We are building an AI-driven camera that uses three 4k sensors mounted on servo controlled arms with independent pan/tilt/roll capabilities. That was the easy part - one Pololu Maestro handles all 9 of those servos. But each lens also has motorized P-Iris, Focus and Zoom controls with very small stepper motors.
At any one instant in time we only need to drive 3 of the nine Iris/Focus/Zoom motors. I think my options are: 1) just use 9 independent stepper drivers or 2) use 3 stepper drivers and multiplex the motor control signals by using a 2 bit motor select input.
The problem with #1 is that we are space constrained - even though the stepper drivers are relatively small, 9 of them would be unwieldy.
The problem with #2 would be figuring out how to multiplex the potentially high current motor control outputs.
Is there any easier way to do this with off the shelf products?
Specs for the stepper motors are:
Iris Control Motor
- 2 phase bipolar drive
- Operating voltage 4V (+/-1)
- Number of steps 75 (open to closed)
- Basic step angle 18°
- Maximum response freq. 1000pps
- Coil resistance 30Ω
- Operation voltage 3.3V (operating range 2.6~4.8V)
- Coil resistance 28.5Ω (±7%)
- Gear ratio 1:1954
- Zoom speed 4 sec (@1000pps)
- Zoom cam rotation 75°
- Focus speed 8.9 sec (@600pps)
You might be able to work out some scheme for multiplexing the motor outputs with MOSFETs or relays, but I doubt that it would save you much space compared to having a separate stepper driver for each motor, which would be a more straight-forward solution anyway.
Thank you. Appreciate the advice and believe you are right. Now just to solve the mechanical and wiring challenges of mounting 9 stepper drivers, a maestro 12, 9 servos and 3 lenses in an IP-67 rated package that can handle getting hit by a baseball at 110 mph.
Is there one of your stepper drivers that might be a best fit for the steppers mentioned in the original post?
From the specifications you provided, it sounds like your motors are designed to run with a coil current of around 100mA. All of our stepper drivers should be able to handle that and run the motors. What kind of power supply do you intend to use? Some of our drivers require a higher supply voltage than others. Do you plan to use microstepping? Is this a battery operated system that you need to save power with and do you need to maintain a holding torque on the motors?
140ma max on current. Power Supply is a 23000maH PowerBank capable of running at 9, 12, 16, 19 or 20V that can deliver a total of 2.1A. It will definitely be microstepped for fine focus control - at least 1/32. No holding torque is required - the steppers hold their position adequately without power. Duty cycle is also VERY low - focus and zoom are set only at the start of an event, then change minimally if at all. Iris changes as needed for lighting conditions, but still very slowly. Also - it is highly unlikely that we will operate more than three motors simultaneously, so total power draw is low.