CAO Ai- ping，LIU Wei- guo，HAN Ying-tao

(Instiute of Automatization．Northwestern Polytechnical University．Xi’an．7l0072．China)

Abstract：With two rare earth permanent magnet BLDC motors as the control objects．this paper presents a control system that uses a SCM and special integrated circuits to drive an electric bed．The control system can work reliably and steady．The cost of the system is low．At the same time it has a function that is used for memorizing the electric bed’S position．It has a wide marketing foreground．

Key words：electric bed；rare earth permanent magnet BLDC motor；the special integrated circuit；SCM

0 Introduction

Electric bed as a medical device, has been widely used in many hospitals, nursing homes, nursing homes and so on. Most electric bed on the market kinematic mechanism employed is brushed DC motors or asynchronous motors. Brushed DC motors presence reliability, commutation spark, mechanical noise, dust and other problems; asynchronous motor requires 220V AC direct drive, and medical devices require the following safety voltage 36V, brushless DC motors to fundamentally solve the above systems problems, low noise, no dust, high reliability.

Domestic electric bed market imports accounted for a large proportion. Better quality but the price is relatively high. This gives domestic electric bed manufacturer presents a challenge: to improve the cost of similar products.

Demand for the application of medical electric beds, proposed a two degree of freedom of the brushless DC motor control system. The control system consists of a control unit 8051 as the core, two MC33035 chips dedicated control two drives, two drives respectively drive two motors to drive two adjustable parts of the electric bed for up and down movement, and thus meet human comfort Attitude requirements. The program of low system cost, good performance, has good prospects. From the control system architecture, circuit design and software driver three aspects of electrical bed with two degrees of freedom brushless DC motor control system.

1 System Design

1.1 Control System Structure

In order to make health care electric bed can be two degrees of freedom of movement, while simplifying the control system structure, the system uses the control structure shown in Figure 1. The control circuit consists of the microcontroller unit and a drive control circuit.

The system control unit consists of a 8051 composition. It also controls the two drives and two motors work. Main function of the electric bed position memory, and at the same time control the motor start / stop and forward / reverse. SCM enter both motor Hall signals, can determine the motor current operating position, and thus can achieve electric bed location information is stored.

Driver circuit consists of two identical drives composed of two drives respectively drive two identical motors. The two degree of freedom control system can realize the function of human comfort, two successive and separate adjustable segments, adapt the body's neck, back and big legs. Two degrees of freedom motion mechanism electric bed schematic shown in Figure 2. These two zones are adjustable bending back part of the body portion and the legs, two controller controls the two motors respectively drive the two parts of the lifting movement up and down to adjust to the optimum position to meet the requirements of human comfort.

For system reliability and cost considerations, the design uses a M0T0R0I A production company dedicated control chip MC33035. The chip peripheral circuits, small, reliable operation, high stability, low cost, and have some special protection features, including: undervoltage lockout, with optional time delay latching shutdown mode circulating current, internal overheating parking , and the only fault output connected to a microprocessor control system and so on.

For system stability considerations, the design uses a speed feedback device MC33039. The device and the Hall of the rotor position sensor signal input terminal is connected, via the F / V converter to obtain a voltage signal proportional to the speed, the RC network filter circuit is fed back to the negative terminal of the differential amplifier MC33035, thus constituting a simple P regulator to achieve the motor running at a steady speed.

The controller works as follows: three-phase position detection signal from the rotor position detector sent, on the one hand away MC33035, through the combination of chip decoder circuit reversing control terminal, a control terminal start-stop, brake control terminal, the current sense input and other control logic channel state, after the operation, resulting in a three-phase upper and lower arm switching devices to six original control signal inverter M0SFET. Under the three-phase and three-phase bridge PWM control signal on the bridge after the driver amplifier control signal is applied to the inverter switching tube, to produce the power needed to run the machine normal-phase PWM voltage.