Selecting controllers for motorised applications

I understand the principles of the technology and their purpose, but selecting the best one for a given application, setting it up and optimising it for a specific purpose is not something with which I feel completely au fait about. Fortunately, help is at hand to go through the basics, and beyond. Maxon Motor UK, based just north of Farnborough, runs training courses twice a month that are free to all, and go through the often misunderstood technology. I decided to sit in on one and report back what I learnt. Maxon offers significant resource in terms of online information and free training courses to help engineers select the correct motors and controllers for applications. The courses are designed to stop the trend of skipping advice from experts and buying direct from online suppliers to try and save on cost, only to run in to problems later down the line in a project. "Whenever you consider buying a controller there are questions that need to be asked," says Mark Gibbons, a technical engineer at Maxon Motors. "Engineers can get vexed as there can be so many, but they need to be addressed so a system works as you want it to. We are not the machine designers. We give designers the tools to succeed in their control applications." The half-day course was on the ESCON controller; a speed and torque controller (with no positioning). It precedes the EPOS session that includes positioning and assumes some knowledge of motor selection. The online help available to analyse duty cycle and advice on choosing motors is open access and excellent for both the novice and expert, so it's worth a look to check you know what you think you know. The basic principles of DC motors – as you should definitely know – is that voltage is proportional to speed at a fixed load and current is proportional to torque at a constant voltage. When you analyse a duty cycle and work out the mechanical requirements of a motor, it can have a significant impact on the controller. You need to be sure that the controller can handle the peaks and the RMS currents. "These first principles are important when choosing a controller," says Gibbons. "You need to look at what your motor is doing in your application, how fast it is turning and what current it is pulling (power = speed x torque or voltage x current). It comes down to the voltage you are applying and what current is available as to whether the controller is fit for application; what type of load is it? Is it continuous or cyclic, for example, and are you going to be dynamically braking or regenerating?" Motors are generators Something that is often overlooked and causes controllers to fail is that motors are generators. Think back to the days of sitting physics exams looking up and seeing everyone pointing their fingers and thumbs to recreate Fleming's left and right hand rules: the electromagnetic principles of field, current, and thrust motion. When a motor is driven by the load (the load is pushing the motor in the direction it is travelling), its inertial load will generate through the motor converting the kinetic energy into electrical energy, creating a backward flow of voltage, back EMF (electromagnetic force). If the power supply is bi-directional, then this energy can be recovered, but many circuits are not and so the energy must be dissipated as heat. "At a particular voltage a capacitive resistive circuit called a shunt is activated which essentially burns off the power," says Gibbons. "If you have a dynamic process where you are slowing down inertia load you have to use a shunt." The practicality of this is that if you are controlling the decent of a load - for example a cam or lever with a load rotating unsymmetrically around an axis – electricity will be generated by the falling load. This needs to be managed. One of the big considerations is a 1-quadrant vs. a 4-quadrant controller. A 1-quandrant controller cannot apply opposing voltage, so it won't brake or slow a motor. A 4-quadrant on the other hand enables the flow of power forwards and backwards and this allows the motor to brake and decelerate using an opposing voltage that can overcome the back EMF. "If you have a motor and a cam arrangement, while lifting a load it is not a problem," says Gibbons. "But, as soon as you go past top dead centre, the load starts pulling down. That generates electricity and if you are trying to maintain speed then you need to decelerate the motor in that downward section. Unless you have a four-quadrant drive, you can't do that." The controller is easily linked to a PC via a USB and the training course talks you through the configuration which uses drop down menus in a windows format. This also features an auto tune function so PID (proportional–integral–derivative) values are automatically calculated. Applications The ESCON can be put to use in a number of practical applications where there is a requirement for things to be moved from one place to another with no need for position. They can be used in a hand tool for example, on a compressor circuit where the motor winds up and fires a nail, or in a normal drill. These can also be used on a conveyor belt, having a stop circuit somewhere along a ball screw. The ESCON has also been used for the traction control of robots, having one on each track. Both are set to a constant speed so the torque will be varied as it goes uphill, downhill and over bumps. "A favourite thing to do is use the ESCON as a dumb servo," says Gibbons. "The encoder is connected to a PLC which calculates the position and sends speed and current inputs the ESCON. That is essentially a low-cost positioning system." If, like me, you feel a little ignorant about the selection of motors and controllers and want to improve your own systems and designs, or just get more up to date with the technology, then perhaps it is time to take a glimpse into the world of controllers.