Perhaps the simplest (and therefore most common) method adopted for dosing applications historically has been via timing. In simple terms one calculates the amount of a particular substance required in relation to the time taken for a given pump to dispense it and then calibrates the pump accordingly.
If 100ml took 60 seconds to dose then 150ml should take 90 seconds.
The problem with this method is that the margin for error can be significant, especially where changes in external factors occur – for example a change in back pressure.
There are several options for how the problems with this method can be resolved.
The first is to use a closed loop system which directly monitors the number of motor turns. This can be quite effective but can also add significant cost and is impractical in many pump applications due to environmental factors. A second option is to use stepper motors which can be much more accurately positioned in order to achieve a specific dose. In peristaltic pumps this is a particularly good solution. However, the power density of stepper motors is particularly poor in comparison to brushless motors. As a result, there are certain applications in which stepper motors are simply impractical due to weight and size constraints.
In this case one option to consider is an open loop sensorless system which can directly count the commutation cycles of the motor as it turns. This system has several key benefits over timed systems;
