Choosing stepper for a delta
- 1 What is specific in delta robots ?
- 2 Stepper characteristics
- 3 The market
- 4 A good practical setup
- 5 Speed, torque and acceleration.
- 6 Bigger is better ?
- 7 Vibration insulators
- 8 Using 0.8° stepper (400 step per rotation)
- 9 Using 24V with 8825 chips
- 10 References
What is specific in delta robots ?
Delta printer have a kinematic very different from Cartesian printers as due to the geometry, while one carriage is in low position, the speed of this carriage is nearly three times higher than the effector speed. So, at maximum printable diameter, the stepper shall run three times faster than for a Cartesian printer.
Steppers have their torque decreasing significantly while their speed increase and above some speed, the torque became so low it can barely turn the motor. The capability of a stepper to run fast is related to its construction and the resistance to changing current in a stepper is translated in a parameter called Inductance, expressed in mH (millihenry).
How to have a stepper run fast: The speed limit is related to the electrical characteristics of the stepper, which makes the electrical current change of direction more or less difficult. To ease the change of current in the stepper, there are two solutions:
- Increase the voltage supply, which is fairly efficient. This is one of the reason you see more high end machines supplied in 24V (there are other motives).
- Have a different winding for the same form factor, say use less turns on the coils, which reduce the inductance, but increase the nominal current.
There is no alternative, stepper faster speed require either higher voltage or higher current (or both, e.g. for CNC machine). See Stepper_torque.
High inductance motors
You find on the market steppers sold for 3D printers, with a torque ranging from 2.6 tof 4.4 kg.cm and a current of 0.4A. This low current appeal constructors as it make the electronic driver heating much less.
However, it came at a cost, which is a very high inductance which varies from 30 to 35mH. That means these motors are totally incapable of any speed. They are unusable for a delta and a bad choice for another printer. A 4.4 kg.cm motor wired for this low current, while having a static torque twice the Fisher motors, simply cannot reach the maximum speed used by the Fisher, effectively having a near zero torque over a given speed. Same motors with a winding giving a nominal current of 1.5 to 2A will be more usable.
Highest usable current
Most drivers cannot practically reach 2A. There is no problem to use a current lower than the nominal current (and you shall, indeed). That will slightly reduce the static torque, but as the high current motors have a low inductance, their effective torque at speed may be higher than for motors designed for lower current and supplied at their maximum. A current lower than the nominal will significantly reduce generated heat.
Don't be fooled by the big torque on the plate, which is the static torque, for the movement motors, what count is the torque at maximum speed (an info which is rarely supplied, anyway).
However, low current high torque stepper have a use for direct drive extruders, as the speed of extruder is very low (except on retract).
A good practical setup
The Fisher, a small delta printer was designed by late RRP company. As for all their printers, they were using small and compact steppers with a torque of 2.2 kg.cm. This is lower than most repraps but is sufficient if there is no mechanical problem (friction).
These small motors have a low nominal static torque, but they also have a low inductance (2.5 mH), while due to their small size, the nominal current remains reasonable (1.2A).
The Fisher is supplied by an external power supply with a voltage of 19V. With low inductance and a higher voltage than most printers, RRP get the best of these small motors and the maximum speed of the Fisher is fair with 180 mm/s at maximum diameter, 250mm/s if diameter is slightly reduced. This is with a geometry where the arm angle at maximum diameter is 12.5°, lower than usual (inducing a carriage speed 4.7 times the effector speed).
Speed, torque and acceleration.
The torque required by a printer movement is due to:
- Mechanical friction (most notable being the belt)
The first one may be limited by careful construction and assembly, and the belt tension shall be sufficiently high for good stiffness, but not too much to limit friction.
However, you can (and shall) play with the acceleration to decrease the torque requirements. While high end machines demonstrated accelerations above 10000 mm/s2, a more common value is 4000 mm/s2. Few software allow different acceleration for travel and printing, which is wise, as larger acceleration may reduce printing time, especially for some infill type as honeycomb.
For that last one, the only way is going in space (which was done in space station, anyway)...
Bigger is better ?
- A bigger motor will generate more heat, which will be difficult to dissipate, as large stepper external area is not much higher than for a small stepper, and not in proportion of the generated heat
- A bigger stepper will makes more noise
- And the biggest problem is that you may experience vibrations with the bigger stepper, which are difficult to fight
- For a given current, a bigger stepper may not have a bigger torque at speed, it is fairly possible that the torque at maximum speed will be indeed, lower.
So, the good choice of current is the current which could be reasonably handled by your board without heating too much (from 1.2 to 1.5A, depending the board), considering the set current may not exceed 85~90% of the stepper nominal current.
It is a common experience to get vibrations from the steppers, generating noise and sometimes, transmitting the vibration to the whole printer structure.
You can find on the market vibration insulators under the reference Astrosyn MY17RMDAMP. Experience shows that they effectively decouple the stepper from its support. They possibly could introduce a slight imprecision in the movement.
You also find cork gaskets sold as vibration insulators, at a much lower cost than the Astrosyn. They are a good thermal insulator, but if the are installed with all 4 screws, the screws transmit the vibrations to the support and there is no vibration decoupling.
However, it is possible to have them acting as real vibration insulator if you untight the two screws that are not in tension. While the belt is in tension, you effectively only need two screw to maintain the motors. Removing the unneeded screws may effectively reduce the stepper motor noise and vibration, the stepper somewhat 'articulating' on the remaining two screws. These cork insulators are quite flexible and the stepper support shall be slightly reclined to take into account the angle due to belt tension.
Using 0.8° stepper (400 step per rotation)
There is a recurring question of the benefit of using 0.9° stepper for delta, as in some position, the delta geometry drive to a loss of precision in some axis.
Indeed, a stepper with twice the steps will be stiffer, as the mechanical stiffness of micro-step is not very high. Experiences show that for light effector and carriage as found in Kossel, there is a benefit, but limited. Larger machine with heavier effectors (dual metal hotend or extruder on effector) may benefit more of 0.9° steppers. While bought new, the price of such steppers is not much higher, so the extra-cost for the stepper is limited.
There is a significant drawback, then. To have the same speed, you need twice the steps. So, your stepper behave nearly like a 1.8° at twice the speed (not completely, as mechanical inertia is unmodified).
So, you are back to the speed problem as explained above and using 0.9° steppers may generally be associated with a 24V power supply. Note that at this voltage, the steppers will be 'singing' at low speed.
It is important to note that as the stepper behaviour is more stiff, the jerk values shall be reduced for 0.9° steppers to avoid loosing steps at corners.
Using 24V with 8825 chips
The drivers we use in 3D printers are all PWM controlled, which means that the current is controlled by switching it off at frequent intervals, which makes the steppers behaving like they are supplied with a voltage controlled driver. The frequency of switching, which may not be constant, is over 20kHz, to not be audible by most humans.
To make that, the current is measured continuously to shut down the supply while the current set point is reached. However, chips do have a minimum time while the current flow is 'open'.
With 24V power supply, the current raise much more quickly than in 12V (this is the purpose using larger voltage) and on the 8825, the minimum on-time is too long and the current will overshoot the target, making the stepper unable to comply with microsteps, overheating and making hissing noises. So drivers based upon 8825 shall be avoided for any voltage over 12V. The problem is less pregnant if you have high inductance stepper. However, using high inductance stepper is a poor choice for a printer as explained above.
The traditional 4988 chip used in a lot of controllers does not suffer from that problem and is a good solution for printer if properly cooled by its body.