Electronic – A4988 and battery power

stepper motor

I have battery power (9v) for stepper motor (auto-tracker for telescope mount). I need micro stepping and high torque (0.1 N*m). I decided to use bipolar stepper and A4988 because there are cheap modules from eBay.

But: I need to rotate stepper very slowly (about 1 step per second), so most of time the motor stay without rotation and A4988 produce a holding current, but I need no holding torque (there are no external torque that can rotate my motor).

How to maximize battery life in my case. Maybe just set "enable" pin of A4988 to 1 between rotations? But I have another questions in this case:

1) Can I lost steps if I will set enable pin to 1 between every steps? In data sheet there is note that if I will use sleep pin, then after wake up A4988 go to HOME state, so I will lost steps. But there are no notes about behavior of enable pin.

2) How microcontroller can know when step have executed (before set enable pin to 1)?

Maybe there are another solutions. I need at least 5 hours of 9v alkaline battery, 0.1 N*m, single step each second.

Best Answer

A4988 data sheet here

(1) The data sheet says that the /ENABLE input does not destroy "state"information. However, as Anindo notes, while there is no external torque, the motor will apply "pull in torque" when depowered. It MAY be possible to use the current limiting setting to reduce holding current to a minimum and to increase it when actually stepping, but this is not a certain solution, and it adds complexity which may make the overall solution unattractive.

On page 10 it says:

Enable Input /ENABLE

This input turns on or off all of the FET outputs.
When set to a logic high, the outputs are disabled.
When set to a logic low, the internal control enables the outputs as required.
The translator inputs STEP, DIR, and MSx, as well as the internal sequencing logic, all remain active, independent of the /enable INPUT STATE.

(2) You may also be able to dynamically alter the effective sense resistors that control maximum current. These MAY latch, on a step by step basis, which would make this method impractical.

Data sheet says:

  • At each step, the current for each full-bridge is set by the value of its external current-sense resistor (RS1 and RS2), a reference voltage (VREF), and the output voltage of its DAC (which in turn is controlled by the output of the translator).
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