powered industrial robots?  One only need look at the specific energies for the different power sources to get an answer. 

--One gallon of gas, burned efficiently in an ICE, has a specific energy of about 10,000 Wh/kg.
--The Chevy Volt’s Li-ion battery has a rated specific energy of about 80 Wh/kg [2].

By weight, a gallon of gas holds over 100 times as much energy as the same weight of a Li-ion battery.  So even if there were some major breakthrough in battery technology that yielded a factor of 10 times the storage capacity over the current Li-ion batteries, it would still fall short of the energy storage capacity of a liquid-fuel based power source by another order of magnitude.

For a liquid-fuel, energy is stored in the chemical bonds of its molecules; therefore every molecule is an energy storage unit.  In contrast, a battery stores energy in the chemical potential of two ions physically held separate.  Thus, a battery will always require the presence of some inert matrix to hold the two ions apart.  A battery also requires the presence of cathode and anode terminals to provide a pathway for the battery's stored energy to reach the outside world.  In other words, a battery will always contain a lot of extra structure that contributes nothing to its energy storage capacity.  For this reason, on a pound-per-pound basis, a battery will never be able to compete with a liquid-fuel.

To avoid violating the WID Rule, once a robot is in the field and working, it needs to be able to run without attention for a full 8-10 hr shift.  This requires a power source based on an energy storage capacity that will forever and always be out of the reach of batteries. 

Sadly for the pro-battery folks, batteries may be a workable solution for robot toys, but they will never be a viable power source for industrial robots that will be required to work in the field for extended periods of time.

Next Post:  New options for liquid-fuel based power sources

[1] When I can, I'll get the calculations behind this estimate posted over at my web site.
[2] For a comparison, a standard 12V lead-acid car battery has a specific energy of about 35 Wh/kg.