AC Motors vs Hydraulic motors for skid-steering robot

Question

I'm designing a mobile robot with tracks and skid-steering driving system and I am evaluating whether an electric motor would be better than a hydraulic one.

I did some calculations, the total weight of the robot is 1200Kg, there are two rubebr tracks and their sprocket had a diameter of 0.3 meters. The maximum velocity should not exceed 0.5 m/s and I do not think the robot will have to face slopes greater than 20%.

If I use the hydraulic system, I would go with: - 1 x 12kW AC Motor, 48VAC, 2900 RPM - 2 x Oleodynamic pumps 8.5 cc/rev, 120Bar working pressure - 2 x OMP 250 hydraulic motors - 1 x AC motor controller 300A - 1 x proportional valve controller - 2 x proportional hydraulic valves - 1 x 19kW/h battery pack

If I use the electric solution, I would go with: - 2 x 2500W AC Motor, 48VAC, 2200 RPM - 2 x gearbox 1:35 - 2 x AC motor controllers 300A - 1 x 19kW/h battery pack

If my calculations are correct, the first solution should be easy to maintain while the second one should be easier to implement.

What can you suggest? I usually would go with the second solution, but since it is a big machine, I would like to save some money and keep the system easy to check, maintain and control.

Answer 1

A hydraulic system requires a hydraulic power unit (hpu), and then you still need a motor everywhere you want an electric motor, and then you've got hydraulic hoses (carrying full hydraulic pressure) running through your robot.

The hydraulic system will not be easier to maintain, will not be easier to install, and I'm pretty sure it'll be dramatically more expensive.

The only time I would consider hydraulics are:

1. If you have space constraints that really require the actuator to be as small as possible, or
2. If you only intend to operate one axis at a time (and thus can essentially have an undersized HPU), or
3. You really need to use a fossil fuel system for motive power, or
4. There's already hydraulics in use for some other aspect.

Especially vs. an AC motor, maybe the only thing you have to do for the AC system is grease a bearing. The hydraulic system has all the hoses to purchase or make up (don't screw that up!), hydraulic reservoir to fill, lines to bleed, etc. And if one of those lines ruptures (because everything fails eventually) then it could ruin the rest of the robot.

And did I mention it's expensive?