Why DC Motors in Robots?
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Let’s take a step back and talk about what a DC motor is. Typically, when thinking about a DC motor, you really are talking about a DC permanent brush type motor. Apply DC power, adjust the voltage and current levels to control it, usually with a simple amplifier, and you have a dc system solution. Add in some type of feedback and you will have pretty good control. There are lots and lots of robot types, but mobile robots are typically going to need to run off of a battery source, thus the need for a DC solution.
Now, let’s look a little closer – even if you run off of a DC source, like a battery, you can use other types of motors along with the appropriate drive electronics to control them. This opens the world of robots to AC Synchronous motors – brushless servo motors that provide the ultimate in precision and control. Yes, I said AC. The drive electronics operate from a DC source, but through PWM (Pulse width modulation) of the dc source through very fast switching transistors, you create the voltage and current waveforms the AC motor desires – and with high resolution feedback, you can precisely control whatever moves you want your robot to make.
Examples of this are the typical Collaborative Robots that have multiple joints and carry payloads of less then 15 Kg. These robots are most likely are utilizing a frameless brushless servo motor solution, running from a 48 Vdc source, or even 120 Vac source which is converted to a lower voltage dc bus to pass to the drives. Are these a better solution then a typical DC motor?
If we go back to the Permanent Magnet DC motor, they require brushes to get the voltage and current to the windings of the rotating armature to commutate the motor – this reduces the burden of the controller to a degree, but there are some concerns – the dc motor has it’s copper windings on the inside – copper heats up and you limit how much you can push the motor since you can’t dissipate that heat as readily. On the other hand, a frameless servo motor solution has the heat producing elements towards the outside – allowing the cobot designer to consider more efficient ways to dissipate the heat. The drive electronics do get more complex – but with todays advanced power devices and control chips, the additional tasks of the drive to commutate the motor are not such a concern.
For larger articulated robots, more conventional framed brushless servo motors are incorporated into the design. This may be through direct coupling with an integrated gearing solution. At this point it is likely going to be powered from 240 AC or even 480 Vac. But even with high powered AC synchronous servo motors, the 3-phase power will be converted to a DC bus and then controlled through some type of switching amplifier, utilizing feedback to close the servo loops for precision control.