November/December 2017

25 www.drivesncontrols.com November/December 2017 SERVODRIVES However, using either of these options won’t give you the necessary control, because the variable-frequency drive 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. This is a crude example, but it makes the point. You could consider a flux vector drive which uses a feedback device – typically an encoder – on the motor to achieve a speed control range of 1:1,000. This would get you much closer to the desired control sensitivity, but even at 0.5 ml/min we are still out by a factor of five. Rule of thumb The speed control range of a servo system is not much of a concern because 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 and, 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 system will meet the demands of the application precisely. For this particular requirement, it’s the obvious choice, but you may want to consider that this is also the most costly option in terms of the initial purchase cost. The servo system does have other advantages though, including high efficiency, that may well be worth the extra initial investment. Considering the speed range of the drive system is only part of the issue. All servodrives are not the same and you need to be aware of the differences. One of the main issues is that most 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. We have a required flow rate range of 0– 500 ml/minute, 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. There are a couple of things that need to be addressed. We will make the assumption that the signal provided over the analogue input is pure analogue. If it is not, you’ll have the same considerations to make on the equipment generating the signal, as with the equipment receiving and processing the signal. What do we mean by this? As an example, think back to the early days of radio, television, record players, and even telephones. They were all pure analogue. The signal was only filtered, enhanced and amplified as it was passed from one component to another before it reached our eyes and ears. With analogue systems, this occurs without the signal being chopped, manipulated, buffered, overlaid and recompiled, as is the case with digital systems. So how does this relate to our application? Well, 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 servomotor running at 3,000 rpm, through a 25:1 gearbox. The 3,000 rpm on the servomotor is therefore the factor that ultimately controls the flow rate of the pump. But to achieve the smallest increment of 0.1 ml/min, the pump's camshaft needs to be turning at 0.024 rpm, which is 0.6 rpm on the motor. This, in turn, means that the analogue signal (±10V DC) needs to be stepped in increments of less than 2mV. But can a servodrive handle such small variations? The short answer is yes, but it depends on the servodrive you are using. If the servodrive does not have a high- resolution analogue input, then you could be in trouble. A significant bit Let’s look at an example where the analogue input is 12-bit resolution. This 12-bit resolution equates to 4,096 increments or approximately 2.5mV. So 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. And remember, this also applies to the output of the equipment providing the demand signal in the first place. There may be work-arounds if your drive does not have an analogue input with a sufficiently fine resolution – such as using a digital interface (serial communications, fieldbuses and so on). Some drives may offer the function of setting the input for a speed value without direction signal – for example, 0–10V – if you are only driving the motor in one direction, as we are doing on this pumping application. How do you determine the smallest increase in speed that you can achieve using servodrives?