Stepper motor control IC not quick enough for low inductance motor ? - Page 1
Forum section: electronics / projects

author: snarkysparky
author_rank: Frequent Contributor
timestamp: June 26, 2023, 11:59:11 am
content: We are using Sanken SLA7073 step motor driver. It has a current reference input and controls the motor winding current accordingly.Except for about 1ms at startup. We tried a new low inductance motor 1.5mh and 1.1 ohm.Power supply is 24V.With this new motor the IC will trip on current fault before it can start controlling the current. That is what it seems to me.Any ideas or thoughts are appreciated.attached scope images have the current as measured at the sense pin in yellow.One image has a near trip and the other an actual trip. Trip occurs when this voltage crosses 0.7VThanks

author: moffy
author_rank: Super Contributor
timestamp: June 26, 2023, 10:52:25 pm
content: Really nice captures. What is your startup sequence? Are all voltages applied before you command the chip to step?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 26, 2023, 11:08:36 pm
content: Low inductance motors have fewer windings in each coil, and therefore need more current for the same torque output.You can also do a test with a 12V (or lower) power supply to reduce the rate in which the "inrush current" changes.

author: thm_w
author_rank: Super Contributor
timestamp: June 27, 2023, 12:10:48 am
content: Why is the new low inductance motor necessary? It might not be suitable for your power supply voltage.https://www.eevblog.com/forum/projects/stepper-motor-question-low-voltage-high-current-vs-low-current-high-voltage/

author: snarkysparky
author_rank: Frequent Contributor
timestamp: June 27, 2023, 07:01:52 am
content: All voltages are applied long before the motor is commanded to run.The purple trace is the Vref Value. Above 2.0 volts puts the driver into standby. So to run the motor Vref is dropped down to the current setpoint.We needed to run the motor much faster for a test. Double the speed. The old motor with inductance of 5.5mh didn't have the torque at speed. It would stall.Consulting the catalog it seems that only lower inductance motors had better torque at speed. We are clocking the IC at 3000 pulses per second in half step mode.I can drop the supply down to 12V and run the motor. But that isn't an option now without a lot of redesigning.I guess I will look again at the motor options. Maybe I can find a motor with higher inductance that has the required torque.

author: moffy
author_rank: Super Contributor
timestamp: June 27, 2023, 08:01:28 am
content: I am assuming that the blue trace is the falling edge of the reset pulse. There is an interesting section in the data sheet under "REF/SLEEP1 PIN":"Ref/Sleep1 PinThe Ref/Sleep1 pin provides access to the following functions:• Standard voltage setting for output current level setting• Output Enable-Disable control inputThese functions are further described in the Truth Table section,and in the discussion of output disabling, above.Range A . In this range, control current value also varies inaccordance with V REF . Therefore, losses in the IC and the senseresistors must be given extra consideration."Could the internal value of Vref take longer to settle? Could you add extra delay between Vref falling and the blue trace falling edge, by a couple of ms?

author: snarkysparky
author_rank: Frequent Contributor
timestamp: June 27, 2023, 08:19:01 pm
content: Finally.Trouble was the Ref pin had a RC filter. Made the transition too slow.Changing the timing so that the ref signal could settle before the mode pins went active fixed it.Thanks all

author: mikerj
author_rank: Super Contributor
timestamp: June 27, 2023, 10:11:27 pm
content: Thanks for posting the solution