View AN266_967532.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

application note AN266/0189 bipolar stepper motor control by pierre payet burin this application note is intended to provide design details for the implementation of a stepper motor control, built around the tea3717. this integrated circuit has been developed to offer control and current regulation of up to 1a, in one win- ding of a bipolar motor. two tea3717s and a minimum of external compo- nents are sufficient to implement the full control fun- ction of a two-phase bipolar stepper motor. the system can be commanded, according to the desired mode of operation, by either fixed or pro- grammable logic. functional description the circuit is organized around a h-bridge configu- rated by four transistors and their integrated free wheel diodes. the ophaseo input controls the switching of the brid- ge transistors and also determines the direction of the current flow in the winding. the signal applied to this input is first gated through a schmidt trigger and then through a delay element so as to avoid a simul- taneous conduction of transistors when direction of current in the bridge is reversed. regulation of winding current is performed by chop- ping action on the power supply for a duration t off de- termined by a monostable. this monostable is triggered by the output level swing of a comparator, to the input of which a volt- age proportional to delivered output current is ap- plied. the current spikes corresponding to the diodes re- verse recovery time are filtered by a low pass filter r c c c to not trigger the comparator. three comparators are available for this purpose : their thresholds are internally fixed ratios of v r input voltage. each of them can be selected individually by using i 1 and i 0 inputs. figure 1 : tea3717 block diagram. 1/13
figure 2 : typical operating sequences. direction of current flow transistors of the bridge are represented by switches : switch open : transistor cut-off application note 2/13
the proposed diagram features two tea3717s each controlling one winding. full-step and fraction-of-a-step operation is performed by combined use of phase and current level selection control inputs. control of bipolar two-phase motor figure 3 : control of a bipolar two-phase motor. application note 3/13
figure 4 : winding currents for rotation in full-and micro-steps. n of steps i a i b 1 2 i i m i i m i i i l 0 -i l -i n of steps i a i b 1 2 3 4 i i i 0 -i -i -i 0 i 0 -i -i -i 0 i i n of steps i a i b 1 2 i i m i m i i i l -i l -i i a ,i b : currents flowing through motor windings. i h ,i m ,i l : current thresholds selected by i o ,i 1 . i:anyofi h ,i m or i l values. i 0 i 1 i 1 0 1 0 1 1 0 0 0 i l i m i h 1/3 step. full step. 1/6 step. n of steps i a i b 1 2 3 4 i i i i i i i i 1/2 step. n of steps i a i b 1 2 i i h i h i h i h i i i m i l -i l -i m -i n of steps i a i b 1 2 i i h i m i i m i h i i i m i l 0 -i l -i m -i 1/8 step. n of steps i a i b 1 2 i i h i m i h i h i m i h i i i m i l i l -i l -i l i m -i 1/4 step. n of steps i a i b 1 2 i i h i m i h i i h i m i h i i i m i l i l 0 -i l -i l -i m -i 1/7 step. 1/5 step. application note 4/13
figure 5 : signals to provide to controller for full-step rotation. full-step operation : this is moto's typical mode of operation. simultaneous power supply to both windings gua- rantees availability of maximum torque. ophaseo inputs determine the direction of current flow in the windings, while inputs i 0 and i 1 at a con- stant level, select the level of this current. this is the simplest type of control implementation. 1 - motor not controlled. 2 - rotation of 7 steps @ i h max. current. 3 - rotation of 6 steps @ i l low current. half-step operation : this mode allows to doublethe motor resolution and also to eliminate certain vibrations. power is applied alternately to one winding and then to both. in half- step position, where only one winding is powered, torque available on motor spindle is at its lowest va- lue. samecontrol signals as thoseused forfull-step ope- ration are applied to ophaseo inputs, and i 0 ,i 1 inputs are used to annul the current in one winding. application note 5/13
figure 6 : signals to provide to controller for full-step rotation. application note 6/13
figure 7 : signals to provide to controller for rotation in 1/8th of a step. micro-step operation: micro-steps of up to 1/8 are obtained by implemen- ting the flow of different supply current levels through both windings. this type of operation may be envisaged if a good rotational regularity is required. some factors reduce the positioning precision of mi- cro-steps : - difference between theoretical value and availa- ble value for winding current, - comparator threshold levels dispersion, - motor winding characteristics dispersion. these factors don't affect full-step position choice of operating frequency motor's rotational speed is determinated by the fre- quency of ph a and ph b signals. this speed is limited by the mechanical characteris- tics and the time constant of l, r circuit formed by the winding. the rate of current increase in the winding depends on v mm . thus, if operation at maximum speed is de- sired, it would be a good practice to work with high supply voltages. figure 8 : phase currents for two differents speeds. application note 7/13
