Let's Make Robots!

"Wild Thumper" 6WD all terrain robot chassis


Vendor's Description: 


It's Here! Now with Video of it in Action on dirt using 34:1 gearboxes and new videos going down and up stairs and over snow with 75:1 gearboxes.

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Now available in silver!

Designed originally here on LMR specifically for robots, this 6WD chassis has wicked spiked 120mm dia. wheels and an anodized aluminium chassis made from 2mm thick plate. The chassis has 4mm dia. holes punched every 10mm to allow easy mounting of PCBs, servos etc. All nuts, bolts and screws are stainless steel. Brass fittings and suspension springs are nickel plated.

Two chassis segments between the wheels have been designed to hold 7.2V sub C battery packs (not included) as used in many RC cars. A total of 4x 7.2V battery packs can be fitted if necessary. These batteries are ideal for driving the 6V motors as well as inexpensive linear regulators to supply 5V.

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These photos show how even with one wheel resting on a large LMR mug (115mm high) all wheels are still touching the ground. It may not be a rock crawler but that's awesome for an off the shelf robot chassis.

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Ground clearance with the suspension lightly loaded is 60mm which is almost half of the total 130mm height when the topdeck is mounted on 25mm spacers.

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As you can see in this photo, with the top deck fitted you have a smooth deck to mount equipment on even when the chassis is flexed. Mounted on brass hex spacers, the top deck gives you room underneath to hide cables and PCBs to give your robot a cleaner more proffesional finish.

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The suspension for the front and rear is a single torsion spring. As the motor housings are connected to each other and not the chassis the front and rear can roll freely with the spring supporting weight and absorbing shock.

The steel cable is used to limit spring travel. The motor housings have 3 holes for each allowing spring tension and travel adjusment depending on the weight of the robot. Rubber grommets are fitted in the spring mounts to eliminate play while allowing the motor housing to roll.

The center suspension is similar but has 2 springs. Each spring connects from the motor housing to the chassis keeping the robot upright.

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The 6 powerful geared motors have steel gears and are fitted with powerful magnets to provide high torque. Top speed is about 6.6km/hr. These motors have a maximum stall current of 5.5A each so with 3 motors per side wired in parallel a dual "H" bridge rated for at least 17A per channel is required.

Rated voltage: 6V DC (min. 2V Max. 7.5V)
Stall current maximum 5.5A
No load current per motor is 350mA
Motor RPM is 10000 +/- 5%
Gearbox ratio is 34:1
Output shaft speed is 295rpm +/- 5%
Stall torque is 4Kg/cm

A 75:1 gearbox is now available!
This gives a top speed of about 3Km/hr and a massive stall torque of 8.8Kg/cm per wheel!!!

The chassis comes pre-assembled and weights 2.6Kg. Shipping cost to America or Europe will be approximately $48 USD.

Now available in 2 colour schemes:
Black with metallic red rims
Silver with chrome rims

Here is a preview of the 4WD version:

4_wheels-1.jpg

 

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Thanks OddBot for your quick response.

I am asking bec I already have a couple of pololu's motors with encoders at the same gear ratio 75:1, so if I repalced the front motors in a 4WD WT with the the couple I have from pololu's; should I notice this power difference? Is that power difference in the delivered tourque or speed? Also in a diffrential drive chassis should this be a problem at all?

Alright so I've got the wild thumper chassis and motor controller up and running. I'm using an arduino and ps2 controller to wirelessly send pwm values to the controller with some cheap rf modules. I currently have the left motors mapped to the left stick, right on the right stick with the sticks centered at 255/2. I'm curious as to what pwm values are safe for the motors (0-255)? I'm using a 7.2V 3800maH nimh. I currently have a deadband between 108-148 for the controllers and I have the bot enter brake mode, and then forward when above 148 and reverse below 108 with scaling factors to make it to 255 for full speed. What is the minimum pwm I should be sending to the motors for safety? I've noticed if it is too low, there is too much torque required and they whine a little bit leading me to believe theres quite a large stall current occuring in that time. Also, while in brake mode and leftpwm=rightpwm, if both are set to say 200/255, will that short be eating up current? While braking, does the pwm need to return to 0? Thanks

 

    //Brake

    if (108 < mydata.left_stick_y < 148)  Leftmode=1;

    if (108 < mydata.right_stick_y < 148)  Rightmode=1;

