I can't control the speed of the servo, can I?
Yes, you can, but not to a very good degree of control
Also, it must have something akin to an h-bridge inside the motor, which surely ups the cost, but the motor will always be geared, which is a plus.
Continuous rotation servo:
- Gearing
- Built in motor driver
- Single pin control (forward, reverse, speed)
- Easy to mount the servo, and to attach movements to the horn
- Hard to 'center' due to temperature drift - ie, without feedback you can't ever fully stop a servo, and even with feedback it continues to hunt for 0, and induces vibration into the mechanism
Motor:
- Cheaper (even with gearing, especially in quantity)
- Greater degree of control with a good motor driver
- Doesn't require a PWM signal
- Smaller
- Lower power consumption for most applications
You can't implement a stable closed-loop system without knowing what the open-loop response looks like.
A simple example might be controlling the brightness of an incandescent lamp using a photo-diode (very fast) as feedback of brightness.
At rest, the system has no-problem then you set a demand that wants to see X watts per square metre produced at 1m. The photodiode will tell you the watts per square metre hitting it but if you don't take into account the time-lag (or inertia) of the lamp your control system will ramp up to maximum power before your lamp has started to glow.
The photo-diode, at some point later registers the correct amount of light and the driving system instantly "levels-out" because it believes the lamp has hit the demand but, the lamp will glow a bit brighter because of thermal lag (or inertia) and then the control loop will switch off and what you'll get is possibly a self-oscillating system and it may take ages before the system settles down.
Along the way you may even destroy the lamp.
What about other control loops for things like linear actuators - you set a demand position and an amplifier starts driving the motor to the correct position but you get overshoot because the motor and mechanism have inertia.
Basically, if you don't respect the open-loop response you have a recipe for disaster.
Best Answer
If you understood what those terms meant you wouldn't be asking this, so I'll explain open loop and closed loop.
Open loop means you pick the set point of a system and whatever comes out comes out. No system is perfectly "stiff", so the output will vary somewhat, sometimes quite a lot, depending on load.
Closed loop is when something is actively watching the output and adjusting the set point to whatever it takes to get the desired output.
Think of the gas pedal in your car. From gas pedal to car speed is a open loop system. You can push the pedal down to a fixed spot, and the car will go faster or slower depending on whether it's going up a hill or not and what else it has recently done. Now consider cruise control. That's puts feedback around the open loop gas pedal to car speed system to regulate the speed. It watches the speed and effectively steps harder on the gas when it notices the car slowing down, and lets up on the gas when it goes faster than desired.
The advantage of a closed loop system should now be obvious. You get to control the parameter you really want to control (the car speed in this example), and it automatically gives the system the right input (the gas pedal setting) to achieve the desired result. Of course this comes at the cost of extra stuff and complexity.
Closed loop control systems also need to be designed carefully so that they are stable. For example, if the cruise control always slammed the gas pedal to the max whenever the car was a little below the set speed, and completely let up when a little above, you'd have a very jerky ride and it would put a lot of stress on various parts of the car. Usually there are some tradeoffs the designer can make between the maximum error, the speed of response to changing conditions, and stability.