FEATURE DRIVES, CONTROLS & MOTORS SPEED CONTROL
What is the smallest speed increment that can be set on a servo motor? Andrew Fallows, managing director of Motor Technology, comments
W
hen designing a drive system, particularly for critical applications,
there are a number of issues that are fundamental to achieving optimum performance, efficiency and accuracy. Speed control and determining the smallest speed increments that can be set on a servo motor is one of these, and is particularly relevant where high precision speed control is critical. Imagine you need a motor to drive a
pump unit and provide a flow rate range from 0 to 500ml/min. Due to the pump’s design the torque required to drive it over the range is relatively constant, but it might be necessary to adjust the flow rate down to 0.1ml/min.
You first need to consider the type
of motor and drive combination best suited to the application. Simple Variable Frequency drive (VFD) units quote speed control ranges down to as low as 1:40; while Open Loop Vector drives enable speed control ranges as low as 1:200. However, the VFD can only run down to 1:40 of its speed range, giving a minimum flow rate of 12.5ml/min; and in the case of the Open Loop Vector drive the closest you could get would be 2.5ml/min. A Flux Vector drive which utilises a feedback device, typically an encoder, on the motor to achieve a speed control range of 1:1000 would get you much closer to the desired control sensitivity, but even at 0.5ml/min we are still out by a factor of 5.
SERVO CONTROL The speed control range of a servo system is not much of a concern as it is substantially better than the other
options mentioned here. Due to the closed-loop control methods and the synchronous, permanent magnet characteristics of the motor, we can achieve full torque at standstill. As a rule of thumb, it is available up to 80% of the motor’s speed range with around 80% of the torque available up to the nominal speed of the motor. To put a figure on the speed control range, it is better than 1:5,000. A simple calculation: 500ml/min ÷ 5000 = 0.1ml/min, indicates that the servo solution will meet the demands of the application exactly. Although the most costly option, the servo system also offers additional advantages such as high efficiency. All servo drives,
however, are not the same. The majority of drives, whether inverter or servo, are digital and all signals to the drive are converted to allow further processing. This
conversion and its resulting data need to be understood before committing to a particular drive solution. So, we have a required flow rate range of 0 to 500ml/min, with the speed demand selectable in increments of 0.1ml/min. Let us suppose that the speed is to be controlled over a ±10V analogue input. We will assume that the signal provided over the analogue input is pure analogue. As an example, think back to the early
As well as
offering speed control, servo drives offer other advantages such as high efficiency
days of radio, television, etc., which were all pure analogue. The signal was only filtered, enhanced and amplified as it was passed from one component to another before it reached us. With analogue this occurs without the signal being chopped, manipulated, buffered overlaid and recompiled, as is the case with digital systems.
In relation to our application, we want to run the pump at up to 500ml per minute, in increments of 0.1ml per minute. Driving the pump input is a servo motor running at 3000rpm, through a 25:1 gearbox. The 3000rpm on the servo motor is therefore the factor ultimately controlling the flow rate of the pump. But to achieve the smallest increment
of 0.1ml/min, the pump’s cam shaft needs to be turning at 0.024rpm, which is 0.6rpm on the motor. This, in turn, means that the analogue signal (±10VDC) needs to be stepped in less than 2mV increments. While a servo drive can handle such small variations, it is dependent on the one you are using. If the drive does not have a high resolution analogue input, then you could be in trouble.
FOR EXAMPLE Let’s look at an example where the analogue input is 12 bit resolution. This resolution equates to 4,096 increments, or
approximately 2.5mV.
We don’t seem far off, but the first bit (the Most Significant Bit, or MSB) may be reserved for defining the direction (Positive or Negative), so our resolution is actually 11 bit for the speed portion of the signal, or 5mV. On this basis we need to select a drive
with at least 14 bit (1.2mV) resolution to achieve the required minimum increment for the application. This also applies to the output of the equipment providing the demand signal in the first place. There may be work-rounds if your drive does not have an analogue input with a sufficiently fine resolution, such as using a digital interface (serial coms, fieldbus, etc). In addition, some drives may offer the function of setting the input for a speed value without direction signal (eg. 0 to 10V), if you are only driving the motor in one direction – as we are doing on this pumping application.
Motor Technology T: 0161 217 7100
www.controlinmotion.com
14 DECEMBER/JANUARY 2018 | DESIGN SOLUTIONS / DESIGNSOLUTIONS
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