To move around the garden, FarmBot sends electronic pulses to its motors. Under normal circumstances, these pulses cause the motors to rotate an exact amount and move FarmBot to exactly where it needs to be.

However, the garden environment is unpredictable, and FarmBot cannot always rely on circumstances being normal. For example, if a tomato plant :tomato: grows across the tracks, FarmBot might get stuck on its vines, causing one or more motors to stall.

In situations like this, FarmBot needs a way to detect the stall so that it can either retry the movement, e-stop, or go back to the home position to reorient itself. If FarmBot had no way of detecting a stall, it would continue operating without realizing its position is incorrect, and potentially destroy the whole garden.

Rotary encoders

Rotary encoders are sensors that can be attached to the back of motors to measure the exact rotation of the motor shafts in real-time. With rotary encoders, FarmBot can detect when a motor has stalled, which could be caused by one of the following reasons:

  • A motor is not moving correctly due to unreasonable speed and/or acceleration settings, a lack of power, or another hardware malfunction.
  • FarmBot has collided with an object such as plant branches :seedling:, a hand :hand:, or an animal :dog:.
  • There is too much resistance or friction for FarmBot to overcome. This could be caused by dirt on the tracks, eccentric spacers that are too tight, belts that are too tight, excessive machine weight, cold temperatures causing cabling and tubing to become stiff, or other factors.
  • The end of an axis has been reached during automatic calibration or homing, or in error.

In addition to detecting stalls, rotary encoders also keep track of FarmBot’s position. This even works when FarmBot is idle and you move an axis by hand.

FarmBot Genesis includes rotary encoders

All of our top-of-the-line Genesis kits include rotary encoders pre-mounted on all four stepper motors.

Back-current sensing stepper drivers

Back-current is a usually small electric current that motors send back against the stepper driver. Some advanced stepper drivers, such as those offered by Trinamic, can sense and measure this back-current, which can indicate if a motor has stalled.

While back-current sensing stepper drivers can detect when stalls occur, they cannot keep track of FarmBot’s exact motor position all the time like rotary encoders can. Thus, they present themselves as the more economical way of performing stall detection.

FarmBot Express includes back-current sensing stepper drivers

All of our more affordable Express kits include Trinamic TMC2130 back-current sensing stepper drivers to power each of the motors.