Turning a 4 Wheel Chassis

[AceOfHearts]

Hi everyone,

I have a chassis that has four wheels. Im finding it difficult to turn the vehicle with any type of differential turning technique ie. increasing power on one set of wheels / one set of wheels go in reverse.

Theres too much drag / resistance when attempting to turn because of the 4 wheel configuration...What is the recommended way of turning?

regards.

 

[3v0]

This is similar to a skid steer loader. The wheels have to skid when it turns. It takes a lot of power to turn this sort of system when traction is good.

Try running it on a surface with less traction.

 

[AceOfHearts]

OK thanks....i have been trying it on my carpet floor. I suppose theres no other way around it but to increase power and reduce friction of the floor.

Looks like Im going to have to power my 7.2V motors with a source nearer to 7.2V as at the moment Im using a 5V regulator...

hmmm...is it unhealthy to drive a 7.2V motor with higher voltage (say 9V) PWM pulses?

I had not envisaged this problem with turning before...I suppose if this the way people are doing it, it must be the only way....I was used to using a two wheel system where this problem did not exist.

 

[Nigel Goodwin]

hmmm...is it unhealthy to drive a 7.2V motor with 9V PWM pulses?

Not at all, you don't want to feed them from 5V, particularly through a regulator which will get red hot and will current limit from the current the motors try and take.

Even without PWM 9V feeding a 7.7V motor is fine - it just goes faster

What is your 9V source anyway?.

 

[AceOfHearts]

Not at all, you don't want to feed them from 5V, particularly through a regulator which will get red hot and will current limit from the current the motors try and take.

Even without PWM 9V feeding a 7.7V motor is fine - it just goes faster

What is your 9V source anyway?.

Hi Nigel,

Im using a 5V 2A regulator mounted on a large heatsink to drive the motors. 2A is way above the current drawn by my motors and the heatsink nicely dissipates the heat.

Im not using 9V source yet....but..if thats the case (about being capable of using higher voltage then rated)...then I might buy a 9V 2A regulator to drive the motors....but I have one concern though, whats the purpose of the rating on the motor then? There must be a reason why its there? Thanks for the help.

 

[AceOfHearts]

Just tried the system on laminated floor...turns like a charm off 5V, could even do with a bit slower. I'll stick with my 5V 2A regulator.

 

[Nigel Goodwin]

Hi Nigel,

Im using a 5V 2A regulator mounted on a large heatsink to drive the motors. 2A is way above the current drawn by my motors and the heatsink nicely dissipates the heat.

Im not using 9V source yet....but..if thats the case (about being capable of using higher voltage then rated)...then I might buy a 9V 2A regulator to drive the motors....but I have one concern though, whats the purpose of the rating on the motor then? There must be a reason why its there? Thanks for the help.

The rating is fairly conservative, and intended for continuous use - in such things as Robot Wars it's normal to run 12V motors at 24V, 36V, or even 48V - where you get masses more power and speed, but no where near continuous running. Apparently car windscreen wiper motors run perfectly well, and continuously, at 24V.

If you can run two motors off a 5V 2A regulator they must be really tiny low power motors?, even normal small model motors easily take over 6A each under heavy load or stall conditions.

 

[3v0]

I would ditch the regulator. The max speed and torque will be higher without the regulator. You can dial that back with PWM to get the same effect (speed) as with the regulator and have lots of reserve power.

Regulating for the motors results is short battery run time.

 

[Krumlink]

The regulator is a waste, simply find a power source near the voltage needed (example, is you need 5VDC so get 4AA's, 4.8).

You should turn an entire side or link the front and back wheels together on one side, so it results in tank drive. The FRC robotic teams do this, or they have omni wheels in the front so it can go forward with lots of traction and turn on itself.

some rather small ones:
**broken link removed**

 

[dknguyen]

If it's indoors and you don't need to climb "step-type objects" usually 2 wheels is best since it's just simpler, more agile. You can either move the wheels closer together, or a front wheel on one side a bit lower and do the same with the rear wheel on the opposite side. Or move all the wheels closer to the center.

Cars only really use 4-wheels because the way the transmission works and it's more stable than 4 wheels.

The linear regulator is a waste because you aren't saving any battery power. The amount of power being drawn is the same with and without the regulator- the only difference is not all of it is getting to the motor because the linear regulator is burning off some of the energy. What you are doing is comparable to sticking a bunch of resistors (or more accurately voltage-dropping diodes) in series with the motor. Motors do not need such an accurate or quiet supply.

 

[govardhan]

Hi everyone,

I have a chassis that has four wheels. Im finding it difficult to turn the vehicle with any type of differential turning technique ie. increasing power on one set of wheels / one set of wheels go in reverse.

Theres too much drag / resistance when attempting to turn because of the 4 wheel configuration...What is the recommended way of turning?

regards.

what if u use a larger power motor 2 giv it a gud torque 2 move on

 

[Sceadwian]

Turn each wheel 45 degrees, this allows a 360 degree turning radius on the spot, if you're using steppers or feedback you can achieve forward motion by 'snakeing' locking one wheel and turning the rest, almost like a walking bot. Ideally you want to control the axis of rotation of each wheel either via a common motor through a series of cams to each wheel axis or some other equivalent system, such a system eliminates having to 'walk' the bot but is mechanically complex. Look at the Mars rover bots for design considerations, they were designed for ultimate maneuverability and high efficiency.

 

[yngndrw]

If you angle the front wheels towards the front slightly, as well as the rear wheels towards the back, that will improve steering.

Remember that by doing this, you are adding to the resistance for forwards/backward movement. It's a trade-off between the two, but a little bit shouldn't hurt.

 

[saturn1bguy]

I have a chassis that has four wheels. I'm finding it difficult to turn the vehicle...What is the recommended way of turning?

Do you mean a 4-wheel drive chassis, one motor per wheel? If so, do what Kumlink says, have it pivot like a (battle) tank. Drive both right side motors FWD, both left side motors REV, and you get a CCW spin, or pivot, in place. No dragging, and it works well on all surfaces.

(See the following URL if interested. The vehicle shown does precisely this, FYI:
**broken link removed**)

 

[AceOfHearts]

Thanks guys. Appreciate the help.

Well my vehicle is really quite small...about the size of the the vehicle in thelink above, not quite the size of the robots in Robot Wars! So my motors I would say are lower in power than Robot Wars thus draws less current.

Yeah, I will consider taking out the regulator for the motor...that will allow the vehicle to turn on rougher terrain as its not feeding from 5V rather from 12V. I am confident this will make very good improvement.

Thanks again.