High Performance Web Guide Mechanism

All our web guiding systems (Figure 1) are equipped with high performance stepper motors based linear actuators driven by high efficiency constant current chopper motor drivers. The performance of the web guiding system is dependent on several factors including the power supply, the stepper motor driver, the motor, and the optimal tuning and setup of each of these components. A few specifications of the web guide mechanisms are as follows: 

Roller Width: 254 mm

Maximum Correction: 52.75 mm

Displacement Resolution: 0.000275 mm/microstep based on the step resolution of the stepper motor, the micro step capability of the driver and the transmission ratio

Linear Speed: 130 mm/sec with a high frequency driver that can generate up to 500,000 microsteps/sec

Time Period for Maximum Correction: 0.425 seconds

Linear Force: With the 24 V power supply the motor can provide a peak force of about 250 N

Figure 1: Roll-2-Roll Technologies LLC web guiding system.
Figure 1: Roll-2-Roll Technologies LLC web guiding system.

Frequency Response

One way to efficiently summarize the performance of the web guiding system is through its frequency response. The driver and the stepper motor are highly non-linear switching systems, therefore a traditional frequency response on a Bode diagram cannot be obtained. However, given all the parameters of the driver and the stepper motor, it is possible to analytically compute the maximum amplitude of travel for a sinusoidal reference input of a given frequency. Figure 2 shows the theoretical frequency response for the high performance guide mechanism. The blue solid line shows the maximum possible correction for frequencies in the range of 1 to 10 Hz. A set of experiments were performed at frequencies between 1 Hz and 10 Hz to verify the theoretical results. The high performance stepper motor driver was used to command the motor to oscillate at the maximum displacement for a given frequency. 

Figure 2: Theoretical and actual measured frequency response of the guide mechanism
Figure 2: Theoretical and actual measured frequency response of the guide mechanism.

The actual movement of the web guiding system was measured using a linear optical quadrature encoder with a resolution of 0.003175 mm/count. The encoder measurement was used to verify that the motor does not stall at high microstep pulse rates1 . The encoder measurement indeed verified the high precision performance of the system which closely matches the theoretical response. Figure 3 shows the time domain data from the experiments. The ordinate shows the web guide displacement in mm measured using the linear optical encoder and the abscissa is timed in seconds. Note that at low frequencies the maximum travel is bound by the stroke length of the linear actuator, while at higher frequencies it is limited by the maximum speed of the motor. A video showing the frequency response of the guide is available here

Figure 3: Actual guide displacement measured at different frequencies
Figure 3: Actual guide displacement measured at different frequencies 

The performance data shows that as the frequency increases above 5 Hz the maximum possible correction drops below 10 mm. It is possible to increase the maximum correction by about 10% without significant change in the price of the system. Additional performance increases are possible with custom actuators and motor drivers which can significantly increase the price of the actuation system. 

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[1] Stalling is a common issue with low quality voltage based stepper drives.