Hi All,, Does anyone have any algorithm for speed control of stepper motor ?? When i run the stepper it steps nicely but gets overheated.. Any suggestion for current limiting or to improve torque ?? Any help would be great. Thankzz && Bye -Rocknmoon
Speed is controlled by changing the interval between steps. You can run a stepper a couple of different ways - either step it at a constant speed, or you can ramp the speed up and down. Many users use the second method, as steppers usually have two speeds listed - initial speed and max speed or max slew rate. If you exceed these step speeds you can stall the motor and you wont get ANY motion. Torque will be governed by how much current you can push through the motor, and by how fast you want to step the unit. Many designs use double or triple the rated voltage - as an example running a 5 volt motor at 10 or 15 volts will increase the torque - though the motor will run hotter. Current limiting can be done simply by using a series resistor. If you want real control, look into the H bridge drivers supplied by ST micro, Allegro and others. Many have a built in current limit circuit, as well as other features. One thing to make sure of if the motor is overheated - make sure all the windings are off when you stop the motor, or if you need to maintain it in a 'locked' fashion, use a transisitor to switch a low ohm series resistor with the winding to reduce the current through the motor. The motor can provide a lot more 'holding' torque even at a reduced current than when it is stepping.
Thanks a lot for a handfull of very usefull information . I have a 10V, 3A reluctance type 3-phase synchronous stepping motor, 96steps/rev. It has 3 separate windings on stator each winding resistance is 2-ohms. I need to run it continously at a fixed speed and need a good torque as well. One end of all the 3 windings is joined together and is connected to +10V. The other end of each winding is connected to a grounded MOSFET. Each winding also has a freewheeling diode connected across it. Now i turn on MOSFET-1 for 5msec followed by a pulse train of 1:3 ratio for next 100msec. Next i stop these pulses at MOSFET-1 and apply a 5msec pulse at MOSFET-2 followed by a pulse train of 1:3 ratio for next 100msec. Similarly for MOSFET-3 and then back to MOSFET-1 and so on. The motor steps nicely but it donot give sufficient torque and becomes heated up in few minutes. Do you see where am i doing anything wrong ?? Thanks in advance. rocknmoon@yahoo.com
The motor steps nicely but it donot give sufficient torque and becomes heated up in few minutes. It is common for stepper motors to drive them with a very high current that after a short time (milliseconds - depending on motor) drops to a "holding current". This method gives more torque and less heat. Erik
Whith Your MOSFET-swithes You don't have any real control about the current. There are a lot of drivers for stepping motors on the market (Infineon, STM, or Ericsson) wich have half step or microstep mode will give You maximal torque. The drivers provide an internal control loop for the current. See on Ericsons website for applications and hints.
See: http://www.cs.uiowa.edu/~jones/step/ This is a particularly good website on stepper motors.