i 1 =i 0 = ph command simultaneous application of control signals to three inputs ph, i 1 and i 0 (or to ph and one of i inputs while the otheris it at high level), is used for half-step ope- ration or unipolar mode. it must be ensured that the time t during which ph = i 1 =i 0 = 1 is higher than the time t min required to annul the winding current. if while the current is still positive, control signals ph :i 0 =i 1 =1 are applied to corresponding inputs,diode d2 or d3 will conduct and cause q1 and q4 to be turned-off,which prevents the current rise in the win- ding and disturbs the proper motor operation. blocking of q1 and q4 is performed by a built-in pro- tection unit and prevents the parasitics generatedby the conductionof d2 and d3 to cause any short-cir- cuit within the bridge. figure 9 : operation with ph = i 1 =i o . normal circuit operation direction of current 1b, 2b identical to 1a, 2a 3b : q1 cut-off due of conduction of d3. slow decrease of current since. v mb v ma = 0 instead of being v mm . 4b 2b 5b 3a 6b 1b. application note 8/13
choice of t off : switching time for current regulation the value of t off determines the quality of the current regulation in one phase. the larger the t off , the more important is the current ripple. value of t off is found from the expression t off = 0.69 r t .c t where 1k w r t 100k w a suitable value of t off for the majority of applications is 30 m s. * t off(max) this is the t off value over which the ripple value be- comes excessive let's k be the desired ripple value and t = l/r the time constant of motor winding, then : t off(max) is given approximately by : t off(max) kt figure 10 : winding current @ off (max). i h ,m,li min k= i h ,m,l l t= r t off (max) =kt application note 9/13
* t off(min) this is the limit under which the current regulation is not guaranteed. even if the current continually exceeds the threshold levels i h ,i m or i l , the device will ensure a minimum conduction time t on(min) which is combination of two periods : -t d : comparator trigger time and transistor desatu- ration time implosed by tea3717 -t' d : this is the time required by v c toreach the com- parator threshold level and is determined by low pass filter r c ,c c . therefore, t off must be selected to be long enough to allow the current to fall to a level below i h ,i m or i l . supply voltage v mm and winding characteristics both determine the value of t off(min) . figure 11 bis : winding current, v e and v c voltages @ t off(min) . figure 11 : winding current @ t off(min) . application note 10/13
selection of r s values three values of motor current i h ,i m and i l are de- terminated by the choice of r s and v r values. the value of r s is calculated such that v ch =r s .i h where v ch = 0.42 5 v r and i h is the maximum motor supply current. achoiceofr s valuearound 1 w will guarantee a fast increase of the winding current and offers the pos- sibility of operation with a voltage around 5 v for v r , and thus is suitable for most applications. it is also possible to vary the motor current in a con- tinuous mode : - by v r adjustment - by feeding back a portion of the voltage drop across r s through a potentiometerwhose wiper is connected to the comparator input. in order to minimize the differential voltage v e -v c due to comparator's input current, care must be ta- ken to avoid the appearance of a large impedance between e and c terminals. appropriates values of pandr c would be : p = 1 k w and r c = 470 w . cabling figure 12 : continuous variation of current level. application note 11/13
sincethe tea3717 operatesin switch mode, it is es- sential to take particular cabling precautions so as to avoid the generation of interferences susceptible to disturb the correct operation of the control elec- tronics. recommended precautions are : - separated ground connection for v mm supply - connection link between rs and the tea3717 must be kept as short as possible - decoupling of v mm by a ceramic capacitor (15 to 150nf) directly connected to the tea3717 and also by an electrolytic of higher value : 10 to 22 m f. - decoupling of v cc . figure 13 : tea3717 pc board layout. application note 12/13
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specifica- tions mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information pre- viously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics all rights reserved sgs-thomson microelectronics group of companies australia - brazil - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thaliand - united kingdom - u.s.a. application note 13/13

▲Up To Search▲

Price & Availability of AN266

	To Download AN266 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .