Half a croissant, on a plate, with a sign in front of it saying '50c'
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Balancing without controlling wheel speed

Move the axle frontward/rearward as needed.
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(+4, -1)
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It's well known that the problem of self balancing can be solved by making use of powered wheels -- and the most well known example is the Segway PT.

However, there is one major drawback of this technique -- since these systems depend on electric motors, the available torque decreases as speed increases, which means that above a certain speed, the vehicle cannot prevent a forward fall. Also, as vehicle speed increases, the vehicle must tilt further and further forward -- bringing the front of the vehicle dangerously close to the ground. In addition, in order to balance at low speeds, low backlash between motor and wheels is required, which could result in the gears binding at high speed due to thermal expansion.

This is an idea for an alternative means of balancing, one which will work at any speed, without tilting the vehicle relative to the ground, without /requiring/ powered wheels. And of course, if the wheels are powered, the system will still keep the vehicle balanced. Even if the wheels are locked (due to brakes or transmission), the system should still be able to keep the vehicle balanced.

Basically, the wheels' axle is attached to an assembly that is below the rest of the vehicle; this assembly is attached to the vehicle using linear bearings which allow it to slide forwards and backwards relative to the rest of the vehicle.

In addition, there is a linear actuator of some sort (hydraulic pistons, or a linear motor, or a rotary motor driving a ball screw, etc.) which moves this assembly frontward or rearward in a controlled manner.

When the vehicle undesirably tilts forward, the assembly is moved toward the front of the vehicle to compensate; when the vehicle undesirably tilts backward, the assembly is moved rearward to compensate.

Determining the tilt of the vehicle should be done by measuring the distance between the vehicle and the ground at the front and rear (probably using a pair of infrared range finders) instead of an accelerometer. This allows the system to keep the vehicle parallel to the ground just as easily on sloped ground as it does on level ground.

Since the ability to move the assembly frontward or rearward is /independent/ of the vehicle's speed, balancing can be done even when the vehicle is moving fast.

In common with a conventional self balancing vehicle, is a PID controller, and perhaps a gyroscope.

goldbb, Mar 23 2011


       I like it. Would be helped if people wore helium filled 10 gallon hats too. They would tend to keep people up the right way:)
saedi, Mar 23 2011

       This is a clever idea.   

       One question (not that I know a lot about Segways). In a Segway, if you tip forward, the wheels do their stuff in such a way as to (a) provide torque to right you and (b) bring the wheels back underneath you. Is this about right? Two effects?   

       In your system, the restoring force is provided by a "see- saw" mechanism (you shift the see-saw's pivot point to correct the tipping). Will this provide enough righting force to prevent tipping?
MaxwellBuchanan, Mar 23 2011

       Why would this system require any less effort to achieve stability than a wheel speed control?   

       The system needs an additional motor to drive the stabiliser mechanism. If that motor's power were added to the drive motor power, you'd achive the same effect.   

       Using some of the power of the main motor for stabilising makes sense. The motor is already there and it doesn't need the weight and bulk of another mechanism.   

       It's also worth mentioning that moving the axle would still require more torque from the main motor - every action has an equal and opposite reaction - so stability would still be speed dependant.
Twizz, Mar 24 2011

       This is very interesting, but you'd have to change the driver support mechanism as this would lead standing people to fly over the handlebars. I do agree with [Twizz] in that this requires an additional motor system that would require a lot of power. So it doesn't seem like a net gain, but interesting enough to earn a bun. (+)
MisterQED, Mar 24 2011

       //Also, as vehicle speed increases, the vehicle must tilt further and further forward//   

       Mmmm, not sure about that...
Ling, Mar 24 2011

       The drawback is weight in that an additional motor of some type is needed.
RayfordSteele, Mar 24 2011

       MB, yes, two effects; however, since the wheels of a Segway aren't any more massive than normal vehicle wheels, it's mostly (a), getting the wheels back under you.   

       If the wheel carrying assembly can travel the full length of the vehicle, then any amount of acceleration that this system cannot compensate for is an amount which would similarly cause a regular car to flip over.   

       Twizz, let's imaging that you've got a self balancing remote controlled robot, which uses only it's wheels to balance. Furthermore, you've got it speeding down a road at it's maximum speed... the force applied to the motors is exactly matched by wind resistance. Now, suppose that a sudden gust of wind gives it a push from behind, and it starts falling forward. What can the robot do to avoid smashing it's face into the road? Nothing!   

       Now, let's imaging a two wheeled robot which balances using the principle described in my idea. It's also moving at it's maximum speed down a road. It will be level to the ground, and have it's axle assembly somewhere behind it's center to balance out wind resistance. A sudden gust of wind gives it a push forward. What will it do? Move it's axle frontwards, and /not/ smash it's front bumper into the ground.   

       And, I don't see what you mean when you say that moving the axle frontward/rearward would need power from the main motor. Explain?   

       MisterQED, this is not for a stand-on vehicle like a Segway... it's move for something car shaped. So if there were a driver/passenger, he/she would be sitting, not standing.   

       Remember, unlike a Segway, this kind of balancing vehicle will remain parallel to the roadway, which means that one can look straight forwards and see where one is going -- there's no need to crane one's neck upwards (one reason why one must stand to use a Segway).   

       And there's no reason why power consumption would be any greater than in a vehicle that balances by means of powered wheels.   

       Ling, don't forget air resistance! :)   

       RS Yes, but how heavy does that additional motor need to be? Electric motors often have a high power to weight ratio.   

       bigsleep: I'm more concerned with getting a vehicle up to highway speeds, than putting a vehicle on a ball.
goldbb, Mar 24 2011

       The trick is never to use the full power of the motor at full speed. Whether you do this by running one big motor at 90% power, or by having a second motor doing nothing until required is up to you. Either way, you need more power than is required to maintain full speed. A single motor is a lighter, more effecient way to do it.   

       Your robot will have a lower maximum speed, as it will be carrying an extra motor and mechanical system to balance.   

       Here's another thing: Torque from the drive motor can be altered almost instantaneously, while the moving axle will take time to travel from mid position to either end of it's travel. This introduces a lag into the system. Lag is a disaster for the control.   

       The additional mechanical system means that power consumption will certainly be greater than a wheel torque stabilised device.   

       I can't see how you expect the thing to remain parallel to the road at speed. Even at 20mph, a runner has to lean forward significantly to remain balanced. What happens if the road is not perfectly flat? This thing will try to keep itself parallel to every pothole and other surface imperfection.
Twizz, Mar 25 2011

       If the wheel motors aren't necessary for balancing, then there's no reason why they need to be electric motors -- a gas engine could power them just fine. And, if a gas engine is used, then torque can't be changed instantaneously.   

       Lag is definitely a potential problem, but how fast will the axle need to be moved frontwards or rearwards?   

       The additional mechanical system will of course use power, but no more power that would otherwise be consumed by the wheel motors the purpose of balancing.   

       A runner at a constant 20mph needs to apply rearward force to the ground to balance wind resistance. To keep his center of gravity "above" his feet (when considering the combined upwards and forwards forces the ground is applying to his feet) he must have his feet behind his center of gravity. The only way for a human to accomplish this is to lean forwards.   

       A vehicle that balances as described in this idea, will, when moving at a constant 20mph, also need to keep it's center of gravity in front of the location it touches the ground. This can be accomplished without tilting the whole vehicle, but instead moving the wheels slightly rearward.   

       And if the road isn't perfectly flat... we can use a moving average of the distance sensors, instead of the raw data, to determine how tilted the vehicle is... using the same type of filtering function that's done with data from the accelerometer in a more conventional balancing vehicle.   

       Thus, the balancing system will ignore potholes while it's moving, but will adapt to the (much less transient) changes in road angle as it ascends or descends a hill.
goldbb, Mar 26 2011

       Before I forget, there's one huge benefit I forgot to mention, in case this is being considered for use on an autonomous robot: Since the vehicle remains parallel to the ground, we don't need to design the cameras to look "upwards" in order to see forwards when moving at speed.
goldbb, Mar 26 2011


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