Stepper vs DC motor

To answer your questions:

1. Power consumption. A stepper will most likely consume much more than a brushed DC motor because a DC motor uses only the energy it needs, whereas the stepper is commutated with a surplus of energy in order to ensure no steps are missed. In theory, you could tune the system so that you use precisely the current the application requires, in which case the stepper may have less consumption since it is built to move with less friction (ball bearings versus copper sleeves). The stepper driver, however, will most likely consume much more power than the DC motor driver because it is a current chopper and the switching losses on a stepper driver will always be higher than on a DC motor driver. A stepper motor is continuously switching 8 FETs, whereas a BDC can be commutated with 4 FETs. If there is no need to change direction, you only need 1 FET to commutate a BDC, whereas you will still need 8 FETs to commutate the stepper regardless. Unipolar steppers, however, will only require 4 FETs.

2. Reliability of drive, in terms of life time. Steppers usually last longer as they are built with ball bearings and they do not have brushes. There are cheapo steppers without ball bearings and in this case such a motor should have a similar mechanical life to a cheapo BDC. However, the BDC will always have the problem of the brushes degrading throughout time, whereas no stepper will ever suffer from this (steppers are Brushless DC motors in nature). If the BDC is a good quality one (AKA $$$Big Bucks$$$), you can always change the brushes and get back in the game with "minimal" cost. Expensive BDC may also be built with ball bearings, in which case the life of such a motor should compare to that of a stepper, as long as the brushes can be replaced.

3. size & weight. Usually, Stepper motors will be heavier and bulkier than BDC, but I am not savvy enough to give you a ratio of size/weight to power. I don't think this is a simple question either. There are so many ways to build these motors, the answer can be anything at the end. I think the BLDC thumb rule, though, is that BLDC will give you better power performance per pound. What does that mean, however, will most likely be a religious discussion among motor manufacturers.

4. Cost of drive. A stepper driver will always be more expensive than a BDC motor driver because you need more power FETs and much more logic to achieve commutation. However, as of today, this extra expense may as well be negligible. It may be that a BDC power stage is $2.00 and a similar stepper driver is $3. A few decades ago, the difference in price was astronomical (most likely there was a micro computer being used to commutate the stepper). Today, cost is really not the major driving factor. In fact, there are drivers which can be used to drive either a DC motor or a stepper, and you pay exactly the same thing!

5. Ease of control. This is a tricky question. To me, it is way easier to control a stepper than a BDC because with the drivers today, all you need to do is apply a known frequency square wave and the motor moves at that speed regardless of load changes (as long as you have enough current). A DC motor is way simpler to commutate than a stepper (all you need to do is apply a voltage and the motor moves at that speed), but you will need to close the loop with a shaft encoder and some microcontroller code if you need the speed to remain constant. So which is simpler? Either! It all depends on what you are most comfortable with. There is enough information out there to make either implementation a 30 minute project to the seasoned motor enthusiast. And for beginners? A weekend should be more than plenty.

In your case, I would go with a geared down BDC. Not certain how big this conveyor is, but if you need to move heavy stuff, current is what you will need. I would look at anything like a DRV8840 or a DRV8842, if 4-5A is enough. If you need more than that, then DRV8402-32 is the way to go.  Don't waste your time with building your own discrete FET based power stage! I see a lot of people trying to play with Smithsonian technologies. I don't get it! Why not embrace the 21st century once and for all?

You can then use a simple MSP430 to generate a PWM and control the speed. If you want to close the loop, you can have a shaft encoder give you a square wave output which is directly proportional to speed and then regulate PWM on a target speed. A simple proportional loop should be more than plenty for an application of this sort. Unless you have some critical process in which case you will need a full PID loop.

Hope this answers some of your questions.

Hello Sandy,

Since your application reqruire speed control and not the position control, I would recommend Brushed DC motor over stepper. Brushed DC motor will have cost advantage due to:

• one H-bridge is required vs two H-bridges for bi-polar stepper
• ease of control, Simple PWM vs Microstepping driver

 As Ryan has mentioned you can also look into BLDC option because it will give you better system efficiency as compared to brushed DC motor.

Best Regards

Milan