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I am trying to assess the pros and cons of steering a robot car using different speeds of 2 or more DC motors versus using a servo and a steering mechanism? From your experience which is better in terms of:

  • Steering accuracy (e.g. prompt responsiveness or skidding while on higher speeds)
  • Efficiency in electrical power consumption
  • Durability and maintenance
  • Control complexity (coding and electronics)

I researched and understood how both approaches work, but I need some practical insight to select the most suitable approach. Any hint or research direction is appreciated.

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    $\begingroup$ Can you define "steering accuracy" and "power efficiency"? $\endgroup$ – Chuck Jul 11 '16 at 19:21
  • $\begingroup$ I modified those two points. Sorry if it wasn't clear, and thanks for pointing it out. $\endgroup$ – Kalaji Jul 12 '16 at 21:34
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I have been working on upgrading an autonomous robot as part of a university project for the past year. As part of my work I have been switching the old skid steering system to a new active steering one, so have some experience with both methods.

Steering accuracy
Skid steering: Fairly good, need to use a PID loop to keep an accurate heading using a compass.

Active steering: Depends on how you are controlling it, if controlling it manually then it will be much worse than skid steering for accurate turning. When used with a control system, assuming you have some way of measuring steering angle and motor speed, it will be much more efficient, probably slower, but more accurate at keeping a heading without the need for a PID loop and compass

Efficiency in electrical power consumption
Skid steering: Not very good, you waste a huge amount of power every time you need to turn or correct your direction while moving. Most power is wasted when the wheels are skidding as it takes a lot of power to do so.

Active steering: Fairly good, since the motors are never fighting themselves it is much more efficient, although you do have to take into account the actuators doing the steering.

Durability and maintenance
Skid steering: Very Robust if mechanics are done properly, the skidding puts huge loads on the axles and motors, so much sure they are up to scratch. Maintenance is mostly making sure the tires haven't completely worn through from all the skidding!

Active steering: In my experience the added complexity leads to a huge amount that can go wrong, but again if the mechanism is well designed and components over-specced appropriately then it can be just as good as skid steering. It depends a lot on how exactly you’re doing the steering - the robot I’ve been working on has steering on the same wheels that are motorised, as well as suspension, so the mechanism is fairly complex (ball joints, double-wishbone suspension etc.) For maintenance you need to check a much larger number of parts and keep the mechanics in good shape.

Control complexity (coding and electronics)
Skid steering: Very easy, both forward to go forward, opposite directions to turn, slow down one side to turn while moving forwards. Best manual control, but does require a PID loop and compass for holding an accurate direction when used with a control system. For drive electronics you just need motor drivers for each wheel, relative power levels are normally sufficient for steering.

Active steering: Insanely complex or easier than skid steering (depending on how you are controlling the robot!) If you are remotely controlling it and can have controls for steering angle and motor speed then it is very simple, although skid steering is still easier to drive manually. However if you want to control the steering with software and do path planning etc, it adds a lot of complexity to the control system. What angle do you turn the wheels? How fast do you turn each wheel for a given steering angle? For drive electronics you will need a way of controlling your steering actuator and a way of knowing what position it is in. You also need to know the speed of each motor so you can control the exact speed of each one (depending on how beefy the mechanics are this might not be necessary). For the robot I am working on if you drive the motors at the wrong speeds then it will probably rip the wishbones right off the robot!

Summary
Skid steering:

  • Simple mechanics
  • Simple control (both manual and autonomous)
  • Easy to construct with good durability
  • Inefficient power-usage

Active steering:

  • Possibly complex mechanics
  • Simple control only with steering angle feedback
  • Lots of component probably required, so lots of maintenance
  • Can be more efficient

While active steering is certainly more impressive when it works properly, it’s a lot harder to achieve with the control system alone being insanely complex. In retrospect to my own projects, I wouldn’t recommend active steering unless it’s absolutely necessary.

I would however suggest some investigation into holonomic robotics. These use special wheels to allow movement in all available degrees of freedom in 2D space with the same mechanics as a skid-steering robot.

Here are some photos of the old and new robots: Tiberius 2 - Skid steering Tiberius 2 - Skid steering Tiberius 3 - Fully independent suspension and steering Tiberius 3 - Fully independent suspension and active steering

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    $\begingroup$ beautiful vehicle $\endgroup$ – holmeski Jul 14 '16 at 18:21
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Automotive style steering can actually be simple. The conventional car has just one motor that powers two rear non-steering wheels through a differential. The differential solves the problem of how to compute a speed for each wheel. The front wheels are not powered so we don't have to compute their speed. Ackerman Steering moves the front wheels so that they are not parallel but have the extended axles of all four wheels meet at a common point. The car drives around this point so path planning is easy except in tight quarters.

The problem with automotive steering is that it is best suited for use by cars on roadways. It is not symmetric forward and backward and getting to some locations and orientations is very hard (try parallel parking a normal car)

Automotive type steering is not complex but some path planning can be. (for example "move left one foot" requires some back and forth motion)

At the other end of the spectrum is four wheel independent steering where each wheel has both a drive motor and a steering motor. This machine can spin in place and drive in any direction.

A related problem is suspension. Its first goal is to keep all four wheel in contact with the ground. Another possible goal for higher speeds is absorbing impacts

Before you can decide what to build you need to think about the kind of TERRAIN the robot will drive over and the SPEED and also the MASS of the robot.

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