SMED

The SMED (Single Minute Exchange of Die) prototype was developed to illustrate a method for reducing changeover time on production lines for currently manually adjusted equipment. The prototype is a box folding machine fitted with linear actuators or electronic rams which transmit position information to a microcontroller, which in turn controls their motion.

Business objective

The prototype will be used to illustrate the reduction in time and manpower for changeovers, and hopefully encourage relevant companies to have frequently adjusted equipment converted to free up engineers for other work and improve output. The outcomes would include a reduced downtime and additional flexibility in batch production.

Method

The box folding machine was constructed with linear actuators instead of a nut-and-bolt method of changing settings. Linear actuators work by a motor attached to a spindle which moves a ram forwards or backwards depending on the polarity. A resistor is incorporated which changes in resistance as the ram moves out, and by putting a voltage across this it is possible to determine the position to a high degree of accuracy. The simplest method of control we decided would be to use a programmable microcontroller, and a type of circuit known as an H-bridge which provides power to the motors based on logical input signals. These are common circuits and are produced in microchip form. I drew up a circuit diagram of the motor controller circuit including a few other components to ensure correct operation. The prototype incorporates four actuators, so I produced a plan and mounted four motor control circuits on stripboard (a plastic grid with holes for components to be mounted easily, and parallel strips of copper to connect individual lines of holes). We used a transformer to convert mains electricity to 24v DC. The microcontroller provides logic signals of 5v to trigger the motor controller circuits which then channel 24v to the actuators. The microcontroller contains a program which accepts controls from a computer via a serial cable and moves all four actuators the correct distance simultaneously. Fine tuning is accomplished by 'nudge' functions which move individual actuators forward or backward by a fraction of a millimetre.

Challenges

The main challenge was the current limit of the H-bridge chips. Each chip includes two H-bridges, and so, in theory, can control two motors, but the actuators draw a higher current than the H-bridge chips can cope with, so the controller circuits had to be paralleled up. By controlling only one actuator from each chip, the current rating of the chips is doubled. Unfortunately, even this was inadequate for when the motors were drawing full power, and two chips burnt out during initial testing. The testing also showed that the actuators were too fast for accurate positioning by the program. Both problems were overcome by rerouting the power supply to the motor control circuits through resistors which would both reduce the current through the chips and slow the actuators down. Fuses were also added to be doubly sure the circuit would be protected in the event of a short.

Results

This prototype has been developed for display at relevant conferences. It will be available for many companies to see, and, if the idea is taken on by companies with a lot of production line work, it could save a lot of money. In the food industry especially, changeover time is becoming a larger proportion of downtime as batch sizes become smaller and flexibility is key to success. The simplicity of the design from the operator's point of view means it can be employed by people with minimal training and at lower risk than previously. Batch sizes currently mean that equipment is used at one setting for an hour and then brought out of action for 10 minutes to prepare for the next batch. Compared to this our prototype will reduce changeover time by at least 90% with proportional cost and manpower savings as well as added flexibility.

The nature of the work has necessitated a better understanding of electronics and computer programming. It has improved my skills in planning as well as electrical circuit and system assembly.

The rig was designed and built to illustrate the rapid changeover concept some years ago, but the control method was more complex. Our prototype provides an easy-to-use, desk-mounted demonstration piece.