November 30th Update: Closed-loop control

Hi Everyone!

      With the holiday this week I expected things to be a little slow, but it was actually a pretty exciting week if you are into technical details. If technical details aren’t your thing, maybe take a pass on the update this week and check back in with us next week.

      I spent the part of this week I didn’t spend eating mashed potatoes working on the motion control system for Maslow. Maslow uses a nuanced control system called a closed-loop control system or a feedback control system. Because I’ve been working on it this week, I’d like to take a second to talk about Maslow’s control system and how it works.

What is a closed-loop or feedback control system?

      A closed-loop control system is a system in which the controller uses information (feedback) about the thing which is being controlled to adjust its response. The classic example of a closed-loop system is the cruse control on a car. When the cruise control is engaged, the onboard computer uses the speedometer to measure the car’s speed. If the car starts to go up a hill and the car slows, the cruise control senses the decrease in speed and applies more gas. If the car starts to go down hill and pick up speed, the controller senses the increase in speed and reduces the gas. The classic diagram of a closed-loop controller (and where the name comes from) looks like this:

You can see that the Sensor block creates a closed-loop around the system. In the case of a car’s cruise control, the sensor would be the speedometer, the system would be car (gas in, movement out), and the controller would be the onboard computer. In the case of Maslow the sensor is the encoder, the system is one of Maslow’s axis, and the controller is the onboard computer. Because Maslow has three separate axes, it is running three separate closed-loop control systems.

Why does Maslow use a closed-loop control system instead of stepper motors?

      The biggest reason that I chose to use a closed-loop system was cost. Keeping the cost of the machine down was a huge priority and a closed-loop system gave simpler hardware (keeping costs down in parts and shipping) at the expense of more complex software (which ships free). Our Maslow’s motors and gearboxes give similar torque to NEMA23 stepper motors with much higher accuracy at a much lower cost. The biggest drawback to using a closed-loop system is that designing a closed-loop system is quite a bit more complicated than designing an open-loop system.

What are some of the other benefits of a closed-loop system?

       While the closed-loop control system was critical to making the machine as low cost as it is, it also brings with it a number of interesting advantages that we can exploit. Because the controller knows the exact speed of each motor and how much voltage is being applied to the motor, the controller in essence has a real time measurement of how much torque is being applied by each motor. This could be used for all kinds of interesting applications like detecting how much force is being used to move the bit through the wood to do real time feed-rate adjustments or detecting if the machine has bumped into something to turn the machine off automatically.

      The first application I’ve begun exploring is using the feedback system to allow the machine to calibrate itself to different motors or different router weights. We expect that many of you will modify your machines or even build your own from different parts entirely and we want to make sure that the machine automatically adapts to give the best performance for your setup. By measuring how much force is required to lift the router, the machine can in essence “weigh” the router sled and adapt its behavior accordingly.

Here you can see the Speed vs Applied Voltage curve for the standard motors as measured by the controller.

Here you can see the Speed vs Applied Voltage curve for the standard motors as measured by the controller.

Thanks for reading all the way to the bottom and have a great week!

-Bar, Hannah, and Tom