 

    //Forward

    if (148 < mydata.left_stick_y) 

    {

     Leftmode=2;

     LeftPWM=(mydata.left_stick_y-128)*2;

    }

    if (148 < mydata.right_stick_y)

    {

     Rightmode=2;

     RightPWM=(mydata.right_stick_y-128)*2;

    }

 

    //Reverse

    if (106 > mydata.left_stick_y)

    {

     Leftmode=0;

     LeftPWM=(255-mydata.left_stick_y*2);

    }  

    if (106 > mydata.right_stick_y)

    {

     Rightmode=0;

     RightPWM=(255-mydata.right_stick_y*2); 

    }

 

No damage is being done but yes it can flatten your battery quicker. Part of the noise you are hearing is the AC component passing through a noise suppression capacitor used to suppress electrical noise.

A technique I developed for low speeds was to have a second PWM at an even lower frequency. For example, lets say that PWM values less than 45 will not move the chassis (on a flat surface with no obstacles). Then what you do is choose a level above that as your change over point. In this case we will use a PWM value of 100 to allow for a slight slope or mildly rough terrain.

When your PWM values are 100 or more then drive the motors normally as you do now.

When your PWM value falls below 100 then toggle your motor PWM between that value and 100 at a relatively low frequency of say 50Hz. Perhaps the simplest way to do this to toggle a value in you main loop. When that value is true set the motor PWM to 100, when it is false set it to the lower value. If your main loop runs very quickly then you may want to use a timer to determine a roughly 50Hz toggle frequency.

The end result is that below a certain speed you start pulsing the motors to prevent them stalling. You will need to experiment with the upper pulse level (100 in my example) and the toggle frequency as a lot will depend on the weight of your robot and the terrain.

I used this technique to make this video of the 4WD Wild Thumper: https://www.youtube.com/watch?v=acdvdwHyogE

It was the only way I could control the chassis at very low speeds to pick up the servo and shot glass. If you listen carefully you can hear the low frequency pulsing of the motor. Gareth has also used this tequnique with a Mr. Tidy for low speed maneuvering without stalling.

Do you mind sending me the code that you've been using for the 4WD Wild Thumper?
I do also have an 75:1, which I wish would go a bit slower.
At the moment It cannot go below 100 on the PWM.

It should be able to go below 50.What sort of battery are you using? how thick are your power wires?

 

I think my battery was a bit low when I tried last time, but now with a charged battery I couldn't go below 65 on my right motors and 80 on my left. ( There's allways about 15 in difference between the both sides).
It's still a bit to fast tough, so I would like it to go slower than that.

I use a 7,2 V, NiMH, 3000 mAh RC Battery and 2,5mm2 cables for the power.

Im using a 6WD, and not a 4WD as you used in the video btw, if that would make anything different.

I'm sorry but I can't find the code I was using.

It was basically the sample code but once your speed got below a certain level it would start pulsing the power. Every time through the loop it would alternate the power between a predetermined value and whatever your speed was. It was effectively pulsing the PWM.

By experimenting with the predetermined value, frequency and even duty cycle you can get the speed down lower than normal without stalling.

Thanks for the tip, I was definitely looking into programming a low speed mode. What is the default pwm frequency? I'm still a little confused about the electronic braking. Is is something I should be using whenever the robot comes to a stop? Currently I have it braking whenever the sticks return to the middle position but this results in a pretty abrupt stop if you have any decent speed going. Would I be better off implementing a short slow down to stop and then brake, or should braking be separate altogether and just return pwm to 0 while sticks are centered? Also, would there be large current draw from the motors during braking if pwm was 255? Thanks for the help, I haven't done much with motors so I'm not sure how to treat them properly!

  1. default PWM fequency is about 490HZ on one motor and 980Hz on the other. These are Arduino defaults. Do not change them!!! People keep trying to change them to high frequencies to make the motor quiet but the motor driver circuit is not designed for high frequencies and will be destroyed!!!!
  2. You don't need the brakes with the 75:1 gear ratio and probably won't need them for the 34:1 unless your robot is really heavy. You can PWM your braking for a gentle brake or just coast the motors.
  3. Refer to #2. If your in brake mode then the PWM is breaking.

Yes you can drive any brushed DC motor as long as the total current does not exceed 15A per channel and you do not use more than 12V for your power supply.