The first thing I want to do is to control a motor. For a mowing robot we need several motors: Those which drive the wheels and the one which drives the cutting-rotor.
Maybe this is the right moment to think about steering: As other robots do, I want to use three wheels. Two wheels at the back which drive the robot and cannot turn. One wheel at the front which is not driven but can be turned around at random. Here is an image: http://www.ratgeberzentrale.de/uploads/tx_news/15260_23529_Bild3.jpg. It does not show the configuration of the mowing robot I have in my mind but it does shows how the three-wheels-concept is meant.
In this configuration steering to the left is done by braking the left wheel or accelerating the right wheel. Steering to the right vica versa. An advantage of this technique is the ability to turn around nearly in place. A disadvantage is the need to control two motors precisely which means to use more gpio-pins than if we would drive our robot like a common car (I will explain the reason for this in an upcoming post).
In this configuration steering to the left is done by braking the left wheel or accelerating the right wheel. Steering to the right vica versa. An advantage of this technique is the ability to turn around nearly in place. A disadvantage is the need to control two motors precisely which means to use more gpio-pins than if we would drive our robot like a common car (I will explain the reason for this in an upcoming post).
So how is a motor controlled by an electronic circuit?
One possibility is to use a stepper-motor. These motors allow to turn a predefined step (for example 1.8°) forward or backwards. Repeatingly doing steps results in a continuous rotation. Speed is controlled by the interval of those steps.
The advantages of those motors are that they are easy to control and the force needed to do a step (even if you are driving uphill) is regulated by them self. Another feature is that they are able to hold their position even if a force tries to change the position. A downside of this is that they consume energy even if they are not rotating. Another disadvantage is that step motors are not built to rotate fast. They are made to move sheets of paper in printers forward and backward or support balanced robots for example. So it might be that this kind of motor is not suitable for our application.
The advantages of those motors are that they are easy to control and the force needed to do a step (even if you are driving uphill) is regulated by them self. Another feature is that they are able to hold their position even if a force tries to change the position. A downside of this is that they consume energy even if they are not rotating. Another disadvantage is that step motors are not built to rotate fast. They are made to move sheets of paper in printers forward and backward or support balanced robots for example. So it might be that this kind of motor is not suitable for our application.
The second possibility is to use back-geared motors. They are quite simple: put voltage on it and the axis starts to rotate. This kind of motor is not able to rotate slowly so the gear slows it down and brings additional power due to its gear ratio. This sounds pretty good but there are downsides!
The first one is the way to control the speed of rotation. This is done by reducing the voltage. If you have a regulator this is done easily but how can this be achieved by Raspberry Pis gpio-pins? The solution for this is PWM (pulse width modulation). This technique is supported by Raspberry Pi nativly but only for one device not two (motors). So we have to build PWM on top on usual gpio-pins. The second downside is that driving uphill would slow down the speed of the motor and voltage has to be increased to compensate the reduction of speed. To do so it is necessary to measure the current speed - also not so easy.
The first one is the way to control the speed of rotation. This is done by reducing the voltage. If you have a regulator this is done easily but how can this be achieved by Raspberry Pis gpio-pins? The solution for this is PWM (pulse width modulation). This technique is supported by Raspberry Pi nativly but only for one device not two (motors). So we have to build PWM on top on usual gpio-pins. The second downside is that driving uphill would slow down the speed of the motor and voltage has to be increased to compensate the reduction of speed. To do so it is necessary to measure the current speed - also not so easy.
As you can see the decision is not easy to come.
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