Adjustment of ramp rates to achieve consistent response to digital inputs
Original Publication Date: 2004-Nov-25
Included in the Prior Art Database: 2004-Nov-25
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Inverters and other motor controllers are often required to stop (or start) a motor in response to a digital input. For some industrial processes it is important that the time between issuing a digital input and the inverter actually stopping (or starting) the motor is always the same. Due to limitations in the processing capability of the CPU's in modern inverters it is impossible to run everything every few µs. Typically the control tasks within the CPU will be separated into a number of timeslices. The current control may run every 500µs, the speed loop every 2ms, the control state machine every 8ms etc. Because of this partitioning of tasks, the response of the inverter to a digital input will depend on the timeslice in which the digital input is read, and then in which timeslice this information is acted upon. For example, if digital inputs are read every 4ms, and the ramp function generator which controls the setpoint to the inverter runs every 2ms, then the response time to the digital input will vary between 0 and 4ms. This is illustrated in figure 1 for an inverter with a ramp time of 20ms. Up to now this limitation always had to be accepted. It is therefore suggested that the time between reading the digital input and acting upon it is measured and used to change the ramp up time of the inverter. By changing the ramp up time it can be assured that the time between the digital input being applied and the motor reaching its setpoint frequency is always the same.